Arithmetic-geometric mean

Arithmetic-geometric mean
You are encouraged to solve this task according to the task description, using any language you may know.

This page uses content from Wikipedia. The original article was at Arithmetic-geometric mean. The list of authors can be seen in the page history. As with Rosetta Code, the text of Wikipedia is available under the GNU FDL. (See links for details on variance)

Task

Write a function to compute the arithmetic-geometric mean of two numbers.

The arithmetic-geometric mean of two numbers can be (usefully) denoted as $\mathrm {agm} (a,g)$, and is equal to the limit of the sequence:

$a_{0}=a;\qquad g_{0}=g$

$a_{n+1}={\tfrac {1}{2}}(a_{n}+g_{n});\quad g_{n+1}={\sqrt {a_{n}g_{n}}}.$

Since the limit of $a_{n}-g_{n}$ tends (rapidly) to zero with iterations, this is an efficient method.

Demonstrate the function by calculating:

$\mathrm {agm} (1,1/{\sqrt {2}})$

Also see

•   mathworld.wolfram.com/Arithmetic-Geometric Mean

11l

Translation of: Python

F agm(a0, g0, tolerance = 1e-10)
   V an = (a0 + g0) / 2.0
   V gn = sqrt(a0 * g0)
   L abs(an - gn) > tolerance
      (an, gn) = ((an + gn) / 2.0, sqrt(an * gn))
   R an
   
print(agm(1, 1 / sqrt(2)))

Output:

0.847213

360 Assembly

For maximum compatibility, this program uses only the basic instruction set.

AGM      CSECT  
         USING  AGM,R13
SAVEAREA B      STM-SAVEAREA(R15)
         DC     17F'0'
         DC     CL8'AGM'
STM      STM    R14,R12,12(R13)
         ST     R13,4(R15)
         ST     R15,8(R13)
         LR     R13,R15
         ZAP    A,K                a=1
         ZAP    PWL8,K
         MP     PWL8,K
         DP     PWL8,=P'2'
         ZAP    PWL8,PWL8(7)
         BAL    R14,SQRT
         ZAP    G,PWL8             g=sqrt(1/2)
WHILE1   EQU    *                  while a!=g
         ZAP    PWL8,A
         SP     PWL8,G
         CP     PWL8,=P'0'         (a-g)!=0
         BE     EWHILE1
         ZAP    PWL8,A
         AP     PWL8,G
         DP     PWL8,=P'2'
         ZAP    AN,PWL8(7)         an=(a+g)/2
         ZAP    PWL8,A
         MP     PWL8,G
         BAL    R14,SQRT
         ZAP    G,PWL8             g=sqrt(a*g)
         ZAP    A,AN               a=an
         B      WHILE1
EWHILE1  EQU    *
         ZAP    PWL8,A
         UNPK   ZWL16,PWL8
         MVC    CWL16,ZWL16
         OI     CWL16+15,X'F0'
         MVI    CWL16,C'+'
         CP     PWL8,=P'0'
         BNM    *+8
         MVI    CWL16,C'-'
         MVC    CWL80+0(15),CWL16
         MVC    CWL80+9(1),=C'.'   /k  (15-6=9)
         XPRNT  CWL80,80           display a
         L      R13,4(0,R13)
         LM     R14,R12,12(R13)
         XR     R15,R15
         BR     R14
         DS     0F
K        DC     PL8'1000000'       10^6 
A        DS     PL8
G        DS     PL8
AN       DS     PL8
* ****** SQRT   *******************
SQRT     CNOP   0,4                function sqrt(x)
         ZAP    X,PWL8
         ZAP    X0,=P'0'           x0=0
         ZAP    X1,=P'1'           x1=1
WHILE2   EQU    *                  while x0!=x1
         ZAP    PWL8,X0
         SP     PWL8,X1
         CP     PWL8,=P'0'         (x0-x1)!=0
         BE     EWHILE2
         ZAP    X0,X1              x0=x1
         ZAP    PWL16,X
         DP     PWL16,X1
         ZAP    XW,PWL16(8)        xw=x/x1
         ZAP    PWL8,X1
         AP     PWL8,XW
         DP     PWL8,=P'2'
         ZAP    PWL8,PWL8(7)
         ZAP    X2,PWL8            x2=(x1+xw)/2
         ZAP    X1,X2              x1=x2
         B      WHILE2
EWHILE2  EQU    *
         ZAP    PWL8,X1            return x1
         BR     R14
         DS     0F
X        DS     PL8
X0       DS     PL8
X1       DS     PL8
X2       DS     PL8
XW       DS     PL8
* end SQRT
PWL8     DC     PL8'0'
PWL16    DC     PL16'0'
CWL80    DC     CL80' '
CWL16    DS     CL16
ZWL16    DS     ZL16
         LTORG  
         YREGS  
         END    AGM

Output:

+00000000.84721

8th

: epsilon  1.0e-12 ;

with: n

: iter  \ n1 n2 -- n1 n2
    2dup * sqrt >r + 2 / r> ;

: agn  \ n1 n2 -- n
    repeat  iter  2dup epsilon ~= not while!  drop ;

"agn(1, 1/sqrt(2)) = " .  1  1 2 sqrt /  agn  "%.10f" s:strfmt . cr

;with
bye

Output:

agn(1, 1/sqrt(2)) = 0.8472130848

Action!

Library: Action! Tool Kit

Library: Action! Real Math

INCLUDE "H6:REALMATH.ACT"

PROC Agm(REAL POINTER a0,g0,result)
  REAL a,g,prevA,tmp,r2
  
  RealAssign(a0,a)
  RealAssign(g0,g)
  IntToReal(2,r2)
  DO
    RealAssign(a,prevA)
    RealAdd(a,g,tmp)
    RealDiv(tmp,r2,a)
    RealMult(prevA,g,tmp)
    Sqrt(tmp,g)
    IF RealGreaterOrEqual(a,prevA) THEN
      EXIT
    FI
  OD
  RealAssign(a,result)
RETURN

PROC Main()
  REAL r1,r2,tmp,g,res

  Put(125) PutE() ;clear screen

  MathInit()
  IntToReal(1,r1)
  IntToReal(2,r2)
  Sqrt(r2,tmp)
  RealDiv(r1,tmp,g)
  Agm(r1,g,res)
  
  Print("agm(") PrintR(r1)
  Print(",") PrintR(g)
  Print(")=") PrintRE(res)
RETURN

Output:

Screenshot from Atari 8-bit computer

agm(1,.7071067873)=.847213085

Ada

with Ada.Text_IO, Ada.Numerics.Generic_Elementary_Functions;

procedure Arith_Geom_Mean is

   type Num is digits 18; -- the largest value gnat/gcc allows
   package N_IO is new Ada.Text_IO.Float_IO(Num);
   package Math is new Ada.Numerics.Generic_Elementary_Functions(Num);

   function AGM(A, G: Num) return Num is
      Old_G: Num;
      New_G: Num := G;
      New_A: Num := A;
   begin
      loop
         Old_G := New_G;
         New_G := Math.Sqrt(New_A*New_G);
         New_A := (Old_G + New_A) * 0.5;
         exit when (New_A - New_G) <= Num'Epsilon;
         -- Num'Epsilon denotes the relative error when performing arithmetic over Num
      end loop;
      return New_G;
   end AGM;

begin
   N_IO.Put(AGM(1.0, 1.0/Math.Sqrt(2.0)), Fore => 1, Aft => 17, Exp => 0);
end Arith_Geom_Mean;

Output:

0.84721308479397909

ALGOL 68

Algol 68 Genie gives IEEE double precision for REAL quantities, 28 decimal digits for LONG REALs and, by default, 63 decimal digits for LONG LONG REAL though this can be made arbitrarily greater with a pragmat.

Printing out the difference between the means at each iteration nicely demonstrates the quadratic convergence.

BEGIN
   PROC agm = (LONG REAL x, y) LONG REAL :
   BEGIN
      IF x < LONG 0.0 OR y < LONG 0.0 THEN -LONG 1.0
      ELIF x + y = LONG 0.0 THEN LONG 0.0		CO Edge cases CO
      ELSE
         LONG REAL a := x, g := y;
         LONG REAL epsilon := a + g;
         LONG REAL next a := (a + g) / LONG 2.0, next g := long sqrt (a * g);
         LONG REAL next epsilon := ABS (a - g);
         WHILE next epsilon < epsilon
         DO
            print ((epsilon, "   ", next epsilon, newline));
            epsilon := next epsilon;
            a := next a; g := next g;
            next a := (a + g) / LONG 2.0; next g := long sqrt (a * g);
            next epsilon := ABS (a - g)
         OD;
         a
      FI
   END;
   printf (($l(-35,33)l$, agm (LONG 1.0, LONG 1.0 / long sqrt (LONG 2.0))))
END

Output:

+1.707106781186547524400844362e  +0   +2.928932188134524755991556379e  -1
+2.928932188134524755991556379e  -1   +1.265697533955921916929670477e  -2
+1.265697533955921916929670477e  -2   +2.363617660269221214237489508e  -5
+2.363617660269221214237489508e  -5   +8.242743980540458935740117000e -11
+8.242743980540458935740117000e -11   +1.002445937606580000000000000e -21
+1.002445937606580000000000000e -21   +4.595001000000000000000000000e -29
+4.595001000000000000000000000e -29   +4.595000000000000000000000000e -29

0.847213084793979086606499123550000

APL

agd←{(⍺-⍵)<10*¯8:⍺⋄((⍺+⍵)÷2)∇(⍺×⍵)*÷2}
1 agd ÷2*÷2

Output:

0.8472130848

AppleScript

By functional composition:

-- ARITHMETIC GEOMETRIC MEAN -------------------------------------------------

property tolerance : 1.0E-5

-- agm :: Num a => a -> a -> a
on agm(a, g)
    script withinTolerance
        on |λ|(m)
            tell m to ((its an) - (its gn)) < tolerance
        end |λ|
    end script
    
    script nextRefinement
        on |λ|(m)
            tell m
                set {an, gn} to {its an, its gn}
                {an:(an + gn) / 2, gn:(an * gn) ^ 0.5}
            end tell
        end |λ|
    end script
    
    an of |until|(withinTolerance, ¬
        nextRefinement, {an:(a + g) / 2, gn:(a * g) ^ 0.5})
end agm

-- TEST ----------------------------------------------------------------------
on run
    
    agm(1, 1 / (2 ^ 0.5))
    
    --> 0.847213084835
    
end run

-- GENERIC FUNCTIONS ---------------------------------------------------------

-- until :: (a -> Bool) -> (a -> a) -> a -> a
on |until|(p, f, x)
    set mp to mReturn(p)
    set v to x
    tell mReturn(f)
        repeat until mp's |λ|(v)
            set v to |λ|(v)
        end repeat
    end tell
    return v
end |until|

-- Lift 2nd class handler function into 1st class script wrapper 
-- mReturn :: Handler -> Script
on mReturn(f)
    if class of f is script then
        f
    else
        script
            property |λ| : f
        end script
    end if
end mReturn

Output:

0.847213084835

Arturo

agm: function [a,g][
    delta: 1e-15
    [aNew, aOld, gOld]: @[0, a, g]

    while [delta < abs aOld - gOld][
        aNew: 0.5 * aOld + gOld
        gOld: sqrt aOld * gOld
        aOld: aNew
    ]
    return aOld
]

print agm 1.0 1.0/sqrt 2.0</lang>

{{out}}

<pre>0.8472130847939792</pre>

=={{header|AutoHotkey}}==
<syntaxhighlight lang="ahk">agm(a, g, tolerance=1.0e-15){
	While abs(a-g) > tolerance
	{
		an := .5 * (a + g)
		g  := sqrt(a*g)
		a  := an
	}
	return a
}
SetFormat, FloatFast, 0.15
MsgBox % agm(1, 1/sqrt(2))

Output:

0.847213084793979

AWK

#!/usr/bin/awk -f
BEGIN {
	printf "%.16g\n", agm(1.0,sqrt(0.5))
}
function agm(a,g) {
	while (1) {
		a0=a
		a=(a0+g)/2
		g=sqrt(a0*g)
		if (abs(a0-a) < abs(a)*1e-15) break
	}
	return a
}
function abs(x) {
	return (x<0 ? -x : x)
}

Output

0.8472130847939792

BASIC

ANSI BASIC

Works with: Decimal BASIC

100 PROGRAM ArithmeticGeometricMean
110 FUNCTION AGM (A, G)
120    DO
130       LET TA = (A + G) / 2
140       LET G = SQR(A * G)
150       LET Tmp = A
160       LET A = TA
170       LET TA = Tmp
180    LOOP UNTIL A = TA
190    LET AGM = A
200 END FUNCTION
210 REM ********************
220 PRINT AGM(1, 1 / SQR(2))
230 END

Output:

 .84721308479398 

Applesoft BASIC

Same code as Commodore BASIC The BASIC solution works without any changes.

