# Mutual recursion

(Redirected from Mutual Recursion)
Mutual recursion
You are encouraged to solve this task according to the task description, using any language you may know.

Two functions are said to be mutually recursive if the first calls the second, and in turn the second calls the first.

Write two mutually recursive functions that compute members of the Hofstadter Female and Male sequences defined as:

{\displaystyle \begin{align} F(0)&=1\ ;\ M(0)=0 \\ F(n)&=n-M(F(n-1)), \quad n>0 \\ M(n)&=n-F(M(n-1)), \quad n>0. \end{align} }

(If a language does not allow for a solution using mutually recursive functions then state this rather than give a solution by other means).

## 8080 Assembly

The 8080 processor has built-in support for recursion, at the instruction level. The processor keeps a stack pointer, called SP, which is a 16-bit register that can be set by the program to point anywhere in the address space. The stack pointer points to the topmost word on the stack. The stack grows downward into memory: when a word is pushed onto the stack, the SP is decremented by 2, and the word written at the new location. When a word is popped from the stack, it is read from the location the SP is pointing to, and afterwards the SP is incremented by 2.

The instruction set includes a call instruction, which pushes the location of the next instruction onto the stack, and then jumps to the given location. Its counterpart is the ret instruction, which pops a location from the stack and jumps there. There are also push and pop instructions, to push and pop the values of register pairs on and off the stack directly. This can be used, among other things, to save 'local variables' in a recursive routine, as the code below does.

	org	100h
jmp	test
;;; Implementation of F(A).
F:	ana	a	; Zero?
jz	one	; Then set A=1
mov	b,a	; Otherwise, set B=A,
push	b	; And put it on the stack
dcr	a	; Set A=A-1
call	F	; Set A=F(A-1)
call	M 	; Set A=M(F(A-1))
pop	b	; Retrieve input value
cma		; (-A)+B is actually one cycle faster
inr	a	; than C=A;A=B;A-=B, and equivalent
ret
one:	mvi	a,1	; Set A to 1,
ret		; and return.
;;; Implementation of M(A).
M:	ana	a	; Zero?
rz		; Then keep it that way and return.
mov	b,a
push	b	; Otherwise, same deal as in F,
dcr	a	; but M and F are called in opposite
call	M 	; order.
call	F
pop	b
cma
inr	a
ret
;;; Demonstration code.
test:	lhld	6	; Set stack pointer to highest usable
sphl		; memory.
;;; Print F([0..15])
lxi	d,fpfx	; Print "F: "
mvi 	c,9
call	5
floop:	push	psw	; Keep N
call	F	; Get value for F(N)
call	pdgt	; Print it
pop	psw	; Restore N
inr 	a	; Next N
cpi	16	; Done yet?
jnz	floop
;;; Print M([0..15])
lxi	d,mpfx	; Print "\r\nM: "
mvi	c,9
call	5
mloop:	push	psw	; same deal as above
call	M
call	pdgt
pop	psw	; Restore N
inr	a
cpi	16
jnz	mloop
rst	0	; Explicit exit, we got rid of system stack
;;; Print digit and space
mov	e,a
mvi	c,2
call	5
mvi	e,' '	; Space
mvi	c,2
jmp	5	; Tail call optimization
fpfx:	db	'F: $' mpfx: db 13,10,'M:$'
Output:
F: 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9
M: 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9

## ABAP

This works for ABAP Version 7.40 and can be implemented in procedural ABAP as well, but with classes it is much more readable. As this allows a method with a returning value to be an input for a subsequent method call.

report z_mutual_recursion.

public section.
class-methods:
f
importing
n             type int4
returning
value(result) type int4,

m
importing
n             type int4
returning
value(result) type int4.
endclass.

method f.
result = cond int4(
when n eq 0
then 1
else n - m( f( n - 1 ) ) ).
endmethod.

method m.
result = cond int4(
when n eq 0
then 0
else n - f( m( n - 1 ) ) ).
endmethod.
endclass.

start-of-selection.
write: |{ reduce string(
init results = |f(0 - 19): { hoffstadter_sequences=>f( 0 ) }|
for i = 1 while i < 20
next results = |{ results }, { hoffstadter_sequences=>f( i ) }| ) }|, /.

write: |{ reduce string(
init results = |m(0 - 19): { hoffstadter_sequences=>m( 0 ) }|
for i = 1 while i < 20
next results = |{ results }, { hoffstadter_sequences=>m( i ) }| ) }|, /.

Output:
f(0 - 19): 1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7, 8, 8, 9, 9, 10, 11, 11, 12

m(0 - 19): 0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6, 7, 7, 8, 9, 9, 10, 11, 11, 12


## ACL2

(mutual-recursion
(defun f (n)
(declare (xargs :mode :program))
(if (zp n)
1
(- n (m (f (1- n))))))

(defun m (n)
(declare (xargs :mode :program))
(if (zp n)
0
(- n (f (m (1- n)))))))


with Ada.Text_Io; use Ada.Text_Io;
procedure Mutual_Recursion is
function M(N : Integer) return Integer;
function F(N : Integer) return Integer is
begin
if N = 0 then
return 1;
else
return N - M(F(N - 1));
end if;
end F;
function M(N : Integer) return Integer is
begin
if N = 0 then
return 0;
else
return N - F(M(N-1));
end if;
end M;
begin
for I in 0..19 loop
Put_Line(Integer'Image(F(I)));
end loop;
New_Line;
for I in 0..19 loop
Put_Line(Integer'Image(M(I)));
end loop;
end Mutual_recursion;

with Ada.Text_Io; use Ada.Text_Io;
procedure Mutual_Recursion is
function M(N: Natural) return Natural;
function F(N: Natural) return Natural;

function M(N: Natural) return Natural is
(if N = 0 then 0 else N – F(M(N–1)));

function F(N: Natural) return Natural is
(if N =0 then 1 else N – M(F(N–1)));
begin
for I in 0..19 loop
Put_Line(Integer'Image(F(I)));
end loop;
New_Line;
for I in 0..19 loop
Put_Line(Integer'Image(M(I)));
end loop;

end Mutual_recursion;


## Aime

Translation of: C
integer F(integer n);
integer M(integer n);

integer F(integer n)
{
integer r;
if (n) {
r = n - M(F(n - 1));
} else {
r = 1;
}
return r;
}

integer M(integer n)
{
integer r;
if (n) {
r = n - F(M(n - 1));
} else {
r = 0;
}
return r;
}

integer main(void)
{
integer i;
i = 0;
while (i < 20) {
o_winteger(3, F(i));
i += 1;
}
o_byte('\n');
i = 0;
while (i < 20) {
o_winteger(3, M(i));
i += 1;
}
o_byte('\n');
return 0;
}

## ALGOL 68

Translation of: C
Works with: ALGOL 68 version Standard - no extensions to language used
Works with: ALGOL 68G version Any - tested with release mk15-0.8b.fc9.i386
Works with: ELLA ALGOL 68 version Any (with appropriate job cards) - tested with release 1.8.8d.fc9.i386
PROC (INT)INT m; # ONLY required for ELLA ALGOL 68RS - an official subset OF full ALGOL 68 #

PROC f = (INT n)INT:
IF n = 0 THEN 1
ELSE n - m(f(n-1)) FI;

m := (INT n)INT:
IF n = 0 THEN 0
ELSE n - f(m(n-1)) FI;

main:
(
FOR i FROM 0 TO 19 DO
print(whole(f(i),-3))
OD;
new line(stand out);
FOR i FROM 0 TO 19 DO
print(whole(m(i),-3))
OD;
new line(stand out)
)
Output:
  1  1  2  2  3  3  4  5  5  6  6  7  8  8  9  9 10 11 11 12
0  0  1  2  2  3  4  4  5  6  6  7  7  8  9  9 10 11 11 12


## ALGOL W

begin
% define mutually recursive funtions F and M that compute the elements   %
% of the Hofstadter Female and Male sequences                            %

integer procedure F ( integer value n ) ;
if n = 0 then 1 else n - M( F( n - 1 ) );

integer procedure M ( integer value n ) ;
if n = 0 then 0 else n - F( M( n - 1 ) );

% print the first few elements of the sequences                          %
i_w := 2; s_w := 1; % set I/O formatting                                 %
write( "F: " );
for i := 0 until 20 do writeon( F( i ) );
write( "M: " );
for i := 0 until 20 do writeon( M( i ) );

end.

## APL

Works with: Dyalog APL
f ← {⍵=0:1 ⋄ ⍵-m∇⍵-1}
m ← {⍵=0:0 ⋄ ⍵-f∇⍵-1}
⍉'nFM'⍪↑(⊢,f,m)¨0,⍳20

Output:
n 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
F 1 1 2 2 3 3 4 5 5 6  6  7  8  8  9  9 10 11 11 12 13
M 0 0 1 2 2 3 4 4 5 6  6  7  7  8  9  9 10 11 11 12 12

## AppleScript

-- f :: Int -> Int
on f(x)
if x = 0 then
1
else
x - m(f(x - 1))
end if
end f

-- m :: Int -> Int
on m(x)
if x = 0 then
0
else
x - f(m(x - 1))
end if
end m

-- TEST
on run
set xs to range(0, 19)

{map(f, xs), map(m, xs)}
end run

-- GENERIC FUNCTIONS

-- map :: (a -> b) -> [a] -> [b]
on map(f, xs)
tell mReturn(f)
set lng to length of xs
set lst to {}
repeat with i from 1 to lng
set end of lst to lambda(item i of xs, i, xs)
end repeat
return lst
end tell
end map

-- 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 lambda : f
end script
end if
end mReturn

-- range :: Int -> Int -> [Int]
on range(m, n)
if n < m then
set d to -1
else
set d to 1
end if
set lst to {}
repeat with i from m to n by d
set end of lst to i
end repeat
return lst
end range

Output:
{{1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7, 8, 8, 9, 9, 10, 11, 11, 12},
{0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6, 7, 7, 8, 9, 9, 10, 11, 11, 12}}


## ARM Assembly

Unlike on the x86 family of processors, the ARM instruction set does not include specialized call and ret instructions. However, the program counter is a visible register (r15, also called pc), so it can be loaded and saved as any other. Nor is there a specialized stack pointer, though the load and store instructions offer pre- and postincrement as well as pre- and postdecrement on the register used as a pointer, making any register usable as a stack pointer.

By convention, r13 is used as the system stack pointer and is therefore also called sp, and r14 is used to store the return address for a function, and is therefore also called the *link register* or lr. The assembler recognizes push {x} and pop {x} instructions, though these are really pseudoinstructions, that generate the exact same machine code as ldmia r13!,{x} and stmdb r13!,{x}, these being, respectively, load with postincrement and store with predecrement on r13.

The link register is slightly special in that there is a family of branch-and-link instructions (bl). These are the same as mov r14,pc ; mov/ldr pc,<destination>, but in one machine instruction instead of two. This is the general way of calling subroutines, meaning no stack access is necessary unless the subroutine wants to call others in turn, in which case the link register must be saved by hand (as the code below shows several ways of doing).

.text
.global _start
@@@	Implementation of F(n), n in R0. n is considered unsigned.
F:	tst	r0,r0		@ n = 0?
moveq	r0,#1		@ In that case, the result is 1
push	{r0,lr}		@ Save link register and argument to stack
sub	r0,r0,#1	@ r0 -= 1    = n-1
bl	F		@ r0 = F(r0) = F(n-1)
bl	M		@ r0 = M(r0) = M(F(n-1))
pop	{r1,lr}		@ Restore link register and argument in r1
sub	r0,r1,r0	@ Result is n-F(M(n-1))

@@@	Implementation of M(n), n in R0. n is considered unsigned.
M:	tst	r0,r0		@ n = 0?
bxeq	lr		@ In that case the result is also 0; return.
push	{r0,lr}		@ Save link register and argument to stack
sub	r0,r0,#1	@ r0 -= 1    = n-1
bl	M		@ r0 = M(r0) = M(n-1)
bl	F		@ r0 = M(r0) = F(M(n-1))
pop	{r1,lr}		@ Restore link register and argument in r1
sub	r0,r1,r0	@ Result is n-M(F(n-1))

@@@	Print F(0..15) and M(0..15)
_start:	ldr	r1,=fmsg	@ Print values for F
ldr	r4,=F
bl	prfn
ldr	r1,=mmsg	@ Print values for M
ldr	r4,=M
bl	prfn
mov	r7,#1		@ Exit process
swi	#0

@@@	Helper function for output: print [r1], then [r4](0..15)
@@@	This assumes [r4] preserves r3 and r4; M and F do.
prfn:	push	{lr}		@ Keep link register
bl	pstr		@ Print the string
mov	r3,#0		@ Start at 0
1:	mov	r0,r3		@ Call the function in r4 with current number
blx	r4
add	r0,r0,#'0	@ Make ASCII digit
ldr	r1,=dgt		@ Store in digit string
strb	r0,[r1]
ldr	r1,=dstr	@ Print result
bl	pstr
cmp	r3,#15		@ Keep going up to and including 15
bls	1b
ldr	r1,=nl		@ Print newline afterwards
bl	pstr
@@@	Print length-prefixed string r1 to stdout
pstr:	push	{lr}		@ Keep link register
mov	r0,#1		@ stdout = 1
ldrb	r2,[r1],#1	@ r2 = length prefix
mov	r7,#4		@ 4 = write syscall
swi	#0
.data
fmsg:	.ascii	"\3F: "
mmsg:	.ascii	"\3M: "
dstr:	.ascii	"\2"
dgt:	.ascii	"* "
nl:	.ascii 	"\1\n"

Output:
F: 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9
M: 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9

## Arturo

f: $[n][ if? n=0 -> 1 else -> n-m f n-1 ] m:$[n][ if? n=0 -> 0 else -> n-f m n-1 ]

loop 0..20 'i [
print ["f(" i ")=" f i]
print ["m(" i ")=" m i]
print ""
]

Output:
f( 0 )= 1
m( 0 )= 0

f( 1 )= 1
m( 1 )= 0

f( 2 )= 2
m( 2 )= 1

f( 3 )= 2
m( 3 )= 2

f( 4 )= 3
m( 4 )= 2

f( 5 )= 3
m( 5 )= 3

f( 6 )= 4
m( 6 )= 4

f( 7 )= 5
m( 7 )= 4

f( 8 )= 5
m( 8 )= 5

f( 9 )= 6
m( 9 )= 6

f( 10 )= 6
m( 10 )= 6

f( 11 )= 7
m( 11 )= 7

f( 12 )= 8
m( 12 )= 7

f( 13 )= 8
m( 13 )= 8

f( 14 )= 9
m( 14 )= 9

f( 15 )= 9
m( 15 )= 9

f( 16 )= 10
m( 16 )= 10

f( 17 )= 11
m( 17 )= 11

f( 18 )= 11
m( 18 )= 11

f( 19 )= 12
m( 19 )= 12

f( 20 )= 13
m( 20 )= 12

## AutoHotkey

Loop 20
i := A_Index-1, t .= "n" i "t   " M(i) "t     " F(i)
MsgBox xtmaletfemalen%t%