BASIC256

print AGM(1, 1 / sqr(2))
end

function AGM(a, g)
	Do
		ta = (a + g) / 2
		g = sqr(a * g)
		x = a
		a = ta
		ta = x
	until a = ta

	return a
end function

Output:

0.84721308479

BBC BASIC

Works with: BBC BASIC for Windows

      *FLOAT 64
      @% = &1010
      PRINT FNagm(1, 1/SQR(2))
      END
      
      DEF FNagm(a,g)
      LOCAL ta
      REPEAT
        ta = a
        a = (a+g)/2
        g = SQR(ta*g)
      UNTIL a = ta
      = a

Output:

0.8472130847939792

Chipmunk Basic

Works with: Chipmunk Basic version 3.6.4

10 print agm(1,1/sqr(2))
20 end
100 sub agm(a,g)
110   do
120     let ta = (a+g)/2
130     let g = sqr(a*g)
140     let x = a
150     let a = ta
160     let ta = x
170   loop until a = ta
180   agm = a
190 end sub

Output:

0.847213

Commodore BASIC

10 A = 1
20 G = 1/SQR(2)
30 GOSUB 100
40 PRINT A
50 END
100 TA = A
110 A = (A+G)/2
120 G = SQR(TA*G)
130 IF A<TA THEN 100
140 RETURN

Craft Basic

let a = 1
let g = 1 / sqrt(2)

do

	let t = (a + g) / 2
	let g = sqrt(a * g)
	let x = a
	let a = t
	let t = x

loopuntil a = t

print a

Output:

0.85

FreeBASIC

' version 16-09-2015
' compile with: fbc -s console

Function agm(a As Double, g As Double) As Double
    
    Dim As Double t_a 
    
    Do
        t_a = (a + g) / 2
        g = Sqr(a * g)
        Swap a, t_a
    Loop Until a = t_a
    
    Return a
    
End Function

' ------=< MAIN >=------

Print agm(1, 1 / Sqr(2) )

' empty keyboard buffer 
While InKey <> "" : Wend
Print : Print "hit any key to end program"
Sleep
End

Output:

 0.8472130847939792

Gambas

Translation of: FreeBASIC

Public Sub Main() 
  
  Print AGM(1, 1 / Sqr(2))

End

Function AGM(a As Float, g As Float) As Float

  Dim t_a As Float
  
  Do 
    t_a = (a + g) / 2 
    g = Sqr(a * g) 
    Swap a, t_a 
  Loop Until a = t_a 
  
  Return a 

End Function

GW-BASIC

10 A = 1
20 G = 1!/SQR(2!)
30 FOR I=1 TO 20  'twenty iterations is plenty
40 B = (A+G)/2
50 G = SQR(A*G)
60 A = B
70 NEXT I
80 PRINT A

IS-BASIC

100 PRINT AGM(1,1/SQR(2))
110 DEF AGM(A,G)
120   DO
130     LET TA=A
140     LET A=(A+G)/2:LET G=SQR(TA*G)
150   LOOP UNTIL A=TA
160   LET AGM=A
170 END DEF

Liberty BASIC

Works with: Just BASIC

print agm(1, 1/sqr(2))
print using("#.#################",agm(1, 1/sqr(2)))
end

function agm(a,g)
    do
        absdiff = abs(a-g)
        an=(a+g)/2
        gn=sqr(a*g)
        a=an
        g=gn
    loop while abs(an-gn)< absdiff
    agm = a
end function

Output:

0.84721308
0.84721308479397904

Minimal BASIC

Translation of: Commodore BASIC

Works with: IS-BASIC

10 LET A = 1
20 LET G = 1 / SQR(2)
30 GOSUB 60
40 PRINT A
50 STOP
60 LET T = A
70 LET A = (A + G) / 2
80 LET G = SQR(T * G)
90 IF A < T THEN 60
100 RETURN
110 END

Output:

 .84721308

MSX Basic

Works with: MSX BASIC version any

The Commodore BASIC solution works without any changes.

The GW-BASIC solution works without any changes.

PureBasic

Procedure.d AGM(a.d, g.d, ErrLim.d=1e-15)
  Protected.d ta=a+1, tg
  While ta <> a 
    ta=a: tg=g
    a=(ta+tg)*0.5
    g=Sqr(ta*tg)
  Wend
  ProcedureReturn a
EndProcedure

If OpenConsole()
  PrintN(StrD(AGM(1, 1/Sqr(2)), 16))
  Input()
  CloseConsole()
EndIf

Output:

 0.8472130847939792

QuickBASIC

Works with: QBasic

PRINT AGM(1, 1 / SQR(2))
END

FUNCTION AGM (a, g)
    DO
        ta = (a + g) / 2
        g = SQR(a * g)
        SWAP a, ta
    LOOP UNTIL a = ta
    
    AGM = a
END FUNCTION

Output:

.8472131

Quite BASIC

Translation of: Commodore BASIC

10 LET A = 1
20 LET G = 1 / SQR(2)
30 GOSUB 100
40 PRINT A
50 END
100 LET T = A
110 LET A = (A + G) / 2
120 LET G = SQR(T * G)
130 IF A < T THEN 100
140 RETURN

Output:

0.8472130847939792

Run BASIC

print agm(1, 1/sqr(2))
print agm(1,1/2^.5)
print using("#.############################",agm(1, 1/sqr(2)))

function agm(agm,g)
 while agm
   an  = (agm + g)/2
   gn  = sqr(agm*g)
   if abs(agm-g) <= abs(an-gn) then exit while
   agm = an
   g   = gn
 wend
end function

Output:

0.847213085
0.847213085
0.8472130847939791165772005376

Sinclair ZX81 BASIC

Translation of: COBOL

Works with 1k of RAM.

The specification calls for a function. Sadly that is not available to us, so this program uses a subroutine: pass the arguments in the global variables A and G, and the result will be returned in AGM. The performance is quite acceptable. Note that the subroutine clobbers A and G, so you should save them if you want to use them again.

Better precision than this is not easily obtainable on the ZX81, unfortunately.

 10 LET A=1
 20 LET G=1/SQR 2
 30 GOSUB 100
 40 PRINT AGM
 50 STOP
100 LET A0=A
110 LET A=(A+G)/2
120 LET G=SQR (A0*G)
130 IF ABS(A-G)>.00000001 THEN GOTO 100
140 LET AGM=A
150 RETURN

Output:

0.84721309

TI-83 BASIC

1→A:1/sqrt(2)→G
While abs(A-G)>e-15
(A+G)/2→B
sqrt(AG)→G:B→A
End
A

Output:

.8472130848

True BASIC

Works with: QBasic

FUNCTION AGM (a, g)
    DO
       LET ta = (a + g) / 2
       LET g = SQR(a * g)
       LET x = a
       LET a = ta
       LET ta = x
    LOOP UNTIL a = ta

    LET AGM = a
END FUNCTION

PRINT AGM(1, 1 / SQR(2))
END

Output:

.84721308

VBA

Private Function agm(a As Double, g As Double, Optional tolerance As Double = 0.000000000000001) As Double
    Do While Abs(a - g) > tolerance
        tmp = a
        a = (a + g) / 2
        g = Sqr(tmp * g)
        Debug.Print a
    Loop
    agm = a
End Function
Public Sub main()
    Debug.Print agm(1, 1 / Sqr(2))
End Sub

Output:

 0,853553390593274 
 0,847224902923494 
 0,847213084835193 
 0,847213084793979 
 0,847213084793979 

VBScript

Translation of: BBC BASIC

Function agm(a,g)
	Do Until a = tmp_a
		tmp_a = a
		a = (a + g)/2
		g = Sqr(tmp_a * g)
	Loop
	agm = a
End Function

WScript.Echo agm(1,1/Sqr(2))

Output:

0.847213084793979

Yabasic

print AGM(1, 1 / sqrt(2))
end

sub AGM(a, g)
	repeat
		ta = (a + g) / 2
		g = sqrt(a * g)
		x = a
		a = ta
		ta = x
	until a = ta

	return a
end sub

Output:

0.847213

Visual Basic .NET

Translation of: C#

Double, Decimal Versions

Imports System.Math
Imports System.Console

Module Module1

    Function CalcAGM(ByVal a As Double, ByVal b As Double) As Double
        Dim c As Double, d As Double = 0, ld As Double = 1
        While ld <> d : c = a : a = (a + b) / 2 : b = Sqrt(c * b)
            ld = d : d = a - b : End While : Return b
    End Function

    Function DecSqRoot(ByVal v As Decimal) As Decimal
        Dim r As Decimal = CDec(Sqrt(CDbl(v))), t As Decimal = 0, d As Decimal = 0, ld As Decimal = 1
        While ld <> d : t = v / r : r = (r + t) / 2
            ld = d : d = t - r : End While : Return t
    End Function

    Function CalcAGM(ByVal a As Decimal, ByVal b As Decimal) As Decimal
        Dim c As Decimal, d As Decimal = 0, ld As Decimal = 1
        While ld <> d : c = a : a = (a + b) / 2 : b = DecSqRoot(c * b)
            ld = d : d = a - b : End While : Return b
    End Function

    Sub Main(ByVal args As String())
        WriteLine("Double  result: {0}", CalcAGM(1.0, DecSqRoot(0.5)))
        WriteLine("Decimal result: {0}", CalcAGM(1D, DecSqRoot(0.5D)))
        If System.Diagnostics.Debugger.IsAttached Then ReadKey()
    End Sub

End Module

Output:

Double  result: 0.847213084793979
Decimal result: 0.8472130847939790866064991235

System.Numerics

Translation of: C#

Library: System.Numerics

Imports System.Math
Imports System.Console
Imports BI = System.Numerics.BigInteger
 
Module Module1
 
    Function BIP(ByVal leadDig As Char, ByVal numDigs As Integer) As BI
        BIP = BI.Parse(leadDig & New String("0"c, numDigs))
    End Function
 
    Function IntSqRoot(ByVal v As BI, ByVal res As BI) As BI ' res is the initial guess of the square root
        Dim d As BI = 0, dl As BI = 1
        While dl <> d :  IntSqRoot = v / res : res = (res + IntSqRoot) / 2
            dl = d : d = IntSqRoot - res : End While
    End Function
 
    Function CalcByAGM(ByVal digits As Integer) As BI
        Dim a As BI = BIP("1"c, digits), ' value is 1, extended to required number of digits
            c as BI, ' a temporary variable for swapping a and b
            diff As BI = 0, ldiff As BI = 1 ' difference of a and b, last difference
        CalcByAGM = BI.Parse(String.Format("{0:0.00000000000000000}", ' initial value of square root of 0.5
            Sqrt(0.5)).Substring(2) & New String("0"c, digits - 17))
        CalcByAGM = IntSqRoot(BIP("5"c, (digits << 1) - 1), CalcByAGM) ' value is now the square root of 0.5
        While ldiff <> diff : c = a : a = (a + CalcByAGM) >> 1 : CalcByAGM = IntSqRoot(c * CalcByAGM, a)
            ldiff = diff : diff = a - CalcByAGM : End While
    End Function
 
    Sub Main(ByVal args As String())
        Dim digits As Integer = 25000
        If args.Length > 0 Then Integer.TryParse(args(0), digits) : _
            If digits < 1 OrElse digits > 999999 Then digits = 25000
        WriteLine("0.{0}", CalcByAGM(digits))
        If System.Diagnostics.Debugger.IsAttached Then ReadKey()
    End Sub
 
End Module

Output:

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48

ZX Spectrum Basic

Translation of: ERRE

10 LET a=1: LET g=1/SQR 2
20 LET ta=a
30 LET a=(a+g)/2
40 LET g=SQR (ta*g)
50 IF a<ta THEN GO TO 20
60 PRINT a

Output:

0.84721309

bc

/* Calculate the arithmethic-geometric mean of two positive
 * numbers x and y.
 * Result will have d digits after the decimal point.
 */
define m(x, y, d) {
    auto a, g, o

    o = scale
    scale = d
    d = 1 / 10 ^ d

    a = (x + y) / 2
    g = sqrt(x * y)
    while ((a - g) > d) {
        x = (a + g) / 2
        g = sqrt(a * g)
        a = x
    }

    scale = o
    return(a)
}    

scale = 20
m(1, 1 / sqrt(2), 20)

Output:

.84721308479397908659

BQN

AGM ← {
    (|𝕨-𝕩) ≤ 1e¯15? 𝕨;
    (0.5×𝕨+𝕩) 𝕊 √𝕨×𝕩  
}

1 AGM 1÷√2

Output:

0.8472130847939792

C

Basic

#include<math.h>
#include<stdio.h>
#include<stdlib.h>

double agm( double a, double g ) {
   /* arithmetic-geometric mean */
   double iota = 1.0E-16;
   double a1, g1;
   
   if( a*g < 0.0 ) {
      printf( "arithmetic-geometric mean undefined when x*y<0\n" );
      exit(1);
   }

   while( fabs(a-g)>iota ) {
      a1 = (a + g) / 2.0;
      g1 = sqrt(a * g);

      a = a1;
      g = g1;
   }
   
   return a;
}

int main( void ) {
   double x, y;
   printf( "Enter two numbers: " );
   scanf( "%lf%lf", &x, &y );
   printf( "The arithmetic-geometric mean is %lf\n", agm(x, y) );
   return 0;
}

Original output:

Enter two numbers: 1.0 2.0
The arithmetic-geometric mean is 1.456791

Task output, the second input (0.707) is 1/sqrt(2) correct to 3 decimal places:

Enter two numbers: 1 0.707
The arithmetic-geometric mean is 0.847155

GMP

/*Arithmetic Geometric Mean of 1 and 1/sqrt(2)