F(n) {
Return n ? n - M(F(n-1)) : 1
}

M(n) {
Return n ? n - F(M(n-1)) : 0
}

Translation of: C

This one is an alternative to the above.

main()
Return

F(n)
{
If (n == 0)
Return 1
Else
Return n - M(F(n-1))
}

M(n)
{
If (n == 0)
Return 0
Else
Return n - F(M(n-1)) ;
}

main()
{
i = 0
While, i < 20
{
male .= M(i) . "n"
female .= F(i) . "n"
i++
}
MsgBox % "male:n" . male
MsgBox % "female:n" . female
}


## AWK

In AWK it is enough that both functions are defined somewhere. It matters not whether the BEGIN block is before or after the function definitions.

cat mutual_recursion.awk:
#!/usr/local/bin/gawk -f

# User defined functions
function F(n)
{ return n == 0 ? 1 : n - M(F(n-1)) }

function M(n)
{ return n == 0 ? 0 : n - F(M(n-1)) }

BEGIN {
for(i=0; i <= 20; i++) {
printf "%3d ", F(i)
}
print ""
for(i=0; i <= 20; i++) {
printf "%3d ", M(i)
}
print ""
}

Output:
$awk -f mutual_recursion.awk 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 13 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 12  ## BaCon ' Mutually recursive FUNCTION F(int n) TYPE int RETURN IIF(n = 0, 1, n - M(F(n -1))) END FUNCTION FUNCTION M(int n) TYPE int RETURN IIF(n = 0, 0, n - F(M(n - 1))) END FUNCTION ' Get iteration limit, default 20 SPLIT ARGUMENT$ BY " " TO arg$SIZE args limit = IIF(args > 1, VAL(arg$[1]), 20)

FOR i = 0 TO limit
PRINT F(i) FORMAT "%2d "
NEXT
PRINT
FOR i = 0 TO limit
PRINT M(i) FORMAT "%2d "
NEXT
PRINT

Output:
prompt$./mutually-recursive 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 13 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 12 ## BASIC Works with: QBasic DECLARE FUNCTION f! (n!) DECLARE FUNCTION m! (n!) FUNCTION f! (n!) IF n = 0 THEN f = 1 ELSE f = m(f(n - 1)) END IF END FUNCTION FUNCTION m! (n!) IF n = 0 THEN m = 0 ELSE m = f(m(n - 1)) END IF END FUNCTION  ### BBC BASIC  @% = 3 : REM Column width PRINT "F sequence:" FOR i% = 0 TO 20 PRINT FNf(i%) ; NEXT PRINT PRINT "M sequence:" FOR i% = 0 TO 20 PRINT FNm(i%) ; NEXT PRINT END DEF FNf(n%) IF n% = 0 THEN = 1 ELSE = n% - FNm(FNf(n% - 1)) DEF FNm(n%) IF n% = 0 THEN = 0 ELSE = n% - FNf(FNm(n% - 1))  Output: F sequence: 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 13 M sequence: 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 12  ### IS-BASIC 100 PROGRAM "Hofstad.bas" 110 PRINT "F sequence:" 120 FOR I=0 TO 20 130 PRINT F(I); 140 NEXT 150 PRINT :PRINT "M sequence:" 160 FOR I=0 TO 20 170 PRINT M(I); 180 NEXT 190 DEF F(N) 200 IF N=0 THEN 210 LET F=1 220 ELSE 230 LET F=N-M(F(N-1)) 240 END IF 250 END DEF 260 DEF M(N) 270 IF N=0 THEN 280 LET M=0 290 ELSE 300 LET M=N-F(M(N-1)) 310 END IF 320 END DEF ## BASIC256 # Rosetta Code problem: http://rosettacode.org/wiki/Mutual_recursion # by Jjuanhdez, 06/2022 n = 24 print "n : "; for i = 0 to n : print ljust(i, 3); : next i print chr(10); ("-" * 78) print "F : "; for i = 0 to n : print ljust(F(i), 3); : next i print chr(10); "M : "; for i = 0 to n : print ljust(M(i), 3); : next i end function F(n) if n = 0 then return 0 else return n - M(F(n-1)) end function function M(n) if n = 0 then return 0 else return n - F(M(n-1)) end function ## Bc cat mutual_recursion.bc: define f(n) { if ( n == 0 ) return(1); return(n - m(f(n-1))); } define m(n) { if ( n == 0 ) return(0); return(n - f(m(n-1))); }  Works with: GNU bc Works with: OpenBSD bc POSIX bc doesn't have the print statement. /* GNU bc */ for(i=0; i < 19; i++) { print f(i); print " "; } print "\n"; for(i=0; i < 19; i++) { print m(i); print " "; } print "\n"; quit  Output: GNU bc mutual_recursion.bc bc 1.06.95 Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc. This is free software with ABSOLUTELY NO WARRANTY. For details type warranty'. 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 13 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 12  ## BCPL get "libhdr" // Mutually recursive functions let f(n) = n=0 -> 1, n - m(f(n-1)) and m(n) = n=0 -> 0, n - f(m(n-1)) // Print f(0..15) and m(0..15) let start() be$(  writes("F:")
for i=0 to 15 do
$( writes(" ") writen(f(i))$)
writes("*NM:")
for i=0 to 15 do
$( writes(" ") writen(m(i))$)
writes("*N")
$) Output: F: 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 M: 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 ## BQN F ← {0:1; 𝕩-M F𝕩-1} M ← {0:0; 𝕩-F M𝕩-1} ⍉"FM"∾>(F∾M)¨↕15 Output: ┌─ ╵ 'F' 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 'M' 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 ┘ ## Bracmat  (F=.!arg:0&1|!arg+-1*M$(F$(!arg+-1))); (M=.!arg:0&0|!arg+-1*F$(M$(!arg+-1))); -1:?n&whl'(!n+1:~>20:?n&put$(F$!n " "))&put$\n
1  1  2  2  3  3  4  5  5  6  6  7  8  8  9  9  10  11  11  12  13

-1:?n&whl'(!n+1:~>20:?n&put$(M$!n " "))&put$\n 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 12 ## Brat female = null #yes, this is necessary male = { n | true? n == 0 { 0 } { n - female male(n - 1) } } female = { n | true? n == 0 { 1 } { n - male female(n - 1 ) } } p 0.to(20).map! { n | female n } p 0.to(20).map! { n | male n } ## C To let C see functions that will be used, it is enough to declare them. Normally this is done in a header file; in this example we do it directly in the code. If we do not declare them explicitly, they get an implicit declaration (if implicit declaration matches the use, everything's fine; but it is better however to write an explicit declaration) #include <stdio.h> #include <stdlib.h> /* let us declare our functions; indeed here we need really only M declaration, so that F can "see" it and the compiler won't complain with a warning */ int F(const int n); int M(const int n); int F(const int n) { return (n == 0) ? 1 : n - M(F(n - 1)); } int M(const int n) { return (n == 0) ? 0 : n - F(M(n - 1)); } int main(void) { int i; for (i = 0; i < 20; i++) printf("%2d ", F(i)); printf("\n"); for (i = 0; i < 20; i++) printf("%2d ", M(i)); printf("\n"); return EXIT_SUCCESS; }  ## C# namespace RosettaCode { class Hofstadter { static public int F(int n) { int result = 1; if (n > 0) { result = n - M(F(n-1)); } return result; } static public int M(int n) { int result = 0; if (n > 0) { result = n - F(M(n - 1)); } return result; } } }  ## C++ C++ has prior declaration rules similar to those stated above for C, if we would use two functions. Instead here we define M and F as static (class) methods of a class, and specify the bodies inline in the declaration of the class. Inlined methods in the class can still call other methods or access fields in the class, no matter what order they are declared in, without any additional pre-declaration. This is possible because all the possible methods and fields are declared somewhere in the class declaration, which is known the first time the class declaration is parsed. #include <iostream> #include <vector> #include <iterator> class Hofstadter { public: static int F(int n) { if ( n == 0 ) return 1; return n - M(F(n-1)); } static int M(int n) { if ( n == 0 ) return 0; return n - F(M(n-1)); } }; using namespace std; int main() { int i; vector<int> ra, rb; for(i=0; i < 20; i++) { ra.push_back(Hofstadter::F(i)); rb.push_back(Hofstadter::M(i)); } copy(ra.begin(), ra.end(), ostream_iterator<int>(cout, " ")); cout << endl; copy(rb.begin(), rb.end(), ostream_iterator<int>(cout, " ")); cout << endl; return 0; }  The following version shows better what's going on and why we seemingly didn't need pre-declaration (like C) when "encapsulating" the functions as static (class) methods. This version is equivalent to the above but does not inline the definition of the methods into the definition of the class. Here the method declarations in the class definition serves as the "pre-declaration" for the methods, as in C. class Hofstadter { public: static int F(int n); static int M(int n); }; int Hofstadter::F(int n) { if ( n == 0 ) return 1; return n - M(F(n-1)); } int Hofstadter::M(int n) { if ( n == 0 ) return 0; return n - F(M(n-1)); }  ## Ceylon Integer f(Integer n) => if (n > 0) then n - m(f(n-1)) else 1; Integer m(Integer n) => if (n > 0) then n - f(m(n-1)) else 0; shared void run() { printAll((0:20).map(f)); printAll((0:20).map(m)); }  Output: 1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7, 8, 8, 9, 9, 10, 11, 11, 12 0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6, 7, 7, 8, 9, 9, 10, 11, 11, 12  ## Clojure (declare F) ; forward reference (defn M [n] (if (zero? n) 0 (- n (F (M (dec n)))))) (defn F [n] (if (zero? n) 1 (- n (M (F (dec n))))))  ## CLU % At the top level, definitions can only see the definitions above. % But if we put F and M in a cluster, they can see each other. mutrec = cluster is F, M rep = null F = proc (n: int) returns (int) if n=0 then return(1) else return(n - M(F(n-1))) end end F M = proc (n: int) returns (int) if n=0 then return(0) else return(n - F(M(n-1))) end end M end mutrec % If we absolutely _must_ have them defined at the top level, % we can then just take them out of the cluster. F = mutrec$F
M = mutrec$M % Print the first few values for F and M print_first_16 = proc (name: string, fn: proctype (int) returns (int)) po: stream := stream$primary_output()
stream$puts(po, name || ":") for i: int in int$from_to(0,15) do
stream$puts(po, " " || int$unparse(fn(i)))
end
stream$putl(po, "") end print_first_16 start_up = proc () print_first_16("F", F) print_first_16("M", M) end start_up Output: F: 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 M: 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 ## CoffeeScript F = (n) -> if n is 0 then 1 else n - M F n - 1 M = (n) -> if n is 0 then 0 else n - F M n - 1 console.log [0...20].map F console.log [0...20].map M  Output: > coffee mutual_recurse.coffee [ 1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7, 8, 8, 9, 9, 10, 11, 11, 12 ] [ 0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6, 7, 7, 8, 9, 9, 10, 11, 11, 12 ]  ## Common Lisp (defun m (n) (if (zerop n) 0 (- n (f (m (- n 1)))))) (defun f (n) (if (zerop n) 1 (- n (m (f (- n 1))))))  ## D import std.stdio, std.algorithm, std.range; int male(in int n) pure nothrow { return n ? n - male(n - 1).female : 0; } int female(in int n) pure nothrow { return n ? n - female(n - 1).male : 1; } void main() { 20.iota.map!female.writeln; 20.iota.map!male.writeln; }  Output: [1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7, 8, 8, 9, 9, 10, 11, 11, 12] [0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6, 7, 7, 8, 9, 9, 10, 11, 11, 12] ## Dart int M(int n) => n==0?1:n-F(M(n-1)); int F(int n) => n==0?0:n-M(F(n-1)); main() { String f="",m=""; for(int i=0;i<20;i++) { m+="${M(i)} ";
f+="${F(i)} "; } print("M:$m");
print("F: $f"); }  ## Delphi unit Hofstadter; interface type THofstadterFemaleMaleSequences = class public class function F(n: Integer): Integer; class function M(n: Integer): Integer; end; implementation class function THofstadterFemaleMaleSequences.F(n: Integer): Integer; begin Result:= 1; if (n > 0) then Result:= n - M(F(n-1)); end; class function THofstadterFemaleMaleSequences.M(n: Integer): Integer; begin Result:= 0; if (n > 0) then Result:= n - F(M(n - 1)); end; end.  ## Déjà Vu F n: if n: - n M F -- n else: 1 M n: if n: - n F M -- n else: 0 for i range 0 10: !.( M i F i ) Output: 0 1 0 1 1 2 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 6  ## Draco /* We need to predeclare M if we want F to be able to see it. * This is done using 'extern', same as if it had been in a * different compilation unit. */ extern M(byte n) byte; /* Mutually recursive functions */ proc F(byte n) byte: if n=0 then 1 else n - M(F(n-1)) fi corp proc M(byte n) byte: if n=0 then 0 else n - F(M(n-1)) fi corp /* Show the first 16 values of each */ proc nonrec main() void: byte i; write("F:"); for i from 0 upto 15 do write(F(i):2) od; writeln(); write("M:"); for i from 0 upto 15 do write(M(i):2) od; writeln() corp Output: F: 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 M: 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 ## Dyalect func f(n) { n == 0 ? 1 : n - m(f(n-1)) } and m(n) { n == 0 ? 0 : n - f(m(n-1)) } print( (0..20).Map(i => f(i)).ToArray() ) print( (0..20).Map(i => m(i)).ToArray() ) ## E In E, nouns (variable names) always refer to preceding definitions, so to have mutual recursion, either one must be forward-declared or we must use a recursive def construct. Either one of these is syntactic sugar for first binding the noun to an E promise (a reference with an undetermined target), then resolving the promise to the value. Recursive def: def [F, M] := [ fn n { if (n <=> 0) { 1 } else { n - M(F(n - 1)) } }, fn n { if (n <=> 0) { 0 } else { n - F(M(n - 1)) } }, ] Forward declaration: def M def F(n) { return if (n <=> 0) { 1 } else { n - M(F(n - 1)) } } bind M(n) { return if (n <=> 0) { 0 } else { n - F(M(n - 1)) } } def M binds M to a promise, and stashes the resolver for that promise where bind can get to it. When def F... is executed, the function F closes over the promise which is the value of M. bind M... uses the resolver to resolve M to the provided definition. The recursive def operates similarly, except that it constructs promises for every variable on the left side ([F, M]), executes the right side ([fn ..., fn ...]) and collects the values, then resolves each promise to its corresponding value. But you don't have to worry about that to use it. ## Eiffel class APPLICATION create make feature make -- Test of the mutually recursive functions Female and Male. do across 0 |..| 19 as c loop io.put_string (Female (c.item).out + " ") end io.new_line across 0 |..| 19 as c loop io.put_string (Male (c.item).out + " ") end end Female (n: INTEGER): INTEGER -- Female sequence of the Hofstadter Female and Male sequences. require n_not_negative: n >= 0 do Result := 1 if n /= 0 then Result := n - Male (Female (n - 1)) end end Male (n: INTEGER): INTEGER -- Male sequence of the Hofstadter Female and Male sequences. require n_not_negative: n >= 0 do Result := 0 if n /= 0 then Result := n - Female (Male (n - 1)) end end end  Output: 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12  ## Elena Translation of: Smalltalk ELENA 4.x : import extensions; import system'collections; F = (n => (n == 0) ? 1 : (n - M(F(n-1))) ); M = (n => (n == 0) ? 0 : (n - F(M(n-1))) ); public program() { var ra := new ArrayList(); var rb := new ArrayList(); for(int i := 0, i <= 19, i += 1) { ra.append(F(i)); rb.append(M(i)) }; console.printLine(ra.asEnumerable()); console.printLine(rb.asEnumerable()) } Output: 1,1,2,2,3,3,4,5,5,6,6,7,8,8,9,9,10,11,11,12 0,0,1,2,2,3,4,4,5,6,6,7,7,8,9,9,10,11,11,12  ## Elixir defmodule MutualRecursion do def f(0), do: 1 def f(n), do: n - m(f(n - 1)) def m(0), do: 0 def m(n), do: n - f(m(n - 1)) end IO.inspect Enum.map(0..19, fn n -> MutualRecursion.f(n) end) IO.inspect Enum.map(0..19, fn n -> MutualRecursion.m(n) end)  Output: [1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7, 8, 8, 9, 9, 10, 11, 11, 12] [0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6, 7, 7, 8, 9, 9, 10, 11, 11, 12]  ## Erlang -module(mutrec). -export([mutrec/0, f/1, m/1]). f(0) -> 1; f(N) -> N - m(f(N-1)). m(0) -> 0; m(N) -> N - f(m(N-1)). mutrec() -> lists:map(fun(X) -> io:format("~w ", [f(X)]) end, lists:seq(0,19)), io:format("~n", []), lists:map(fun(X) -> io:format("~w ", [m(X)]) end, lists:seq(0,19)), io:format("~n", []).  ## Euphoria integer idM, idF function F(integer n) if n = 0 then return 1 else return n - call_func(idM,{F(n-1)}) end if end function idF = routine_id("F") function M(integer n) if n = 0 then return 0 else return n - call_func(idF,{M(n-1)}) end if end function idM = routine_id("M") ## F# let rec f n = match n with | 0 -> 1 | _ -> n - (m (f (n-1))) and m n = match n with | 0 -> 0 | _ -> n - (f (m (n-1)))  Like OCaml, the let rec f .. and m ... construct indicates that the functions call themselves (rec) and each other (and). ## Factor In Factor, if you need a word before it's defined, you have to DEFER: it. DEFER: F : M ( n -- n' ) dup 0 = [ dup 1 - M F - ] unless ; : F ( n -- n' ) dup 0 = [ drop 1 ] [ dup 1 - F M - ] if ;  ## FALSE [$[$1-f;!m;!-1-]?1+]f: [$[$1-m;!f;!- ]? ]m: [0[$20\>][\$@$@!." "1+]#%%]t:
f; t;!"
"m; t;!