  Nigel_Galloway
  February 7th., 2012.
*/

#include "gmp.h"

void agm (const mpf_t in1, const mpf_t in2, mpf_t out1, mpf_t out2) {
	mpf_add (out1, in1, in2);
	mpf_div_ui (out1, out1, 2);
	mpf_mul (out2, in1, in2);
	mpf_sqrt (out2, out2);
}

int main (void) {
	mpf_set_default_prec (65568);
	mpf_t x0, y0, resA, resB;

	mpf_init_set_ui (y0, 1);
	mpf_init_set_d (x0, 0.5);
	mpf_sqrt (x0, x0);
	mpf_init (resA);
	mpf_init (resB);

	for(int i=0; i<7; i++){
		agm(x0, y0, resA, resB);
		agm(resA, resB, x0, y0);
	}
	gmp_printf ("%.20000Ff\n", x0);
	gmp_printf ("%.20000Ff\n\n", y0);

	return 0;
}

The first couple of iterations produces:

0.853
0.840

Then 7 iterations produces:

0.847213084793979086606499123482191636481445910326942185060579372659734004834134759723200293994611229942122285625233410963097962665830871059699713635983384251176326814289060389706768601616650048281188721897713309411767462019944392962902167289194499507231677897346863947606671057980557852173140349398304200422119216039839553595098193641293716340646029599967970599434351602031842648756950242174863855405981954581601742417887854192758804162719012085587685648326834140431218400804035809204559494313877815120926522254574397124286820766340954733674599621792665535348625686118543308626287287287563010835563193570668714785639088982115108836352147696979612621832943228417868113768445170018146021913694027020945996683513596327880804274345481744587363220025153952936265806614198365616491626259607434723706616902353080017375312847852558430631907454274934152685790655269406003147591020332746719686124796325510554648902820855297439651249940096625528660675804487353892185701401167716976535014084952476848993257321337028984668939194665861873752966387562266045914777044204681089256584408380320409106190031537067341195941010074743310599055058205243260099516927924174782169767810616836977141107392733439215501430220070873673659622721492587761928510523803670268904639096219076636442355380859029452340651900133423451058383417121805142550039237011113254111446126289062541335505266436535958245521562933975182514706501346410470569793556813066063293733450387109770972948759171790158173202815782884871499313408154933423677970447127859376185950851466773645546792016159342239971429840707888822790326567515965284358177957272848083564899635044041407342261101833835469759626633304220849998523007427039302772434749797179732645525465430198316949684610986907439050680137661192529197709384412997070158894931666611619945922650113111839663525025305616464315872084545229887754751772727476567216489829182392388952072076428397108847059603569219929218319015481412807665926982944644571492396663299730758139049576224389624231752095073190184244624423709864272811495111808228260538624846176751801409831274972576519837564923569028002161749055314272081534395405955635763711272816570597373374429700390560401563886630722257003892301591123769601215800817790778633512408624310735715837659265045466527873378744448344063102447570396812554539822664303534164130356138016341655752655897529445211668734512201912274667331915712407637538211069681410769263900748331757433967523196603308649735713838741960989838322028826948821913028193669499544222406972761686213695116578388850121990961606554546115432531481642493326947970041594914763231129205935165189979433500459762882172926259180894055084314663937825483351395501906533708720620640240770560758487964998436515927282645344286366154191425857771067561850172780332871751951893050318055052454260223355229007714181287986543511879180063562795936247682677864122494603381260826282540988953125276775346562432792145112295555160318184331336929617230417838551571255674049834166659269695800089537245730576945422753721602096871914703988784663672432627061911270717165908246400416799411204056571036408300024192943985530739946565396778104927010554103595133394321999250666762020783946955537605517964010097492188563113010178138885787938131720959480625392013009836502879176958279859052799477219417979970249430621584194688853281154977215799601944096234776861440850757392842988237593968232236705803341347746231128976258593243766317789749110772619097044895222045096307255155900938249040213648077920347672150485684460225544099928261631743126422857876289833806507220230103717531492635046310601885737725670066183812905806389545081270313113710437161358334880658339554312179013483988332164130576352447125115394720666703301013487165163241138288176398396295261211412632197959650986567867552507607604240959075175230219461045325643332496149012535333292237238689481278850201359663053760558493589283916304694038878549600274714871978014576595790495858022600660995249673643249668334617601066081567069751423818665036108388522097616550025160731149921612947757901997292486896382206038087602762816723701668191066335857751546503813342367223476420265585655884641601021054048985561871147358849763784064864267981865044863190774703822867114351511230036070865742988647714667473375011434581885279700605621172469217484718069486625119947289344427037830462070735493805287272062156063071882868580564521110696708028569906982576917722099867195996850779068144349493280497681154368046325993869307623507099951829512958112123570724538335482619075239515827309824818054966589790916886798407170779370595904577584091047341310960419411135775662072733779783320379730113767265853574771027978140972130961214239385473746276961504130795283737288205065871915225976508402779699176117539300672549249122984508236297556872271106584943553385049453263873648980460665597995436016950309279009245005785647723587619884898603441219534079536900299641197454906074160097885953766072290516077242859007090115663913836429904122082676962979786764903235649998199076599743987054864876909102491192709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00

The limit (19,740) is imposed by the accuracy (65568). Using 6 iterations would produce a less accurate result. At 7 iterations increasing the 65568 would mean we already have 38,000 or so digits accurate.

C#

namespace RosettaCode.ArithmeticGeometricMean
{
    using System;
    using System.Collections.Generic;
    using System.Globalization;

    internal static class Program
    {
        private static double ArithmeticGeometricMean(double number,
                                                      double otherNumber,
                                                      IEqualityComparer<double>
                                                          comparer)
        {
            return comparer.Equals(number, otherNumber)
                       ? number
                       : ArithmeticGeometricMean(
                           ArithmeticMean(number, otherNumber),
                           GeometricMean(number, otherNumber), comparer);
        }

        private static double ArithmeticMean(double number, double otherNumber)
        {
            return 0.5 * (number + otherNumber);
        }

        private static double GeometricMean(double number, double otherNumber)
        {
            return Math.Sqrt(number * otherNumber);
        }

        private static void Main()
        {
            Console.WriteLine(
                ArithmeticGeometricMean(1, 0.5 * Math.Sqrt(2),
                                        new RelativeDifferenceComparer(1e-5)).
                    ToString(CultureInfo.InvariantCulture));
        }

        private class RelativeDifferenceComparer : IEqualityComparer<double>
        {
            private readonly double _maximumRelativeDifference;

            internal RelativeDifferenceComparer(double maximumRelativeDifference)
            {
                _maximumRelativeDifference = maximumRelativeDifference;
            }

            public bool Equals(double number, double otherNumber)
            {
                return RelativeDifference(number, otherNumber) <=
                       _maximumRelativeDifference;
            }

            public int GetHashCode(double number)
            {
                return number.GetHashCode();
            }

            private static double RelativeDifference(double number,
                                                     double otherNumber)
            {
                return AbsoluteDifference(number, otherNumber) /
                       Norm(number, otherNumber);
            }

            private static double AbsoluteDifference(double number,
                                                     double otherNumber)
            {
                return Math.Abs(number - otherNumber);
            }

            private static double Norm(double number, double otherNumber)
            {
                return 0.5 * (Math.Abs(number) + Math.Abs(otherNumber));
            }
        }
    }
}

Output:

0.847213084835193

Note that the last 5 digits are spurious, as maximumRelativeDifference was only specified to be 1e-5. Using 1e-11 instead will give the result 0.847213084793979, which is as far as double can take it.

Using Decimal Type

using System;
 
class Program {
 
    static Decimal DecSqRoot(Decimal v) {
        Decimal r = (Decimal)Math.Sqrt((double)v), t = 0, d = 0, ld = 1; 
        while (ld != d) { t = v / r; r = (r + t) / 2;
            ld = d; d = t - r; } return t; }
 
    static Decimal CalcAGM(Decimal a, Decimal b) {
        Decimal c, d = 0, ld = 1; while (ld != d) { ld = d; c = a;
            d = (a = (a + b) / 2) - (b = DecSqRoot(c * b)); } return b; }
 
    static void Main(string[] args) {
        Console.WriteLine(CalcAGM(1M, DecSqRoot(0.5M)));
        if (System.Diagnostics.Debugger.IsAttached) Console.ReadKey();
    }
}

Output:

0.8472130847939790866064991235

C# with System.Numerics

Library: System.Numerics

Even though the System.Numerics library directly supports only BigInteger (and not big rationals or big floating point numbers), it can be coerced into making this calculation. One just has to keep track of the decimal place and multiply by a very large constant.

using static System.Math;
using static System.Console;
using BI = System.Numerics.BigInteger;
 
class Program {
 
    static BI BIP(char leadDig, int numDigs) { // makes big constant 
        return BI.Parse(leadDig + new string('0', numDigs)); }
 
    static BI IntSqRoot(BI v, BI res) { // res is the initial guess
        BI term = 0, d = 0, dl = 1; while (dl != d) { term = v / res; res = (res + term) >> 1;
            dl = d; d = term - res; } return term; }
 
    static BI CalcByAGM(int digs) { // note: a and b are hard-coded for this RC task
        BI a = BIP('1', digs), // value of 1, extended to required number of digits 
            b = BI.Parse(string.Format("{0:0.00000000000000000}", Sqrt(0.5)).Substring(2) +
                new string('0', digs - 17)), // initial guess for square root of 0.5
            c, // temporary variable to swap a and b
            diff = 0, ldiff = 1; // difference of a and b, last difference
        b = IntSqRoot(BIP('5', (digs << 1) - 1), b); // value of square root of 0.5
        while (ldiff != diff) { ldiff = diff; c = a; a = (a + b) >> 1;
            diff = a - (b = IntSqRoot(c * b, a)); } return b; }
 
    static void Main(string[] args) {
        int digits = 25000; if (args.Length > 0) {
            int.TryParse(args[0], out digits);
            if (digits < 1 || digits > 999999) digits = 25000; }
        WriteLine("0.{0}", CalcByAGM(digits));
        if (System.Diagnostics.Debugger.IsAttached) ReadKey(); }
}

Output:

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48

C++

#include<bits/stdc++.h>
using namespace std;
#define _cin	ios_base::sync_with_stdio(0);	cin.tie(0);
#define rep(a, b)	for(ll i =a;i<=b;++i)

double agm(double a, double g)		//ARITHMETIC GEOMETRIC MEAN
{	double epsilon = 1.0E-16,a1,g1;
	if(a*g<0.0)
	{	cout<<"Couldn't find arithmetic-geometric mean of these numbers\n";
		exit(1);
	}
	while(fabs(a-g)>epsilon)
	{	a1 = (a+g)/2.0;
		g1 = sqrt(a*g);
		a = a1;
		g = g1;
	}
	return a;
}

int main()
{	_cin;    //fast input-output
	double x, y;
	cout<<"Enter X and Y: ";	//Enter two numbers
	cin>>x>>y;
	cout<<"\nThe Arithmetic-Geometric Mean of "<<x<<" and "<<y<<" is "<<agm(x, y);
return 0;
}

Enter X and Y: 1.0 2.0 
The Arithmetic-Geometric Mean of 1.0 and 2.0 is 1.45679103104690677028543177584651857614517211914062

Clojure

(ns agmcompute
  (:gen-class))

; Java Arbitray Precision Library
(import '(org.apfloat Apfloat ApfloatMath))

(def precision 70)
(def one (Apfloat. 1M precision))
(def two (Apfloat. 2M precision))
(def half (Apfloat. 0.5M precision))
(def isqrt2 (.divide one  (ApfloatMath/pow two half)))
(def TOLERANCE (Apfloat. 0.000000M precision))

(defn agm [a g]
  " Simple AGM Loop calculation "
       (let [THRESH 1e-65                 ; done when error less than threshold or we exceed max loops
             MAX-LOOPS 1000000]
        (loop [[an gn] [a g], cnt 0]
            (if (or (< (ApfloatMath/abs (.subtract an gn)) THRESH)
                    (> cnt MAX-LOOPS))
              an
              (recur [(.multiply (.add an gn) half) (ApfloatMath/pow (.multiply an gn) half)]
                     (inc cnt))))))

(println  (agm one isqrt2))

Output:

8.47213084793979086606499123482191636481445910326942185060579372659734e-1

COBOL

IDENTIFICATION DIVISION.
PROGRAM-ID. ARITHMETIC-GEOMETRIC-MEAN-PROG.
DATA DIVISION.
WORKING-STORAGE SECTION.
01  AGM-VARS.
    05 A       PIC 9V9(16).
    05 A-ZERO  PIC 9V9(16).
    05 G       PIC 9V9(16).
    05 DIFF    PIC 9V9(16) VALUE 1.
* Initialize DIFF with a non-zero value, otherwise AGM-PARAGRAPH
* is never performed at all.
PROCEDURE DIVISION.
TEST-PARAGRAPH.
    MOVE    1 TO A.
    COMPUTE G = 1 / FUNCTION SQRT(2).
* The program will run with the test values. If you would rather
* calculate the AGM of numbers input at the console, comment out
* TEST-PARAGRAPH and un-comment-out INPUT-A-AND-G-PARAGRAPH.
* INPUT-A-AND-G-PARAGRAPH.
*     DISPLAY 'Enter two numbers.'
*     ACCEPT  A.
*     ACCEPT  G.
CONTROL-PARAGRAPH.
    PERFORM AGM-PARAGRAPH UNTIL DIFF IS LESS THAN 0.000000000000001.
    DISPLAY A.
    STOP RUN.
AGM-PARAGRAPH.
    MOVE     A TO A-ZERO.
    COMPUTE  A = (A-ZERO + G) / 2.
    MULTIPLY A-ZERO BY G GIVING G.
    COMPUTE  G = FUNCTION SQRT(G).
    SUBTRACT A FROM G GIVING DIFF.
    COMPUTE  DIFF = FUNCTION ABS(DIFF).