## Fantom

class Main
{
static Int f (Int n)
{
if (n <= 0) // ensure n > 0
return 1
else
return n - m(f(n-1))
}

static Int m (Int n)
{
if (n <= 0) // ensure n > 0
return 0
else
return n - f(m(n-1))
}

public static Void main ()
{
50.times |Int n| { echo (f(n)) }
}
}

## FOCAL

01.01 C--PRINT F(0..15) AND M(0..15)
01.10 T "F(0..15)"
01.20 F X=0,15;S N=X;D 4;T %1,N
01.30 T !"M(0..15)"
01.40 F X=0,15;S N=X;D 5;T %1,N
01.50 T !
01.60 Q

04.01 C--N = F(N)
04.10 I (N(D)),4.11,4.2
04.11 S N(D)=1;R
04.20 S D=D+1;S N(D)=N(D-1)-1;D 4;D 5
04.30 S D=D-1;S N(D)=N(D)-N(D+1)

05.01 C--N = M(N)
05.10 I (N(D)),5.11,5.2
05.11 R
05.20 S D=D+1;S N(D)=N(D-1)-1;D 5;D 4
05.30 S D=D-1;S N(D)=N(D)-N(D+1)
Output:
F(0..15)= 1= 1= 2= 2= 3= 3= 4= 5= 5= 6= 6= 7= 8= 8= 9= 9
M(0..15)= 0= 0= 1= 2= 2= 3= 4= 4= 5= 6= 6= 7= 7= 8= 9= 9

## Forth

Forward references required for mutual recursion may be set up using DEFER.

defer m

: f ( n -- n )
dup 0= if 1+ exit then
dup 1- recurse m - ;

:noname ( n -- n )
dup 0= if exit then
dup 1- recurse f - ;
is m

: test ( xt n -- ) cr 0 do i over execute . loop drop ;

' m defer@ 20 test \ 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12
' f 20 test        \ 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12


## Fortran

As long as the code of the two functions is inside the same "block" (module or program) we don't need special care. Otherwise, we should "load" at least the interface of the other function (each module will load mutually the other; of course the compiler won't enter in a infinite loop), e.g. by using a "use" (we do that if M and F function are inside different modules)

Works with: Fortran version 95 and later
module MutualRec
implicit none
contains
pure recursive function m(n) result(r)
integer :: r
integer, intent(in) :: n
if ( n == 0 ) then
r = 0
return
end if
r = n - f(m(n-1))
end function m

pure recursive function f(n) result(r)
integer :: r
integer, intent(in) :: n
if ( n == 0 ) then
r = 1
return
end if
r = n - m(f(n-1))
end function f

end module


I've added the attribute pure so that we can use them in a forall statement.

program testmutrec
use MutualRec
implicit none

integer :: i
integer, dimension(20) :: a = (/ (i, i=0,19) /), b = (/ (i, i=0,19) /)
integer, dimension(20) :: ra, rb

forall(i=1:20)
ra(i) = m(a(i))
rb(i) = f(b(i))
end forall

write(*,'(20I3)') rb
write(*,'(20I3)') ra

end program testmutrec


## FreeBASIC

' FB 1.05.0 Win64

' Need forward declaration of M as it's used
' by F before its defined
Declare Function M(n As Integer) As Integer

Function F(n As Integer) As Integer
If n = 0 Then
Return 1
End If
Return n - M(F(n - 1))
End Function

Function M(n As Integer) As Integer
If n = 0 Then
Return 0
End If
Return n - F(M(n - 1))
End Function

Dim As Integer n = 24
Print "n :";
For i As Integer = 0 to n : Print Using "###"; i;    : Next
Print
Print String(78, "-")
Print "F :";
For i As Integer = 0 To n : Print Using "###"; F(i); : Next
Print
Print "M :";
For i As Integer = 0 To n : Print Using "###"; M(i); : Next
Print
Print "Press any key to quit"
Sleep

Output:
n :  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
------------------------------------------------------------------------------
F :  1  1  2  2  3  3  4  5  5  6  6  7  8  8  9  9 10 11 11 12 13 13 14 14 15
M :  0  0  1  2  2  3  4  4  5  6  6  7  7  8  9  9 10 11 11 12 12 13 14 14 15


## Fōrmulæ

Fōrmulæ programs are not textual, visualization/edition of programs is done showing/manipulating structures but not text. Moreover, there can be multiple visual representations of the same program. Even though it is possible to have textual representation —i.e. XML, JSON— they are intended for storage and transfer purposes more than visualization and edition.

Programs in Fōrmulæ are created/edited online in its website, However they run on execution servers. By default remote servers are used, but they are limited in memory and processing power, since they are intended for demonstration and casual use. A local server can be downloaded and installed, it has no limitations (it runs in your own computer). Because of that, example programs can be fully visualized and edited, but some of them will not run if they require a moderate or heavy computation/memory resources, and no local server is being used.