Output:

0.8472130847939792

Common Lisp

(defun agm (a0 g0 &optional (tolerance 1d-8))
  (loop for a = a0 then (* (+ a g) 5d-1)
     and g = g0 then (sqrt (* a g))
     until (< (abs (- a g)) tolerance)
     finally (return a)))

Output:

CL-USER> (agm 1d0 (/ 1d0 (sqrt 2d0)))
0.8472130848351929d0
CL-USER> (agm 1d0 (/ 1d0 (sqrt 2d0)) 1d-10)
0.8472130848351929d0
CL-USER> (agm 1d0 (/ 1d0 (sqrt 2d0)) 1d-12)
0.8472130847939792d0

D

import std.stdio, std.math, std.meta, std.typecons;

real agm(real a, real g, in int bitPrecision=60) pure nothrow @nogc @safe {
    do {
        //{a, g} = {(a + g) / 2.0, sqrt(a * g)};
        AliasSeq!(a, g) = tuple((a + g) / 2.0, sqrt(a * g));
    } while (feqrel(a, g) < bitPrecision);
    return a;
}

void main() @safe {
    writefln("%0.19f", agm(1, 1 / sqrt(2.0)));
}

Output:

0.8472130847939790866

All the digits shown are exact.

Delphi

Library: System.SysUtils

Translation of: C#

program geometric_mean;

{$APPTYPE CONSOLE}

uses
  System.SysUtils;

function Fabs(value: Double): Double;
begin
  Result := value;
  if Result < 0 then
    Result := -Result;
end;

function agm(a, g: Double):Double;
var
  iota, a1, g1: Double;
begin
  iota := 1.0E-16;
  if a * g < 0.0 then
  begin
    Writeln('arithmetic-geometric mean undefined when x*y<0');
    exit(1);
  end;

  while Fabs(a - g) > iota do
  begin
    a1 := (a + g) / 2.0;
    g1 := sqrt(a * g);
    a := a1;
    g := g1;
  end;
  Exit(a);
end;

var
  x, y: Double;

begin
  Write('Enter two numbers:');
  Readln(x, y);
  writeln(format('The arithmetic-geometric mean is  %.6f', [agm(x, y)]));
  readln;
end.

Output:

Enter two numbers:1
2
The arithmetic-geometric mean is  1,456791

dc

>>> 200 k ? sbsa [lalb +2/ lalb *vsb dsa lb - 0!=:]ds:xlap
?> 1 1 2 v /

Output:

.8472130847939790866064991234821916364814459103269421850605793726597\
34004834134759723200293994611229942122285625233410963097962665830871\
05969971363598338425117632681428906038970676860161665004828118868

You can change the precision (200 by default)

EasyLang

Translation of: AWK

func agm a g .
   repeat
      a0 = a
      a = (a0 + g) / 2
      g = sqrt (a0 * g)
      until abs (a0 - a) < abs (a) * 1e-15
   .
   return a
.
numfmt 16 0
print agm 1 sqrt 0.5

EchoLisp

We use the (~= a b) operator which tests for |a - b| < ε = (math-precision).

(lib 'math)

(define (agm a g) 
    (if (~= a g) a 
       (agm (// (+ a g ) 2) (sqrt (* a g)))))

(math-precision)
    → 0.000001 ;; default
(agm 1 (/ 1 (sqrt 2)))
    → 0.8472130848351929
(math-precision 1.e-15)
    → 1e-15
(agm 1 (/ 1 (sqrt 2)))
    → 0.8472130847939792

Elixir

defmodule ArithhGeom do
  def mean(a,g,tol) when abs(a-g) <= tol, do: a
  def mean(a,g,tol) do
    mean((a+g)/2,:math.pow(a*g, 0.5),tol)
  end
end

IO.puts ArithhGeom.mean(1,1/:math.sqrt(2),0.0000000001)

Output:

0.8472130848351929

Erlang

%% Arithmetic Geometric Mean of 1 and 1 / sqrt(2)
%% Author: Abhay Jain

-module(agm_calculator).
-export([find_agm/0]).
-define(TOLERANCE, 0.0000000001).

find_agm() ->
    A = 1,
    B = 1 / (math:pow(2, 0.5)),
    AGM = agm(A, B),
    io:format("AGM = ~p", [AGM]).
    
agm (A, B) when abs(A-B) =< ?TOLERANCE ->
    A;
agm (A, B) ->
    A1 = (A+B) / 2,
    B1 = math:pow(A*B, 0.5),
    agm(A1, B1).

Output:

AGM = 0.8472130848351929

ERRE

PROGRAM AGM

!
! for rosettacode.org
!

!$DOUBLE

PROCEDURE AGM(A,G->A)
   LOCAL TA
   REPEAT
      TA=A
      A=(A+G)/2
      G=SQR(TA*G)
   UNTIL A=TA
END PROCEDURE

BEGIN
   AGM(1.0,1/SQR(2)->A)
   PRINT(A)
END PROGRAM

F#

Translation of: OCaml

let rec agm a g precision  =
    if precision > abs(a - g) then a else
    agm (0.5 * (a + g)) (sqrt (a * g)) precision

printfn "%g" (agm 1. (sqrt(0.5)) 1e-15)

Output

0.847213

Factor

USING: kernel math math.functions prettyprint ;
IN: rosetta-code.arithmetic-geometric-mean

: agm ( a g -- a' g' ) 2dup [ + 0.5 * ] 2dip * sqrt ;

1 1 2 sqrt / [ 2dup - 1e-15 > ] [ agm ] while drop .

Output:

0.8472130847939792

Forth

: agm ( a g -- m )
  begin
    fover fover f+ 2e f/
    frot frot f* fsqrt
    fover fover 1e-15 f~
  until
  fdrop ;

1e  2e -0.5e f**  agm f.   \ 0.847213084793979

Fortran

A Fortran 77 implementation

      function agm(a,b)
      implicit none
      double precision agm,a,b,eps,c
      parameter(eps=1.0d-15)
   10 c=0.5d0*(a+b)
      b=sqrt(a*b)
      a=c
      if(a-b.gt.eps*a) go to 10
      agm=0.5d0*(a+b)
      end
      program test
      implicit none
      double precision agm
      print*,agm(1.0d0,1.0d0/sqrt(2.0d0))
      end

Futhark

This example is incorrect. Please fix the code and remove this message. Details: Futhark's syntax has changed, so this example will not compile

import "futlib/math"

fun agm(a: f64, g: f64): f64 =
  let eps = 1.0E-16
  loop ((a,g)) = while f64.abs(a-g) > eps do
    ((a+g) / 2.0,
     f64.sqrt (a*g))
  in a

fun main(x: f64, y: f64): f64 =
  agm(x,y)

Go

package main

import (
    "fmt"
    "math"
)

const ε = 1e-14

func agm(a, g float64) float64 {
    for math.Abs(a-g) > math.Abs(a)*ε {
        a, g = (a+g)*.5, math.Sqrt(a*g)
    }
    return a
}

func main() {
    fmt.Println(agm(1, 1/math.Sqrt2))
}

Output:

0.8472130847939792

Groovy

Translation of: Java

Solution:

double agm (double a, double g) {
    double an = a, gn = g
    while ((an-gn).abs() >= 10.0**-14) { (an, gn) = [(an+gn)*0.5, (an*gn)**0.5] }
    an
}

Test:

println "agm(1, 0.5**0.5) = agm(1, ${0.5**0.5}) = ${agm(1, 0.5**0.5)}"
assert (0.8472130847939792 - agm(1, 0.5**0.5)).abs() <= 10.0**-14

Output:

agm(1, 0.5**0.5) = agm(1, 0.7071067811865476) = 0.8472130847939792

Haskell

-- Return an approximation to the arithmetic-geometric mean of two numbers.
-- The result is considered accurate when two successive approximations are
-- sufficiently close, as determined by "eq".
agm :: (Floating a) => a -> a -> ((a, a) -> Bool) -> a
agm a g eq = snd $ until eq step (a, g)
  where
    step (a, g) = ((a + g) / 2, sqrt (a * g))

-- Return the relative difference of the pair.  We assume that at least one of
-- the values is far enough from 0 to not cause problems.
relDiff :: (Fractional a) => (a, a) -> a
relDiff (x, y) =
  let n = abs (x - y)
      d = (abs x + abs y) / 2
  in n / d

main :: IO ()
main = do
  let equal = (< 0.000000001) . relDiff
  print $ agm 1 (1 / sqrt 2) equal

Output:

0.8472130847527654

Icon and Unicon

procedure main(A)
    a := real(A[1]) | 1.0
    g := real(A[2]) | (1 / 2^0.5)
    epsilon := real(A[3])
    write("agm(",a,",",g,") = ",agm(a,g,epsilon))
end

procedure agm(an, gn, e)
    /e := 1e-15
    while abs(an-gn) > e do {
       ap := (an+gn)/2.0
       gn := (an*gn)^0.5
       an := ap
       }
    return an
end

Output:

->agm
agm(1.0,0.7071067811865475) = 0.8472130847939792
->

J

This one is probably worth not naming, in J, because there are so many interesting variations.

First, the basic approach (with display precision set to 16 digits, which slightly exceeds the accuracy of 64 bit IEEE floating point arithmetic):

mean=: +/ % #
   (mean , */ %:~ #)^:_] 1,%%:2
0.8472130847939792 0.8472130847939791

This is the limit -- it stops when values are within a small epsilon of previous calculations. We can ask J for unique values (which also means -- unless we specify otherwise -- values within a small epsilon of each other, for floating point values):

   ~.(mean , */ %:~ #)^:_] 1,%%:2
0.8472130847939792

Another variation would be to show intermediate values, in the limit process:

   (mean, */ %:~ #)^:a: 1,%%:2
                 1 0.7071067811865475
0.8535533905932737 0.8408964152537145
0.8472249029234942 0.8472012667468915
0.8472130848351929 0.8472130847527654
0.8472130847939792 0.8472130847939791

Arbitrary Precision

Another variation would be to use arbitrary precision arithmetic in place of floating point arithmetic.

Borrowing routines from that page, but going with a default of approximately 100 digits of precision:

DP=:101

round=: DP&$: : (4 : 0)
 b %~ <.1r2+y*b=. 10x^x
)

sqrt=: DP&$: : (4 : 0) " 0
 assert. 0<:y
 %/ <.@%: (2 x: (2*x) round y)*10x^2*x+0>.>.10^.y
)

ln=: DP&$: : (4 : 0) " 0
 assert. 0<y
 m=. <.0.5+2^.y
 t=. (<:%>:) (x:!.0 y)%2x^m
 if. x<-:#":t do. t=. (1+x) round t end.
 ln2=. 2*+/1r3 (^%]) 1+2*i.>.0.5*(%3)^.0.5*0.1^x+>.10^.1>.m
 lnr=. 2*+/t   (^%]) 1+2*i.>.0.5*(|t)^.0.5*0.1^x
 lnr + m * ln2
)

exp=: DP&$: : (4 : 0) " 0
 m=. <.0.5+y%^.2
 xm=. x+>.m*10^.2
 d=. (x:!.0 y)-m*xm ln 2
 if. xm<-:#":d do. d=. xm round d end.
 e=. 0.1^xm
 n=. e (>i.1:) a (^%!@]) i.>.a^.e [ a=. |y-m*^.2 
 (2x^m) * 1++/*/\d%1+i.n
)

We are also going to want a routine to display numbers with this precision, and we are going to need to manage epsilon manually, and we are going to need an arbitrary root routine:

fmt=:[: ;:inv DP&$: : (4 :0)&.>
  x{.deb (x*2j1)":y
)

root=: ln@] exp@% [

epsilon=: 1r9^DP

Some example uses:

   fmt sqrt 2
1.414213562373095048801688724209698078569671875376948073176679737990732478462107038850387534327641572
   fmt *~sqrt 2
2.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
   fmt epsilon
0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000418
   fmt 2 root 2
1.414213562373095048801688724209698078569671875376948073176679737990732478462107038850387534327641572

Note that 2 root 2 is considerably slower than sqrt 2. The price of generality. So, while we could define geometric mean generally, a desire for good performance pushes us to use a routine specialized for two numbers:

geomean=: */ root~ #
geomean2=: [: sqrt */

A quick test to make sure these can be equivalent:

   fmt geomean 3 5
3.872983346207416885179265399782399610832921705291590826587573766113483091936979033519287376858673517
   fmt geomean2 3 5
3.872983346207416885179265399782399610832921705291590826587573766113483091936979033519287376858673517

Now for our task example:

   fmt (mean, geomean2)^:(epsilon <&| -/)^:a: 1,%sqrt 2
1.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 0.707106781186547524400844362104849039284835937688474036588339868995366239231053519425193767163820786
0.853553390593273762200422181052424519642417968844237018294169934497683119615526759712596883581910393 0.840896415253714543031125476233214895040034262356784510813226085974924754953902239814324004199292536
0.847224902923494152615773828642819707341226115600510764553698010236303937284714499763460443890601464 0.847201266746891460403631453693352397963981013612000500823295747923488191871327668107581434542353536
0.847213084835192806509702641168086052652603564606255632688496879079896064578021083935520939216477500 0.847213084752765366704298051779902070392110656059452583317776227659438896688518556753569298762449381
0.847213084793979086607000346473994061522357110332854108003136553369667480633269820344545118989463440 0.847213084793979086605997900490389211440534858586261300461413929971399281619068666682569108141224710
0.847213084793979086606499123482191636481445984459557704232275241670533381126169243513557113565344075 0.847213084793979086606499123482191636481445836194326665888883503648934628542100275932846717790147361
0.847213084793979086606499123482191636481445910326942185060579372659734004834134759723201915677745718 0.847213084793979086606499123482191636481445910326942185060579372659734004834134759723198672311476741
0.847213084793979086606499123482191636481445910326942185060579372659734004834134759723200293994611229 0.847213084793979086606499123482191636481445910326942185060579372659734004834134759723200293994611229

We could of course extract out only a representative final value, but it's obvious enough, and showing how rapidly this converges is fun.

Java

/*
 * Arithmetic-Geometric Mean of 1 & 1/sqrt(2)
 * Brendan Shaklovitz
 * 5/29/12
 */
public class ArithmeticGeometricMean {

    public static double agm(double a, double g) {
        double a1 = a;
        double g1 = g;
        while (Math.abs(a1 - g1) >= 1.0e-14) {
            double arith = (a1 + g1) / 2.0;
            double geom = Math.sqrt(a1 * g1);
            a1 = arith;
            g1 = geom;
        }
        return a1;
    }

    public static void main(String[] args) {
        System.out.println(agm(1.0, 1.0 / Math.sqrt(2.0)));
    }
}

Output:

0.8472130847939792

JavaScript

ES5

function agm(a0, g0) {
    var an = (a0 + g0) / 2,
        gn = Math.sqrt(a0 * g0);
    while (Math.abs(an - gn) > tolerance) {
        an = (an + gn) / 2, gn = Math.sqrt(an * gn)
    }
    return an;
}

agm(1, 1 / Math.sqrt(2));

ES6

(() => {
    'use strict';

    // ARITHMETIC-GEOMETRIC MEAN

    // agm :: Num a => a -> a -> a
    let agm = (a, g) => {
            let abs = Math.abs,
                sqrt = Math.sqrt;

            return until(
                    m => abs(m.an - m.gn) < tolerance,
                    m => {
                        return {
                            an: (m.an + m.gn) / 2,
                            gn: sqrt(m.an * m.gn)
                        };
                    }, {
                        an: (a + g) / 2,
                        gn: sqrt(a * g)
                    }
                )
                .an;
        },

        // GENERIC

        // until :: (a -> Bool) -> (a -> a) -> a -> a
        until = (p, f, x) => {
            let v = x;
            while (!p(v)) v = f(v);
            return v;
        };


    // TEST

    let tolerance = 0.000001;


    return agm(1, 1 / Math.sqrt(2));

})();

Output:

0.8472130848351929

jq

Works with: jq version 1.4

Naive version that assumes tolerance is appropriately specified:

def naive_agm(a; g; tolerance):
  def abs: if . < 0 then -. else . end;
  def _agm:
     # state [an,gn]
     if ((.[0] - .[1])|abs) > tolerance 
     then [add/2, ((.[0] * .[1])|sqrt)] | _agm 
     else .
     end;
  [a, g] | _agm | .[0] ;

This version avoids an infinite loop if the requested tolerance is too small:

def agm(a; g; tolerance):
  def abs: if . < 0 then -. else . end;
  def _agm:
     # state [an,gn, delta]
     ((.[0] - .[1])|abs) as $delta
     | if $delta == .[2] and $delta < 10e-16 then .
       elif $delta > tolerance
       then [ .[0:2]|add / 2, ((.[0] * .[1])|sqrt), $delta] | _agm
       else . 
       end;
  if tolerance <= 0 then error("specified tolerance must be > 0")
  else [a, g, 0] | _agm | .[0]
  end ;
 
# Example:
agm(1; 1/(2|sqrt); 1e-100)

Output:

$ jq -n -f Arithmetic-geometric_mean.jq
0.8472130847939792

Julia

Works with: Julia version 1.2

function agm(x, y, e::Real = 5)
    (x ≤ 0 || y ≤ 0 || e ≤ 0) && throw(DomainError("x, y must be strictly positive"))
    g, a = minmax(x, y)
    while e * eps(x) < a - g
        a, g = (a + g) / 2, sqrt(a * g)
    end
    a
end
 
x, y = 1.0, 1 / √2
println("# Using literal-precision float numbers:")
@show agm(x, y)

println("# Using half-precision float numbers:")
x, y = Float32(x), Float32(y)
@show agm(x, y)

println("# Using ", precision(BigFloat), "-bit float numbers:")
x, y = big(1.0), 1 / √big(2.0)
@show agm(x, y)

The ε for this calculation is given as a positive integer multiple of the machine ε for x.

Output:

# Using literal-precision float numbers:
agm(x, y) = 0.8472130847939792
# Using half-precision float numbers:
agm(x, y) = 0.84721315f0
# Using 256-bit float numbers:
agm(x, y) = 8.472130847939790866064991234821916364814459103269421850605793726597340048341323e-01

Klingphix

Translation of: Oforth

include ..\Utilitys.tlhy

:agm [ over over + 2 / rot rot * sqrt ] [ over over tostr swap tostr # ] while drop ;

1 1 2 sqrt / agm

pstack

" " input

Translation of: F#

include ..\Utilitys.tlhy

:agm %a %g %p !p !g !a
	$p $a $g - abs > ( [$a] [.5 $a $g + * $a $g * sqrt $p agm] ) if ;

1 .5 sqrt 1e-15 agm

pstack

" " input

Output:

(0.847213)

Kotlin

// version 1.0.5-2

fun agm(a: Double, g: Double): Double {
    var aa = a             // mutable 'a'
    var gg = g             // mutable 'g'
    var ta: Double         // temporary variable to hold next iteration of 'aa'
    val epsilon = 1.0e-16  // tolerance for checking if limit has been reached

    while (true) {
        ta = (aa + gg) / 2.0
        if (Math.abs(aa - ta) <= epsilon) return ta
        gg = Math.sqrt(aa * gg)
        aa = ta
    }
}

fun main(args: Array<String>) {
    println(agm(1.0, 1.0 / Math.sqrt(2.0)))
}

Output:

0.8472130847939792

Lambdatalk

{def eps 1e-15}
-> eps

{def agm
  {lambda {:a :g}
   {if {> {abs {- :a :g}} {eps}}
    then {agm {/ {+ :a :g} 2} 
              {sqrt {* :a :g}}}
    else :a }}} 
-> agm

{agm 1 {/ 1 {sqrt 2}}}
-> 0.8472130847939792

Multi-precision version using the lib_BN library

{BN.DEC 70} 
->   70 digits
{def EPS {BN./ 1 {BN.pow 10 45}}}
-> EPS 
 
{def AGM
  {lambda {:a :g}
   {if {= {BN.compare {BN.abs {BN.- :a :g}} {EPS}} 1}
    then {AGM {BN./ {BN.+ :a :g} 2}
              {BN.sqrt {BN.* :a :g}}}
    else :a }}}  
-> AGM

{AGM 1 {BN./ 1 {BN.sqrt 2}}}
-> 0.8472130847939790866064991234821916364814459103269421850605793726597339

LFE

(defun agm (a g)
  (agm a g 1.0e-15))

(defun agm (a g tol)
  (if (=< (- a g) tol)
    a
    (agm (next-a a g)
         (next-g a g)
         tol)))

(defun next-a (a g)
  (/ (+ a g) 2))

(defun next-g (a g)
  (math:sqrt (* a g)))

Usage:

> (agm 1 (/ 1 (math:sqrt 2)))
0.8472130847939792

LiveCode

function agm aa,g
    put abs(aa-g) into absdiff 
    put (aa+g)/2 into aan
    put sqrt(aa*g) into gn
    repeat while abs(aan - gn) < absdiff
        put abs(aa-g) into absdiff 
        put (aa+g)/2 into aan
        put sqrt(aa*g) into gn
        put aan into aa
        put gn into g
    end repeat
    return aa
end agm

Example

put agm(1, 1/sqrt(2))
-- ouput
-- 0.847213

LLVM

; This is not strictly LLVM, as it uses the C library function "printf".
; LLVM does not provide a way to print values, so the alternative would be
; to just load the string into memory, and that would be boring.

; Additional comments have been inserted, as well as changes made from the output produced by clang such as putting more meaningful labels for the jumps

$"ASSERTION" = comdat any
$"OUTPUT" = comdat any

@"ASSERTION" = linkonce_odr unnamed_addr constant [48 x i8] c"arithmetic-geometric mean undefined when x*y<0\0A\00", comdat, align 1
@"OUTPUT" = linkonce_odr unnamed_addr constant [42 x i8] c"The arithmetic-geometric mean is %0.19lf\0A\00", comdat, align 1

;--- The declarations for the external C functions
declare i32 @printf(i8*, ...)
declare void @exit(i32) #1
declare double @sqrt(double) #1

declare double @llvm.fabs.f64(double) #2

;----------------------------------------------------------------
;-- arithmetic geometric mean
define double @agm(double, double) #0 {
    %3 = alloca double, align 8                     ; allocate local g
    %4 = alloca double, align 8                     ; allocate local a
    %5 = alloca double, align 8                     ; allocate iota
    %6 = alloca double, align 8                     ; allocate a1
    %7 = alloca double, align 8                     ; allocate g1
    store double %1, double* %3, align 8            ; store param g in local g
    store double %0, double* %4, align 8            ; store param a in local a
    store double 1.000000e-15, double* %5, align 8  ; store 1.0e-15 in iota (1.0e-16 was causing the program to hang)

    %8 = load double, double* %4, align 8           ; load a
    %9 = load double, double* %3, align 8           ; load g
    %10 = fmul double %8, %9                        ; a * g
    %11 = fcmp olt double %10, 0.000000e+00         ; a * g < 0.0
    br i1 %11, label %enforce, label %loop

enforce:
    %12 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([48 x i8], [48 x i8]* @"ASSERTION", i32 0, i32 0))
    call void @exit(i32 1) #6
    unreachable

loop:
    %13 = load double, double* %4, align 8          ; load a
    %14 = load double, double* %3, align 8          ; load g
    %15 = fsub double %13, %14                      ; a - g
    %16 = call double @llvm.fabs.f64(double %15)    ; fabs(a - g)
    %17 = load double, double* %5, align 8          ; load iota
    %18 = fcmp ogt double %16, %17                  ; fabs(a - g) > iota
    br i1 %18, label %loop_body, label %eom

loop_body:
    %19 = load double, double* %4, align 8          ; load a
    %20 = load double, double* %3, align 8          ; load g
    %21 = fadd double %19, %20                      ; a + g
    %22 = fdiv double %21, 2.000000e+00             ; (a + g) / 2.0
    store double %22, double* %6, align 8           ; store %22 in a1

    %23 = load double, double* %4, align 8          ; load a
    %24 = load double, double* %3, align 8          ; load g
    %25 = fmul double %23, %24                      ; a * g
    %26 = call double @sqrt(double %25) #4          ; sqrt(a * g)
    store double %26, double* %7, align 8           ; store %26 in g1

    %27 = load double, double* %6, align 8          ; load a1
    store double %27, double* %4, align 8           ; store a1 in a

    %28 = load double, double* %7, align 8          ; load g1
    store double %28, double* %3, align 8           ; store g1 in g

    br label %loop

eom:
    %29 = load double, double* %4, align 8          ; load a
    ret double %29                                  ; return a
}

;----------------------------------------------------------------
;-- main
define i32 @main() #0 {
    %1 = alloca double, align 8                     ; allocate x
    %2 = alloca double, align 8                     ; allocate y

    store double 1.000000e+00, double* %1, align 8  ; store 1.0 in x

    %3 = call double @sqrt(double 2.000000e+00) #4  ; calculate the square root of two
    %4 = fdiv double 1.000000e+00, %3               ; divide 1.0 by %3
    store double %4, double* %2, align 8            ; store %4 in y

    %5 = load double, double* %2, align 8           ; reload y
    %6 = load double, double* %1, align 8           ; reload x
    %7 = call double @agm(double %6, double %5)     ; agm(x, y)

    %8 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([42 x i8], [42 x i8]* @"OUTPUT", i32 0, i32 0), double %7)

    ret i32 0                                       ; finished
}

attributes #0 = { noinline nounwind optnone uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { noreturn "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #2 = { nounwind readnone speculatable }
attributes #4 = { nounwind }
attributes #6 = { noreturn }

Output:

The arithmetic-geometric mean is 0.8472130847939791654

Logo

to about :a :b
  output and [:a - :b < 1e-15] [:a - :b > -1e-15]
end
to agm :arith :geom
  if about :arith :geom [output :arith]
  output agm (:arith + :geom)/2  sqrt (:arith * :geom)
end

show agm 1 1/sqrt 2

Lua

function agm(a, b, tolerance)
    if not tolerance or tolerance < 1e-15 then
        tolerance = 1e-15
    end
    repeat
        a, b = (a + b) / 2, math.sqrt(a * b)
    until math.abs(a-b) < tolerance
    return a
end

print(string.format("%.15f", agm(1, 1 / math.sqrt(2))))

Output:

   0.847213084793979

M2000 Interpreter

Module Checkit {
      Function Agm {
      \\ new stack constructed at calling the Agm() with two values
            Repeat {
                  Read a0, b0
                  Push  Sqrt(a0*b0), (a0+b0)/2
                  ' last pushed first read 
            } Until Stackitem(1)==Stackitem(2)
            =Stackitem(1)
            \\ stack deconstructed at exit of function
      } 
      Print Agm(1,1/Sqrt(2))
}
Checkit

Maple

Maple provides this function under the name GaussAGM. To compute a floating point approximation, use evalf.