## Go

It just works. No special pre-declaration is necessary.

package main
import "fmt"

func F(n int) int {
if n == 0 { return 1 }
return n - M(F(n-1))
}

func M(n int) int {
if n == 0 { return 0 }
return n - F(M(n-1))
}

func main() {
for i := 0; i < 20; i++ {
fmt.Printf("%2d ", F(i))
}
fmt.Println()
for i := 0; i < 20; i++ {
fmt.Printf("%2d ", M(i))
}
fmt.Println()
}


## Groovy

Solution:

def f, m  // recursive closures must be declared before they are defined
f = { n -> n == 0 ? 1 : n - m(f(n-1)) }
m = { n -> n == 0 ? 0 : n - f(m(n-1)) }


Test program:

println 'f(0..20): ' + (0..20).collect { f(it) }
println 'm(0..20): ' + (0..20).collect { m(it) }

Output:
f(0..20): [1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7, 8, 8, 9, 9, 10, 11, 11, 12, 13]
m(0..20): [0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6, 7, 7, 8, 9, 9, 10, 11, 11, 12, 12]

Haskell's definitions constructs (at the top level, or inside a let or where construct) are always mutually-recursive:

f 0 = 1
f n | n > 0 = n - m (f $n-1) m 0 = 0 m n | n > 0 = n - f (m$ n-1)

main = do
print $map f [0..19] print$ map m [0..19]


## Icon and Unicon

procedure main(arglist)
every write(F(!arglist))   # F of all arguments
end

procedure F(n)
if integer(n) >= 0 then
return (n = 0, 1) |  n - M(F(n-1))
end

procedure M(n)
if integer(n) >= 0 then
return (0 = n) | n - F(M(n-1))
end


## Idris

mutual {
F : Nat -> Nat
F Z = (S Z)
F (S n) = (S n) minus M(F(n))

M : Nat -> Nat
M Z = Z
M (S n) = (S n) minus F(M(n))
}


## Io

f := method(n, if( n == 0, 1, n - m(f(n-1))))
m := method(n, if( n == 0, 0, n - f(m(n-1))))

Range
0 to(19) map(n,f(n)) println
0 to(19) map(n,m(n)) println

Output:
list(1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7, 8, 8, 9, 9, 10, 11, 11, 12)
list(0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6, 7, 7, 8, 9, 9, 10, 11, 11, 12)

## J

F =: 1:(-  M @ $: @ <:) @.* M."0 M =: 0:(- F @$: @ <:) @.* M."0


Example use:

   F i. 20
1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12


That said, note that numbers are defined recursively, so some other approaches using numbers which give equivalent results should be acceptable.

## Java

Replace translation (that doesn't compile) with a Java native implementation.

import java.util.HashMap;
import java.util.Map;

public class MutualRecursion {

public static void main(final String args[]) {
int max = 20;
System.out.printf("First %d values of the Female sequence:  %n", max);
for (int i = 0; i < max; i++) {
System.out.printf("  f(%d) = %d%n", i, f(i));
}
System.out.printf("First %d values of the Male sequence:  %n", max);
for (int i = 0; i < 20; i++) {
System.out.printf("  m(%d) = %d%n", i, m(i));
}
}

private static Map<Integer,Integer> F_MAP = new HashMap<>();

private static int f(final int n) {
if ( F_MAP.containsKey(n) ) {
return F_MAP.get(n);
}
int fn = n == 0 ? 1 : n - m(f(n - 1));
F_MAP.put(n, fn);
return fn;
}

private static Map<Integer,Integer> M_MAP = new HashMap<>();

private static int m(final int n) {
if ( M_MAP.containsKey(n) ) {
return M_MAP.get(n);
}
int mn = n == 0 ? 0 : n - f(m(n - 1));
M_MAP.put(n, mn);
return mn;
}

}

Output:

First 20 values of the Female sequence:

 f(0) = 1
f(1) = 1
f(2) = 2
f(3) = 2
f(4) = 3
f(5) = 3
f(6) = 4
f(7) = 5
f(8) = 5
f(9) = 6
f(10) = 6
f(11) = 7
f(12) = 8
f(13) = 8
f(14) = 9
f(15) = 9
f(16) = 10
f(17) = 11
f(18) = 11
f(19) = 12


First 20 values of the Male sequence:

 m(0) = 0
m(1) = 0
m(2) = 1
m(3) = 2
m(4) = 2
m(5) = 3
m(6) = 4
m(7) = 4
m(8) = 5
m(9) = 6
m(10) = 6
m(11) = 7
m(12) = 7
m(13) = 8
m(14) = 9
m(15) = 9
m(16) = 10
m(17) = 11
m(18) = 11
m(19) = 12


## JavaScript

function f(num) {
return (num === 0) ? 1 : num - m(f(num - 1));
}

function m(num) {
return (num === 0) ? 0 : num - f(m(num - 1));
}

function range(m, n) {
return Array.apply(null, Array(n - m + 1)).map(
function (x, i) { return m + i; }
);
}

var a = range(0, 19);

//return a new array of the results and join with commas to print
console.log(a.map(function (n) { return f(n); }).join(', '));
console.log(a.map(function (n) { return m(n); }).join(', '));

Output:
1,1,2,2,3,3,4,5,5,6,6,7,8,8,9,9,10,11,11,12
0,0,1,2,2,3,4,4,5,6,6,7,7,8,9,9,10,11,11,12

ES6 implementation

var f = num => (num === 0) ? 1 : num - m(f(num - 1));
var m = num => (num === 0) ? 0 : num - f(m(num - 1));

function range(m, n) {
return Array.apply(null, Array(n - m + 1)).map(
function (x, i) { return m + i; }
);
}

var a = range(0, 19);

//return a new array of the results and join with commas to print
console.log(a.map(n => f(n)).join(', '));
console.log(a.map(n => m(n)).join(', '));


More ES6 implementation

var range = (m, n) => Array(... Array(n - m + 1)).map((x, i) => m + i)


## jq

jq supports mutual recursion but requires functions to be defined before they are used. In the present case, this can be accomplished by defining an inner function.

He we define F and M as arity-0 filters:

def M:
def F: if . == 0 then 1 else . - ((. - 1) | F | M) end;
if . == 0 then 0 else . - ((. - 1) | M | F) end;

def F:
if . == 0 then 1 else . - ((. - 1) | F | M) end;
Example:
[range(0;20) | F],
[range(0;20) | M]
$jq -n -c -f Mutual_recursion.jq [1,1,2,2,3,3,4,5,5,6,6,7,8,8,9,9,10,11,11,12] [0,0,1,2,2,3,4,4,5,6,6,7,7,8,9,9,10,11,11,12]  ## Jsish /* Mutual recursion, is jsish */ function f(num):number { return (num === 0) ? 1 : num - m(f(num - 1)); } function m(num):number { return (num === 0) ? 0 : num - f(m(num - 1)); } function range(n=10, start=0, step=1):array { var a = Array(n).fill(0); for (var i in a) a[i] = start+i*step; return a; } var a = range(21); puts(a.map(function (n) { return f(n); }).join(', ')); puts(a.map(function (n) { return m(n); }).join(', ')); /* =!EXPECTSTART!= 1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7, 8, 8, 9, 9, 10, 11, 11, 12, 13 0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6, 7, 7, 8, 9, 9, 10, 11, 11, 12, 12 =!EXPECTEND!= */  Output: prompt$ jsish -u mutual-recursion.jsi
[PASS] mutual-recursion.jsi