> evalf( GaussAGM( 1, 1 / sqrt( 2 ) ) ); # default precision is 10 digits
                              0.8472130847

> evalf[100]( GaussAGM( 1, 1 / sqrt( 2 ) ) ); # to 100 digits
0.847213084793979086606499123482191636481445910326942185060579372659\
    7340048341347597232002939946112300

Alternatively, if one or both arguments is already a float, Maple will compute a floating point approximation automatically.

> GaussAGM( 1.0, 1 / sqrt( 2 ) );            
                              0.8472130847

Mathematica/Wolfram Language

To any arbitrary precision, just increase PrecisionDigits

PrecisionDigits = 85;
AGMean[a_, b_] := FixedPoint[{ Tr@#/2, Sqrt[Times@@#] }&, N[{a,b}, PrecisionDigits]]〚1〛

AGMean[1, 1/Sqrt[2]]
0.8472130847939790866064991234821916364814459103269421850605793726597340048341347597232

MATLAB / Octave

function [a,g]=agm(a,g)
%%arithmetic_geometric_mean(a,g)
	while (1)
		a0=a;
		a=(a0+g)/2;
		g=sqrt(a0*g);
	if (abs(a0-a) < a*eps) break; end; 
	end;
end

octave:26> agm(1,1/sqrt(2))
ans =  0.84721

Maxima

agm(a, b) := %pi/4*(a + b)/elliptic_kc(((a - b)/(a + b))^2)$

agm(1, 1/sqrt(2)), bfloat, fpprec: 85;
/* 8.472130847939790866064991234821916364814459103269421850605793726597340048341347597232b-1 */

МК-61/52

П1	<->	П0	1	ВП	8	/-/	П2	ИП0	ИП1
-	ИП2	-	/-/	x<0	31	ИП1	П3	ИП0	ИП1
*	КвКор	П1	ИП0	ИП3	+	2	/	П0	БП
08	ИП0	С/П

Modula-2

Translation of: C

MODULE AGM;
FROM EXCEPTIONS IMPORT AllocateSource,ExceptionSource,GetMessage,RAISE;
FROM LongConv IMPORT ValueReal;
FROM LongMath IMPORT sqrt;
FROM LongStr IMPORT RealToStr;
FROM Terminal IMPORT ReadChar,Write,WriteString,WriteLn;

VAR
    TextWinExSrc : ExceptionSource;

PROCEDURE ReadReal() : LONGREAL;
VAR
    buffer : ARRAY[0..63] OF CHAR;
    i : CARDINAL;
    c : CHAR;
BEGIN
    i := 0;

    LOOP
        c := ReadChar();
        IF ((c >= '0') AND (c <= '9')) OR (c = '.') THEN
            buffer[i] := c;
            Write(c);
            INC(i)
        ELSE
            WriteLn;
            EXIT
        END
    END;

    buffer[i] := 0C;
    RETURN ValueReal(buffer)
END ReadReal;

PROCEDURE WriteReal(r : LONGREAL);
VAR
    buffer : ARRAY[0..63] OF CHAR;
BEGIN
    RealToStr(r, buffer);
    WriteString(buffer)
END WriteReal;

PROCEDURE AGM(a,g : LONGREAL) : LONGREAL;
CONST iota = 1.0E-16;
VAR a1, g1 : LONGREAL;
BEGIN
    IF a * g < 0.0 THEN
        RAISE(TextWinExSrc, 0, "arithmetic-geometric mean undefined when x*y<0")
    END;

    WHILE ABS(a - g) > iota DO
        a1 := (a + g) / 2.0;
        g1 := sqrt(a * g);

        a := a1;
        g := g1
    END;

    RETURN a
END AGM;

VAR
    x, y, z: LONGREAL;
BEGIN
    WriteString("Enter two numbers: ");
    x := ReadReal();
    y := ReadReal();
    WriteReal(AGM(x, y));
    WriteLn
END AGM.

Output:

Enter two numbers: 1.0
2.0
1.456791031046900

Enter two numbers: 1.0
0.707
0.847154622368330

NetRexx

Translation of: Java

/* NetRexx */
options replace format comments java crossref symbols nobinary

numeric digits 18
parse arg a_ g_ .
if a_ = '' | a_ = '.' then a0 = 1
                      else a0 = a_
if g_ = '' | g_ = '.' then g0 = 1 / Math.sqrt(2)
                      else g0 = g_

say agm(a0, g0)

return

-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
method agm(a0, g0) public static returns Rexx
  a1 = a0
  g1 = g0
  loop while (a1 - g1).abs() >= Math.pow(10, -14)
    temp = (a1 + g1) / 2
    g1 = Math.sqrt(a1 * g1)
    a1 = temp
    end
  return a1 + 0

Output:

0.8472130847939792

NewLISP

(define (a-next a g) (mul 0.5 (add a g)))

(define (g-next a g) (sqrt (mul a g)))

(define (amg a g tolerance)
	(if (<= (sub a g) tolerance)
		a
		(amg (a-next a g) (g-next a g) tolerance)
	)
)

(define quadrillionth 0.000000000000001)

(define root-reciprocal-2 (div 1.0 (sqrt 2.0)))

(println 
	"To the nearest one-quadrillionth, "
	"the arithmetic-geometric mean of "
	"1 and the reciprocal of the square root of 2 is "
	(amg 1.0 root-reciprocal-2 quadrillionth)
)

Nim

import math

proc agm(a, g: float,delta: float = 1.0e-15): float =
  var 
    aNew: float = 0
    aOld: float = a
    gOld: float = g
  while (abs(aOld - gOld) > delta):
    aNew = 0.5 * (aOld + gOld)
    gOld = sqrt(aOld * gOld)
    aOld = aNew
  result = aOld

echo agm(1.0,1.0/sqrt(2.0))

Output:

8.4721308479397917e-01

See first 24 iterations:

from math import sqrt
from strutils import parseFloat, formatFloat, ffDecimal

proc agm(x,y: float): tuple[resA,resG: float] =
  var
    a,g: array[0 .. 23,float]

  a[0] = x
  g[0] = y

  for n in 1 .. 23:
    a[n] = 0.5 * (a[n - 1] + g[n - 1])
    g[n] = sqrt(a[n - 1] * g[n - 1])

  (a[23], g[23])

var t = agm(1, 1/sqrt(2.0))

echo("Result A: " & formatFloat(t.resA, ffDecimal, 24))
echo("Result G: " & formatFloat(t.resG, ffDecimal, 24))

Oberon-2

Works with: oo2c

MODULE Agm;
IMPORT
  Math := LRealMath,
  Out;

CONST
  epsilon = 1.0E-15;

PROCEDURE Of*(a,g: LONGREAL): LONGREAL;
VAR
  na,ng,og: LONGREAL;
BEGIN
  na := a; ng := g;
  LOOP
    og := ng;
    ng := Math.sqrt(na * ng); 
    na := (na + og) * 0.5;
    IF na - ng <= epsilon THEN EXIT END
  END;
  RETURN ng;
END Of;

BEGIN 
  Out.LongReal(Of(1,1 / Math.sqrt(2)),0,0);Out.Ln
END Agm.

Output:

8.4721308479397905E-1

Objeck

Translation of: Java

class ArithmeticMean {
  function : Amg(a : Float, g : Float) ~ Nil {
    a1 := a;
    g1 := g;
    while((a1-g1)->Abs() >= Float->Power(10, -14)) {
        tmp := (a1+g1)/2.0;
        g1 := Float->SquareRoot(a1*g1);
        a1 := tmp;
    };
    a1->PrintLine();
  }
  
  function : Main(args : String[]) ~ Nil {
    Amg(1,1/Float->SquareRoot(2));
  }
}

Output:

0.847213085

OCaml

let rec agm a g tol =
  if tol > abs_float (a -. g) then a else
  agm (0.5*.(a+.g)) (sqrt (a*.g)) tol

let _ = Printf.printf "%.16f\n" (agm 1.0 (sqrt 0.5) 1e-15)

Output

0.8472130847939792

Oforth

: agm   \ a b -- m
   while( 2dup <> ) [ 2dup + 2 / -rot * sqrt ] drop ;

Usage :

1 2 sqrt inv agm

Output:

0.847213084793979

OOC

import math // import for sqrt() function

amean: func (x: Double, y: Double) -> Double {
  (x + y) / 2.
}
gmean: func (x: Double, y: Double) -> Double {
  sqrt(x * y)
}
agm: func (a: Double, g: Double) -> Double {
  while ((a - g) abs() > pow(10, -12)) {
    (a1, g1) := (amean(a, g), gmean(a, g))
    (a, g) = (a1, g1)
  }
  a
}

main: func {
  "%.16f" printfln(agm(1., sqrt(0.5)))
}

Output

0.8472130847939792

ooRexx

numeric digits 20
say agm(1, 1/rxcalcsqrt(2,16))

::routine agm
  use strict arg a, g
  numeric digits 20

  a1 = a
  g1 = g

  loop while abs(a1 - g1) >= 1e-14
      temp = (a1 + g1)/2
      g1 = rxcalcsqrt(a1*g1,16)
      a1 = temp
  end
  return a1+0

::requires rxmath LIBRARY

Output:

0.8472130847939791968

PARI/GP

Built-in:

agm(1,1/sqrt(2))

Iteration:

agm2(x,y)=if(x==y,x,agm2((x+y)/2,sqrt(x*y))

Pascal

Works with: Free_Pascal

Library: GMP

Port of the C example:

Program ArithmeticGeometricMean;

uses
  gmp;
  
procedure agm (in1, in2: mpf_t; var out1, out2: mpf_t);
begin
  mpf_add (out1, in1, in2);
  mpf_div_ui (out1, out1, 2);
  mpf_mul (out2, in1, in2);
  mpf_sqrt (out2, out2);
end;
 
const
  nl = chr(13)+chr(10);
var
  x0, y0, resA, resB: mpf_t;
  i: integer;
begin
  mpf_set_default_prec (65568);

  mpf_init_set_ui (y0, 1);
  mpf_init_set_d (x0, 0.5);
  mpf_sqrt (x0, x0);
  mpf_init (resA);
  mpf_init (resB);

  for i := 0 to 6 do
  begin
    agm(x0, y0, resA, resB);
    agm(resA, resB, x0, y0);
  end;
  mp_printf ('%.20000Ff'+nl, @x0);
  mp_printf ('%.20000Ff'+nl+nl, @y0);
end.

Output is as long as the C example.

Perl

#!/usr/bin/perl -w

my ($a0, $g0, $a1, $g1);

sub agm($$) {
    $a0 = shift;
    $g0 = shift;
    do { 
        $a1 = ($a0 + $g0)/2; 
        $g1 = sqrt($a0 * $g0); 
        $a0 = ($a1 + $g1)/2; 
        $g0 = sqrt($a1 * $g1); 
    } while ($a0 != $a1); 
    return $a0;
}

print agm(1, 1/sqrt(2))."\n";

Output:

0.847213084793979

Phix

function agm(atom a, atom g, atom tolerance=1.0e-15)
    while abs(a-g)>tolerance do
        {a,g} = {(a + g)/2,sqrt(a*g)}
        printf(1,"%0.15g\n",a)
    end while
    return a
end function
?agm(1,1/sqrt(2))   -- (rounds to 10 d.p.)

Output:

0.853553390593274
0.847224902923494
0.847213084835193
0.847213084793979
0.8472130848

Phixmonti

include ..\Utilitys.pmt

1.0e-15 var tolerance

def test
    over over - abs tolerance >
enddef

def agm /# n1 n2 -- n3 #/
    test while
        over over + 2 / rot rot * sqrt
    test endwhile
enddef

1 1 2 sqrt / agm tostr ?

PHP

define('PRECISION', 13);

function agm($a0, $g0, $tolerance = 1e-10)                                                                                                    
{
    // the bc extension deals in strings and cannot convert
    // floats in scientific notation by itself - hence
    // this manual conversion to a string
    $limit = number_format($tolerance, PRECISION, '.', '');
    $an    = $a0;
    $gn    = $g0;
    do {
        list($an, $gn) = array(
            bcdiv(bcadd($an, $gn), 2), 
            bcsqrt(bcmul($an, $gn)),
        );  
    } while (bccomp(bcsub($an, $gn), $limit) > 0); 

    return $an;
}

bcscale(PRECISION);
echo agm(1, 1 / bcsqrt(2));

Output:

0.8472130848350

Picat

main =>
    println(agm(1.0, 1/sqrt(2))).

agm(A,G) = A, A-G < 1.0e-10 => true.
agm(A,G) = agm((A+G)/2, sqrt(A*G)).