prompt$jsish mutual-recursion.jsi 1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7, 8, 8, 9, 9, 10, 11, 11, 12 0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6, 7, 7, 8, 9, 9, 10, 11, 11, 12 ## Julia F(n) = n < 1 ? one(n) : n - M(F(n - 1)) M(n) = n < 1 ? zero(n) : n - F(M(n - 1))  Output: julia> [F(i) for i = 0:19], [M(i) for i = 0:19] ([1,1,2,2,3,3,4,5,5,6,6,7,8,8,9,9,10,11,11,12],[0,0,1,2,2,3,4,4,5,6,6,7,7,8,9,9,10,11,11,12])  ## Kotlin // version 1.0.6 fun f(n: Int): Int = when { n == 0 -> 1 else -> n - m(f(n - 1)) } fun m(n: Int): Int = when { n == 0 -> 0 else -> n - f(m(n - 1)) } fun main(args: Array<String>) { val n = 24 print("n :") for (i in 0..n) print("%3d".format(i)) println() println("-".repeat(78)) print("F :") for (i in 0..24) print("%3d".format(f(i))) println() print("M :") for (i in 0..24) print("%3d".format(m(i))) println() }  Output: n : 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 ------------------------------------------------------------------------------ F : 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 13 13 14 14 15 M : 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 12 13 14 14 15  ## Lambdatalk {def F {lambda {:n} {if {= :n 0} then 1 else {- :n {M {F {- :n 1}}}} }}} {def M {lambda {:n} {if {= :n 0} then 0 else {- :n {F {M {- :n 1}}}} }}} {map F {serie 0 19}} -> 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 {map M {serie 0 19}} -> 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12  The naïve version is very slow, {F 80} requires 3800 ms on a recent laptop, so let's memoize: {def cache {def cache.F {#.new}} {def cache.M {#.new}} {lambda {:f :n} {let { {:f :f} {:n :n} {:cx {if {equal? :f MF} then {cache.F} else {cache.M}}} } {if {equal? {#.get :cx :n} undefined} then {#.get {#.set! :cx :n {:f :n}} :n} else {#.get :cx :n}}}}} -> cache {def MF {lambda {:n} {if {= :n 0} then 1 else {- :n {MM {cache MF {- :n 1}}}}}}} -> MF {def MM {lambda {:n} {if {= :n 0} then 0 else {- :n {MF {cache MM {- :n 1}}}}}}} -> MM {MF 80} -> 50 (requires 55 ms) {map MF {serie 0 100}} (requires75ms) -> 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 13 13 14 14 15 16 16 17 17 18 19 19 20 21 21 22 22 23 24 24 25 25 26 27 27 28 29 29 30 30 31 32 32 33 34 34 35 35 36 37 37 38 38 39 40 40 41 42 42 43 43 44 45 45 46 46 47 48 48 49 50 50 51 51 52 53 53 54 55 55 56 56 57 58 58 59 59 60 61 61 62  ## Liberty BASIC print "F sequence." for i = 0 to 20 print f(i);" "; next print print "M sequence." for i = 0 to 20 print m(i);" "; next end function f(n) if n = 0 then f = 1 else f = n - m(f(n - 1)) end if end function function m(n) if n = 0 then m = 0 else m = n - f(m(n - 1)) end if end function Output: F sequence. 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 13 M sequence. 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 12 ## LibreOffice Basic '// LibreOffice Basic Implementation of Hofstadter Female-Male sequences '// Utility functions sub setfont(strfont) ThisComponent.getCurrentController.getViewCursor.charFontName = strfont end sub sub newline oVC = thisComponent.getCurrentController.getViewCursor oText = oVC.text oText.insertControlCharacter(oVC, com.sun.star.text.ControlCharacter.PARAGRAPH_BREAK, False) end sub sub out(sString) oVC = ThisComponent.getCurrentController.getViewCursor oText = oVC.text oText.insertString(oVC, sString, false) end sub sub outln(optional sString) if not ismissing (sString) then out(sString) newline end sub function intformat(n as integer,nlen as integer) as string dim nstr as string nstr = CStr(n) while len(nstr) < nlen nstr = " " & nstr wend intformat = nstr end function '// Hofstadter Female-Male function definitions function F(n as long) as long if n = 0 Then F = 1 elseif n > 0 Then F = n - M(F(n - 1)) endif end function function M(n) if n = 0 Then M = 0 elseif n > 0 Then M = n - F(M(n - 1)) endif end function '// Hofstadter Female Male sequence demo routine sub Hofstadter_Female_Male_Demo '// Introductory Text setfont("LM Roman 10") outln("Rosetta Code Hofstadter Female and Male Sequence Challenge") outln out("Two functions are said to be mutually recursive if the first calls the second,") outln(" and in turn the second calls the first.") out("Write two mutually recursive functions that compute members of the Hofstadter") outln(" Female and Male sequences defined as:") outln setfont("LM Mono Slanted 10") outln(chr(9)+"F(0) = 1 ; M(0)=0") outln(chr(9)+"F(n) = n - M(F(n-1)), n > 0") outln(chr(9)+"M(n) = n - F(M(n-1)), n > 0") outln '// Sequence Generation const nmax as long = 20 dim n as long setfont("LM Mono 10") out("n = " for n = 0 to nmax out(" " + intformat(n, 2)) next n outln out("F(n) = " for n = 0 to nmax out(" " + intformat(F(n),2)) next n outln out("M(n) = " for n = 0 to nmax out(" " + intformat(M(n), 2)) next n outln end sub ------------------------------ Output ------------------------------ n = 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 F(n) = 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 13 M(n) = 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 12 ## Logo Like Lisp, symbols in Logo are late-bound so no special syntax is required for forward references. to m :n if 0 = :n [output 0] output :n - f m :n-1 end to f :n if 0 = :n [output 1] output :n - m f :n-1 end show cascade 20 [lput m #-1 ?] [] [1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12] show cascade 20 [lput f #-1 ?] [] [0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12] ## LSL To test it yourself; rez a box on the ground, and add the following as a New Script. integer iDEPTH = 100; integer f(integer n) { if(n==0) { return 1; } else { return n-m(f(n - 1)); } } integer m(integer n) { if(n==0) { return 0; } else { return n-f(m(n - 1)); } } default { state_entry() { integer x = 0; string s = ""; for(x=0 ; x<iDEPTH ; x++) { s += (string)(f(x))+" "; } llOwnerSay(llList2CSV(llParseString2List(s, [" "], []))); s = ""; for(x=0 ; x<iDEPTH ; x++) { s += (string)(m(x))+" "; } llOwnerSay(llList2CSV(llParseString2List(s, [" "], []))); } }  Output: 1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7, 8, 8, 9, 9, 10, 11, 11, 12, 13, 13, 14, 14, 15, 16, 16, 17, 17, 18, 19, 19, 20, 21, 21, 22, 22, 23, 24, 24, 25, 25, 26, 27, 27, 28, 29, 29, 30, 30, 31, 32, 32, 33, 34, 34, 35, 35, 36, 37, 37, 38, 38, 39, 40, 40, 41, 42, 42, 43, 43, 44, 45, 45, 46, 46, 47, 48, 48, 49, 50, 50, 51, 51, 52, 53, 53, 54, 55, 55, 56, 56, 57, 58, 58, 59, 59, 60, 61, 61 0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6, 7, 7, 8, 9, 9, 10, 11, 11, 12, 12, 13, 14, 14, 15, 16, 16, 17, 17, 18, 19, 19, 20, 20, 21, 22, 22, 23, 24, 24, 25, 25, 26, 27, 27, 28, 29, 29, 30, 30, 31, 32, 32, 33, 33, 34, 35, 35, 36, 37, 37, 38, 38, 39, 40, 40, 41, 42, 42, 43, 43, 44, 45, 45, 46, 46, 47, 48, 48, 49, 50, 50, 51, 51, 52, 53, 53, 54, 54, 55, 56, 56, 57, 58, 58, 59, 59, 60, 61, 61 ## Lua function m(n) return n > 0 and n - f(m(n-1)) or 0 end function f(n) return n > 0 and n - m(f(n-1)) or 1 end  It is important to note, that if m and f are to be locally scoped functions rather than global, that they would need to be forward declared: local m,n function m(n) return n > 0 and n - f(m(n-1)) or 0 end function f(n) return n > 0 and n - m(f(n-1)) or 1 end  ## M2000 Interpreter A function can call a global function and must be global to call it again by the second function A group's function can call sibling function from same group. We can use This.F() or simply .f() to use group's f() member. We can use subroutines, which can call each other, in a module, and we can use the modules stack of values to get results from subs. Subs running as parts of module, and see same variables and same stack of values. Arguments are local to sub, and we can define local variables too. Last module export to clipboard and that used for output here. \\ set console 70 characters by 40 lines Form 70, 40 Module CheckSubs { Flush Document one$, two$For i =0 to 20 Print format$("{0::-3}",i);
f(i)
\\  number pop then top value of stack
one$=format$("{0::-3}",number)
m(i)
two$=format$("{0::-3}",number)
Next i
Print
Print one$Print two$
Sub f(x)
if x<=0 then Push 1 : Exit sub
f(x-1)  ' leave result to for m(x)
m()
push x-number
End Sub
Sub m(x)
if x<=0 then Push 0 : Exit sub
m(x-1)
f()
push x-number
End Sub
}
CheckSubs