Output:

0.847213084835193

PicoLisp

(scl 80)

(de agm (A G)
   (do 7
      (prog1 (/ (+ A G) 2)
         (setq G (sqrt A G)  A @) ) ) )

(round
   (agm 1.0 (*/ 1.0 1.0 (sqrt 2.0 1.0)))
   70 )

Output:

-> "0.8472130847939790866064991234821916364814459103269421850605793726597340"

PL/I

arithmetic_geometric_mean:           /* 31 August 2012 */
   procedure options (main);
   declare (a, g, t) float (18);

   a = 1; g = 1/sqrt(2.0q0);
   put skip list ('The arithmetic-geometric mean of ' || a || ' and ' || g || ':');
   do until (abs(a-g) < 1e-15*a);
      t = (a + g)/2; g = sqrt(a*g);
      a = t;
      put skip data (a, g);
   end;
   put skip list ('The result is:', a);
end arithmetic_geometric_mean;

Results:

The arithmetic-geometric mean of  1.00000000000000000E+0000 and  7.07106781186547524E-0001: 
A= 8.53553390593273762E-0001                    G= 8.40896415253714543E-0001;
A= 8.47224902923494153E-0001                    G= 8.47201266746891460E-0001;
A= 8.47213084835192807E-0001                    G= 8.47213084752765367E-0001;
A= 8.47213084793979087E-0001                    G= 8.47213084793979087E-0001;
The result is:           8.47213084793979087E-0001 

Potion

Input values should be floating point

sqrt = (x) :
   xi = 1
   7 times :
      xi = (xi + x / xi) / 2
   .
   xi
.

agm = (x, y) :
   7 times :
      a = (x + y) / 2
      g = sqrt(x * y)
      x = a
      y = g
   .
   x
.

PowerShell

function agm ([Double]$a, [Double]$g) {
    [Double]$eps = 1E-15
    [Double]$a1 = [Double]$g1 = 0
    while([Math]::Abs($a - $g) -gt $eps) {
        $a1, $g1 = $a, $g
        $a = ($a1 + $g1)/2
        $g = [Math]::Sqrt($a1*$g1)
    }
    [pscustomobject]@{
        a = "$a"
        g = "$g"
    }
}
agm 1 (1/[Math]::Sqrt(2))

Output:

a                                                      g                                                     
-                                                      -                                                     
0.847213084793979                                      0.847213084793979                                                                       

Prolog

agm(A,G,A) :- abs(A-G) < 1.0e-15, !.
agm(A,G,Res) :- A1 is (A+G)/2.0, G1 is sqrt(A*G),!, agm(A1,G1,Res).

?- agm(1,1/sqrt(2),Res).
Res = 0.8472130847939792.

Python

The calculation generates two new values from two existing values which is the classic example for the use of assignment to a list of values in the one statement, so ensuring a_n g_n are only calculated from a_n-1 g_n-1.

Basic Version

from math import sqrt

def agm(a0, g0, tolerance=1e-10):
    """
    Calculating the arithmetic-geometric mean of two numbers a0, g0.

    tolerance     the tolerance for the converged 
                  value of the arithmetic-geometric mean
                  (default value = 1e-10)
    """
    an, gn = (a0 + g0) / 2.0, sqrt(a0 * g0)
    while abs(an - gn) > tolerance:
        an, gn = (an + gn) / 2.0, sqrt(an * gn)
    return an

print agm(1, 1 / sqrt(2))

Output:

 0.847213084835

Multi-Precision Version

from decimal import Decimal, getcontext

def agm(a, g, tolerance=Decimal("1e-65")):
    while True:
        a, g = (a + g) / 2, (a * g).sqrt()
        if abs(a - g) < tolerance:
            return a

getcontext().prec = 70
print agm(Decimal(1), 1 / Decimal(2).sqrt())

Output:

0.847213084793979086606499123482191636481445910326942185060579372659734

All the digits shown are correct.

Quackery

  [ $ "bigrat.qky" loadfile ] now!

  [ temp put
    [ 2over 2over temp share approx=
      iff 2drop done 
      2over 2over v*
      temp share vsqrt drop 
      dip [ dip [ v+ 2 n->v v/ ] ]
      again ] 
    base share temp take ** round ]  is agm ( n/d n/d n --> n/d )

  1 n->v 
  2 n->v 125 vsqrt drop 1/v 
  125 agm
  2dup
  125 point$ echo$ cr cr 
  swap say "Num: " echo cr
  say      "Den: " echo

Output:

Rational approximation good to 125 decimal places.

0.84721308479397908660649912348219163648144591032694218506057937265973400483413475972320029399461122994212228562523341096309796

Num: 25070388762104643854110087231213532104992429267859552974434367463980830062627660152123462048041692668477424160883635235463565
Den: 29591597689029002472001305353032599592592702596663142670993392754036951453351898973702304260474345315746065192782388085181246

R

arithmeticMean <- function(a, b) { (a + b)/2 }
geometricMean <- function(a, b) { sqrt(a * b) }

arithmeticGeometricMean <- function(a, b) {
  rel_error <- abs(a - b) / pmax(a, b) 
  if (all(rel_error < .Machine$double.eps, na.rm=TRUE)) {
    agm <- a
    return(data.frame(agm, rel_error));
  }
  Recall(arithmeticMean(a, b), geometricMean(a, b))  
}

agm <- arithmeticGeometricMean(1, 1/sqrt(2))
print(format(agm, digits=16))

Output:

                 agm             rel_error
1 0.8472130847939792 1.310441309927519e-16

This function also works on vectors a and b (following the spirit of R):

a <- c(1, 1, 1)
b <- c(1/sqrt(2), 1/sqrt(3), 1/2)
agm <- arithmeticGeometricMean(a, b)
print(format(agm, digits=16))

Output:

                 agm             rel_error
1 0.8472130847939792 1.310441309927519e-16
2 0.7741882646460426 0.000000000000000e+00
3 0.7283955155234534 0.000000000000000e+00

Racket

This version uses Racket's normal numbers:

#lang racket
(define (agm a g [ε 1e-15])
  (if (<= (- a g) ε)
      a
      (agm (/ (+ a g) 2) (sqrt (* a g)) ε)))

(agm 1 (/ 1 (sqrt 2)))

Output:

0.8472130847939792

This alternative version uses arbitrary precision floats:

#lang racket
(require math/bigfloat)
(bf-precision 200)
(bfagm 1.bf (bf/ (bfsqrt 2.bf)))

Output:

(bf #e0.84721308479397908660649912348219163648144591032694218506057918)

Raku

(formerly Perl 6)

sub agm( $a is copy, $g is copy ) {
    ($a, $g) = ($a + $g)/2, sqrt $a * $g until $a ≅ $g;
    return $a;
}
 
say agm 1, 1/sqrt 2;

Output:

0.84721308479397917

It's also possible to write it recursively:

sub agm( $a, $g ) {
    $a ≅ $g ?? $a !! agm(|@$_)
        given ($a + $g)/2, sqrt $a * $g;
}

say agm 1, 1/sqrt 2;

We can also get a bit fancy and use a converging sequence of complex numbers:

sub agm {
  ($^z, {(.re+.im)/2 + (.re*.im).sqrt*1i} ... * ≅ *)
  .tail.re
}
 
say agm 1 + 1i/2.sqrt

Raven

define agm  use  $a, $g, $errlim
    # $errlim $g $a "%d %g %d\n" print
    $a 1.0  +   as $t
    repeat $a 1.0 * $g - abs -15 exp10 $a *  >   while
        $a $g + 2 /   as $t
        $a $g * sqrt  as $g
        $t as $a
        $g $a $t  "t: %g a: %g g: %g\n" print
    $a


16   1 2 sqrt /   1   agm   "agm: %.15g\n" print

Output:

t: 0.853553 a: 0.853553 g: 0.840896
t: 0.847225 a: 0.847225 g: 0.847201
t: 0.847213 a: 0.847213 g: 0.847213
t: 0.847213 a: 0.847213 g: 0.847213
agm: 0.847213084793979

Relation

function agm(x,y)
set a = x
set g = y
while abs(a - g) > 0.00000000001
set an = (a + g)/2
set gn = sqrt(a * g)
set a = an
set g = gn
set i = i + 1
end while
set result = g
end function

set x = 1
set y = 1/sqrt(2)
echo (x + y)/2
echo sqrt(x+y)
echo agm(x,y)

0.853553391
0.840896415
0.847213085

REXX

Also, this version of the AGM REXX program has three   short circuits   within it for an equality case and for two zero cases.

REXX supports arbitrary precision, so the default digits can be changed if desired.

/*REXX program calculates the  AGM  (arithmetic─geometric mean)  of two (real) numbers. */
parse arg a b digs .                             /*obtain optional numbers from the C.L.*/
if digs=='' | digs==","  then digs= 120          /*No DIGS specified?  Then use default.*/
numeric digits digs                              /*REXX will use lots of decimal digits.*/
if    a=='' |    a==","  then a= 1               /*No A specified?  Then use the default*/
if    b=='' |    b==","  then b= 1 / sqrt(2)     /* " B     "         "   "   "     "   */
call AGM a,b                                     /*invoke the  AGM  function.           */
say  '1st # ='      a                            /*display the   A   value.             */
say  '2nd # ='      b                            /*   "     "    B     "                */
say  '  AGM ='  agm(a, b)                        /*   "     "   AGM    "                */
exit 0                                           /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
agm:  procedure: parse arg x,y;  if x=y  then return x       /*is this an equality case?*/
                                 if y=0  then return 0       /*is   Y   equal to zero ? */
                                 if x=0  then return y/2     /* "   X     "    "   "    */
      d= digits()                                /*obtain the  current  decimal digits. */
      numeric digits d + 5                       /*add 5 more digs to ensure convergence*/
      tiny= '1e-'  ||  (digits() - 1)            /*construct a pretty tiny REXX number. */
      ox= x + 1                                  /*ensure that   the old X  ¬=  new X.  */
                do  while ox\=x  &  abs(ox)>tiny /*compute until the old X   ≡  new X.  */
                ox= x;    oy= y                  /*save    the  old  value of  X and Y. */
                x=     (ox + oy)  *  .5          /*compute  "   new    "    "  X.       */
                y= sqrt(ox * oy)                 /*   "     "    "     "    "  Y.       */
                end   /*while*/

      numeric digits d                           /*restore the original decimal digits. */
      return x / 1                               /*normalize  X  to new    "       "    */
/*──────────────────────────────────────────────────────────────────────────────────────*/
sqrt: procedure; parse arg x; if x=0  then return 0; d=digits(); m.=9; numeric form; h=d+6
      numeric digits; parse value format(x,2,1,,0) 'E0'  with  g 'E' _ .;  g=g *.5'e'_ % 2
        do j=0  while h>9;      m.j=h;               h=h % 2  + 1;  end /*j*/
        do k=j+5  to 0  by -1;  numeric digits m.k;  g=(g+x/g)*.5;  end /*k*/;    return g

output   when using the default input:

1st # = 1
2nd # = 0.707106781186547524400844362104849039284835937688474036588339868995366239231053519425193767163820786367506923115456148513
  AGM = 0.847213084793979086606499123482191636481445910326942185060579372659734004834134759723200293994611229942122285625233410963

Ring

decimals(9)
see agm(1, 1/sqrt(2)) + nl
see agm(1,1/pow(2,0.5)) + nl

func agm agm,g
       while agm
             an  = (agm + g)/2
             gn  = sqrt(agm*g)
             if fabs(agm-g) <= fabs(an-gn) exit ok
             agm = an
             g   = gn
       end
       return gn

RPL

≪ 1E-10 → epsilon
   ≪ WHILE DUP2 - ABS epsilon > REPEAT
        DUP2 + 2 / ROT ROT * √ 
      END DROP
≫ ≫ ‘AGM’ STO   

Input:

 1 2 / √ AGM

Output:

 1: .847213084835

Ruby

Flt Version

The thing to note about this implementation is that it uses the Flt library for high-precision math. This lets you adapt context (including precision and epsilon) to a ridiculous-in-real-life degree.

# The flt package (http://flt.rubyforge.org/) is useful for high-precision floating-point math.
# It lets us control 'context' of numbers, individually or collectively -- including precision
# (which adjusts the context's value of epsilon accordingly).

require 'flt'
include Flt

BinNum.Context.precision = 512  # default 53 (bits)

def agm(a,g)
  new_a = BinNum a
  new_g = BinNum g
  while new_a - new_g > new_a.class.Context.epsilon do
    old_g = new_g
    new_g = (new_a * new_g).sqrt
    new_a = (old_g + new_a) * 0.5
  end
  new_g
end

puts agm(1, 1 / BinNum(2).sqrt)

Output:

0.84721308479397908660649912348219163648144591032694218506057937265973400483413475972320029399461122994212228562523341096309796266583087105969971363598338426

Adjusting the precision setting (at about line 9) will of course affect this. :-)

BigDecimal Version

Ruby has a BigDecimal class in standard library

require 'bigdecimal'

PRECISION = 100
EPSILON = 0.1 ** (PRECISION/2)
BigDecimal::limit(PRECISION)

def agm(a,g)
  while a - g > EPSILON
    a, g = (a+g)/2, (a*g).sqrt(PRECISION)
  end
  [a, g]
end

a = BigDecimal(1)
g = 1 / BigDecimal(2).sqrt(PRECISION)
puts agm(a, g)

Output:

0.847213084793979086606499123482191636481445910326942185060579372659734004834134759723201915677745718E0
0.8472130847939790866064991234821916364814459103269421850605793726597340048341347597231986723114767413E0

Rust

// Accepts two command line arguments
// cargo run --name agm arg1 arg2

fn main () {
    let mut args = std::env::args();

    let x = args.nth(1).expect("First argument not specified.").parse::<f32>().unwrap();
    let y = args.next().expect("Second argument not specified.").parse::<f32>().unwrap();

    let result = agm(x,y);
    println!("The arithmetic-geometric mean is {}", result);
}

fn agm (x: f32, y: f32) -> f32 {
    let e: f32 = 0.000001;
    let mut a = x;
    let mut g = y;
    let mut a1: f32;
    let mut g1: f32;

    if a * g < 0f32 { panic!("The arithmetric-geometric mean is undefined for numbers less than zero!"); }
    else {
        loop {
            a1 = (a + g) / 2.;
            g1 = (a * g).sqrt();
            a = a1;
            g = g1;
            if (a - g).abs() < e {  return a; }
        }
    }
}

Output:

Output of running with arguments 1, 0.70710678:

The arithmetic-geometric mean is 1.456791

Scala

  def agm(a: Double, g: Double, eps: Double): Double = {
    if (math.abs(a - g) < eps) (a + g) / 2
    else agm((a + g) / 2, math.sqrt(a * g), eps)
  }

  agm(1, math.sqrt(2)/2, 1e-15)

Scheme

(define agm
  (case-lambda
    ((a0 g0) ; call again with default value for tolerance
     (agm a0 g0 1e-8))
    ((a0 g0 tolerance) ; called with three arguments
     (do ((a a0 (* (+ a g) 1/2))
          (g g0 (sqrt (* a g))))
       ((< (abs (- a g)) tolerance) a)))))

(display (agm 1 (/ 1 (sqrt 2)))) (newline)

Output:

0.8472130848351929

Seed7

$ include "seed7_05.s7i";
  include "float.s7i";
  include "math.s7i";

const func float: agm (in var float: a, in var float: g) is func
  result
    var float: agm is 0.0;
  local
    const float: iota is 1.0E-7;
    var float: a1 is 0.0;
    var float: g1 is 0.0;
  begin
    if a * g < 0.0 then
      raise RANGE_ERROR;
    else
      while abs(a - g) > iota do
        a1 := (a + g) / 2.0;
        g1 := sqrt(a * g);
        a := a1;
        g := g1;
      end while;
      agm := a;
    end if;
  end func;
 
const proc: main is func
  begin
    writeln(agm(1.0, 2.0) digits 6);
    writeln(agm(1.0, 1.0 / sqrt(2.0)) digits 6);
  end func;

Output:

1.456791
0.847213

SequenceL

import <Utilities/Math.sl>;

agm(a, g) :=
    let
        iota := 1.0e-15;
        arithmeticMean := 0.5 * (a + g);
        geometricMean := sqrt(a * g);
    in
        a when abs(a-g) < iota
    else
        agm(arithmeticMean, geometricMean);

main := agm(1.0, 1.0 / sqrt(2));

Output:

0.847213

Sidef

func agm(a, g) {
    loop {
        var (a1, g1) = ((a+g)/2, sqrt(a*g))
        [a1,g1] == [a,g] && return a
        (a, g) = (a1, g1)
    }
}

say agm(1, 1/sqrt(2))

Output:

0.8472130847939790866064991234821916364814

Smalltalk

Works with: Smalltalk/X

That is simply a copy/paste of the already existing agm method in the Number class:

agm:y
    "return the arithmetic-geometric mean agm(x, y) 
     of the receiver (x) and the argument, y.
     See https://en.wikipedia.org/wiki/Arithmetic-geometric_mean"

    |ai an gi gn epsilon delta|

    ai := (self + y) / 2.
    gi := (self * y) sqrt.
    epsilon := self ulp.

    [
        an := (ai + gi) / 2.
        gn := (ai * gi) sqrt.
        delta := (an - ai) abs.
        ai := an.
        gi := gn.
    ] doUntil:[ delta < epsilon ].
    ^ ai

Transcript showCR: (24 agm:6).
Transcript showCR: ( (1/2) agm:(1/6) ).
Transcript showCR: (1 agm:(1 / 2 sqrt)).

Output:

13.4581714817256
0.310602797207483
0.847213084793979

SQL

Works with: oracle version 11.2 and higher

The solution uses recursive WITH clause (aka recursive CTE, recursive query, recursive factored subquery). Some, perhaps many, but not all SQL dialects support recursive WITH clause. The solution below was written and tested in Oracle SQL - Oracle has supported recursive WITH clause since version 11.2.

with
  rec (rn, a, g, diff) as (
    select  1, 1, 1/sqrt(2), 1 - 1/sqrt(2)
      from  dual
    union all
    select  rn + 1, (a + g)/2, sqrt(a * g), (a + g)/2 - sqrt(a * g)
      from  rec
      where diff > 1e-38
  )
select *
from   rec
where  diff <= 1e-38
;

Output:

RN                                         A                                          G                                       DIFF
-- ----------------------------------------- ------------------------------------------ ------------------------------------------
 6 0.847213084793979086606499123482191636480 0.8472130847939790866064991234821916364792 0.0000000000000000000000000000000000000008

Standard ML

fun agm(a, g) = let
  fun agm'(a, g, eps) = 
    if Real.abs(a-g) < eps then
      a
    else
      agm'((a+g)/2.0, Math.sqrt(a*g), eps)
  in agm'(a, g, 1e~15)
  end;

Output:

agm(1.0, 1.0/Math.sqrt(2.0)) => 0.847213084794

Stata

mata 

real scalar agm(real scalar a, real scalar b) {
	real scalar c
	do {
		c=0.5*(a+b)
		b=sqrt(a*b)
		a=c
	} while (a-b>1e-15*a)
	return(0.5*(a+b))
}

agm(1,1/sqrt(2))
end

Output:

.8472130848

Swift

import Darwin

enum AGRError : Error {
	case undefined
}

func agm(_ a: Double, _ g: Double, _ iota: Double = 1e-8) throws -> Double {
	var a = a
	var g = g
	var a1: Double = 0
	var g1: Double = 0
	
	guard a * g >= 0 else {
		throw AGRError.undefined
	}
	
	while abs(a - g) > iota {
		a1 = (a + g) / 2
		g1 = sqrt(a * g)
		a = a1
		g = g1
	}
	
	return a
}

do {
	try print(agm(1, 1 / sqrt(2)))
} catch {
	print("agr is undefined when a * g < 0")
}

Output:

0.847213084835193

Tcl

The tricky thing about this implementation is that despite the finite precision available to IEEE doubles (which Tcl uses in its implementation of floating point arithmetic, in common with many other languages) the sequence of values does not quite converge to a single value; it gets to within a ULP and then errors prevent it from getting closer. This means that an additional termination condition is required: once a value does not change (hence the old_b variable) we have got as close as we can. Note also that we are using exact equality with floating point; this is reasonable because this is a rapidly converging sequence (it only takes 4 iterations in this case).

proc agm {a b} {
    set old_b [expr {$b<0?inf:-inf}]
    while {$a != $b && $b != $old_b} {
	set old_b $b
	lassign [list [expr {0.5*($a+$b)}] [expr {sqrt($a*$b)}]] a b
    }
    return $a
}

puts [agm 1 [expr 1/sqrt(2)]]

Output:

0.8472130847939792

TI SR-56

Texas Instruments SR-56 Program Listing for "Arithmetic-geometric mean"

Display	Key	Display	Key	Display	Key	Display	Key
00 33	STO	25 03	3	50		75
01 02	2	26 12	INV	51		76
02 32	x<>t	27 44	EE	52		77
03 64	×	28 41	R/S	53		78
04 32	x<>t	29		54		79
05 94	=	30		55		80
06 48	*√x	31		56		81
07 32	x<>t	32		57		82
08 84	+	33		58		83
09 34	RCL	34		59		84
10 02	2	35		60		85
11 94	=	36		61		86
12 54	÷	37		62		87
13 02	2	38		63		88
14 94	=	39		64		89
15 33	STO	40		65		90
16 02	2	41		66		91
17 44	EE	42		67		92
18 94	=	43		68		93
19 32	x<>t	44		69		94
20 44	EE	45		70		95
21 94	=	46		71		96
22 12	INV	47		72		97
23 37	*x=t	48		73		98
24 00	0	49		74		99

Asterisk denotes 2nd function key.

Register allocation

0: Unused	1: Unused	2: Previous Term	3: Unused	4: Unused
5: Unused	6: Unused	7: Unused	8: Unused	9: Unused

Annotated listing:

STO 2 x<>t                   // x := term a, t := R2 := term g
× x<>t = √x                  // Calculate term g'
x<>t + RCL 2 = / 2 = STO 2   // Calculate term a'
EE = x<>t EE =               // Round terms to ten digits
INV x=t 0 3                  // Loop if unequal
INV EE                       // Exit scientific notation
R/S                          // End

Usage:

Enter term a, press x<>t, then enter term g. Finally, press RST R/S to run the program.

Input:

1 x<>t 2 √x 1/x RST R/S

Output:

.8472130848

UNIX Shell

Works with: ksh93

ksh is one of the few unix shells that can do floating point arithmetic (bash does not).

function agm {
    float a=$1 g=$2 eps=${3:-1e-11} tmp
    while (( abs(a-g) > eps )); do
        print "debug: a=$a\tg=$g"
        tmp=$(( (a+g)/2.0 ))
        g=$(( sqrt(a*g) ))
        a=$tmp
    done
    echo $a
}

agm $((1/sqrt(2))) 1

Output:

debug: a=0.7071067812	g=1
debug: a=0.8535533906	g=0.8408964153
debug: a=0.8472249029	g=0.8472012668
debug: a=0.8472130848	g=0.8472130847
debug: a=0.8472130848	g=0.8472130848
debug: a=0.8472130848	g=0.8472130848
0.8472130848

You can get a more approximate convergence by changing the while condition to compare the numbers as strings: change

while (( abs(a-g) > eps ))

to

while [[ $a != $g ]]

V (Vlang)

import math
 
const ep = 1e-14
 
fn agm(aa f64, gg f64) f64 {
    mut a, mut g := aa, gg
    for math.abs(a-g) > math.abs(a)*ep {
        t := a
        a, g = (a+g)*.5, math.sqrt(t*g)
    }
    return a
}
 
fn main() {
    println(agm(1.0, 1.0/math.sqrt2))
}

Using standard math module

import math.stats
import math

fn main() {
    println(stats.geometric_mean<f64>([1.0, 1.0/math.sqrt2]))
}

Output:

0.8408964152537145

Wren

Translation of: Go

var eps = 1e-14

var agm = Fn.new { |a, g|
    while ((a-g).abs > a.abs * eps) {
        var t = a
        a = (a+g)/2
        g = (t*g).sqrt
    }
    return a
}

System.print(agm.call(1, 1/2.sqrt))

Output:

0.84721308479398

XPL0

include c:\cxpl\codesi;
real A, A1, G;
[Format(0, 16);
A:= 1.0;  G:= 1.0/sqrt(2.0);
repeat	A1:= (A+G)/2.0;
	G:= sqrt(A*G);
	A:= A1;
	RlOut(0, A);  RlOut(0, G);  RlOut(0, A-G);  CrLf(0);
until	A=G;
]

Output:

 8.5355339059327400E-001 8.4089641525371500E-001 1.2656975339559100E-002
 8.4722490292349400E-001 8.4720126674689100E-001 2.3636176602726000E-005
 8.4721308483519300E-001 8.4721308475276500E-001 8.2427509262572600E-011
 8.4721308479397900E-001 8.4721308479397900E-001 0.0000000000000000E+000

zkl

Translation of: XPL0

a:=1.0; g:=1.0/(2.0).sqrt();
while(not a.closeTo(g,1.0e-15)){
   a1:=(a+g)/2.0; g=(a*g).sqrt(); a=a1; 
   println(a,"  ",g," ",a-g);
}

Output:

0.853553  0.840896 0.012657
0.847225  0.847201 2.36362e-05
0.847213  0.847213 8.24275e-11
0.847213  0.847213 1.11022e-16

Or, using tail recursion

fcn(a=1.0, g=1.0/(2.0).sqrt()){ println(a," ",g," ",a-g);
   if(a.closeTo(g,1.0e-15)) return(a) else return(self.fcn((a+g)/2.0, (a*g).sqrt()));
}()

Output:

1 0.707107 0.292893
0.853553 0.840896 0.012657
0.847225 0.847201 2.36362e-05
0.847213 0.847213 8.24275e-11
0.847213 0.847213 1.11022e-16