Module Checkit {
Function global f(n) {
if n=0 then =1: exit
if n>0 then  =n-m(f(n-1))
}
Function global m(n) {
if n=0 then =0
if n>0 then  =n-f(m(n-1))

}
Document one$, two$
For i =0 to 20
Print format$("{0::-3}",i); one$=format$("{0::-3}",f(i)) two$=format$("{0::-3}",m(i)) Next i Print Print one$
Print two$} Checkit Module Checkit2 { Group Alfa { function f(n) { if n=0 then =1: exit if n>0 then =n-.m(.f(n-1)) } function m(n) { if n=0 then =0 if n>0 then =n-.f(.m(n-1)) } } Document one$, two$For i =0 to 20 Print format$("{0::-3}",i);
one$=format$("{0::-3}",Alfa.f(i))
two$=format$("{0::-3}",Alfa.m(i))
Next i
Print
Print one$Print two$
Clipboard one$+{ }+two$
}
Checkit2
Output:
  1  1  2  2  3  3  4  5  5  6  6  7  8  8  9  9 10 11 11 12 13
0  0  1  2  2  3  4  4  5  6  6  7  7  8  9  9 10 11 11 12 12


## M4

define(female',ifelse(0,$1,1,eval($1 - male(female(decr($1))))')')dnl define(male',ifelse(0,$1,0,eval($1 - female(male(decr($1))))')')dnl
define(loop',ifelse($1,$2,,$3($1) loop(incr($1),$2,$3')')')dnl loop(0,20,female') loop(0,20,male') ## MAD By default, functions in MAD are not reentrant. There is also no variable scope, all variables are always global. Functions can call other functions, but on the old IBM mainframes this was done by storing the return address in a special location (one per function); should a function call itself (either directly or indirectly), the return address would be overwritten. MAD does include a stack mechanism, but it is entirely manual. The programmer must allocate memory for it himself and activate it by hand, by default there is no stack. The command for this is SET LIST TO array. Once this is done, however, variables can be pushed and popped (using the SAVE and RESTORE commands). Furthermore, SAVE RETURN and RESTURE RETURN can be used to push and pop the current return address, enabling proper recursion, as long as the programmer is careful. The downside to this is that it does not play well with argument passing. All variables are still global. This means that passing arguments to a recursive function has to be done either by pushing them on the stack beforehand, or by setting global variables that the functions will push and pop themselves. (This program does the latter.) At the same time, the language syntax demands that all functions take at least one argument, so a dummy argument must be passed. To obtain a recursive function that uses the argument it is given, it is necessary to write a front-end function that uses its argument to pass it through the actual function in the manner described above. This is also shown. In this program, F. and M. are the front ends, taking an argument and using it to set N, then calling either FREC. or MREC., which are the actual recursive functions, with a dummy zero argument.  NORMAL MODE IS INTEGER R SET UP STACK SPACE DIMENSION STACK(100) SET LIST TO STACK R DEFINE RECURSIVE FUNCTIONS R INPUT ARGUMENT ASSUMED TO BE IN N INTERNAL FUNCTION(DUMMY) ENTRY TO FREC. WHENEVER N.LE.0, FUNCTION RETURN 1 SAVE RETURN SAVE DATA N N = N-1 N = FREC.(0) X = MREC.(0) RESTORE DATA N RESTORE RETURN FUNCTION RETURN N-X END OF FUNCTION INTERNAL FUNCTION(DUMMY) ENTRY TO MREC. WHENEVER N.LE.0, FUNCTION RETURN 0 SAVE RETURN SAVE DATA N N = N-1 N = MREC.(0) X = FREC.(0) RESTORE DATA N RESTORE RETURN FUNCTION RETURN N-X END OF FUNCTION R DEFINE FRONT-END FUNCTIONS THAT CAN BE CALLED WITH ARGMT INTERNAL FUNCTION(NN) ENTRY TO F. N = NN FUNCTION RETURN FREC.(0) END OF FUNCTION INTERNAL FUNCTION(NN) ENTRY TO M. N = NN FUNCTION RETURN MREC.(0) END OF FUNCTION R PRINT F(0..19) AND M(0..19) THROUGH SHOW, FOR I=0, 1, I.GE.20 SHOW PRINT FORMAT FMT,I,F.(I),I,M.(I) VECTOR VALUES FMT = 0$2HF(,I2,4H) = ,I2,S8,2HM(,I2,4H) = ,I2*$END OF PROGRAM Output: F( 0) = 1 M( 0) = 0 F( 1) = 1 M( 1) = 0 F( 2) = 2 M( 2) = 1 F( 3) = 2 M( 3) = 2 F( 4) = 3 M( 4) = 2 F( 5) = 3 M( 5) = 3 F( 6) = 4 M( 6) = 4 F( 7) = 5 M( 7) = 4 F( 8) = 5 M( 8) = 5 F( 9) = 6 M( 9) = 6 F(10) = 6 M(10) = 6 F(11) = 7 M(11) = 7 F(12) = 8 M(12) = 7 F(13) = 8 M(13) = 8 F(14) = 9 M(14) = 9 F(15) = 9 M(15) = 9 F(16) = 10 M(16) = 10 F(17) = 11 M(17) = 11 F(18) = 11 M(18) = 11 F(19) = 12 M(19) = 12 ## Maple female_seq := proc(n) if (n = 0) then return 1; else return n - male_seq(female_seq(n-1)); end if; end proc; male_seq := proc(n) if (n = 0) then return 0; else return n - female_seq(male_seq(n-1)); end if; end proc; seq(female_seq(i), i=0..10); seq(male_seq(i), i=0..10); Output: 1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6 0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6 ## Mathematica/Wolfram Language Without caching: f[0]:=1 m[0]:=0 f[n_]:=n-m[f[n-1]] m[n_]:=n-f[m[n-1]]  With caching: f[0]:=1 m[0]:=0 f[n_]:=f[n]=n-m[f[n-1]] m[n_]:=m[n]=n-f[m[n-1]]  Example finding f(1) to f(30) and m(1) to m(30): m /@ Range[30] f /@ Range[30]  gives back: {0,1,2,2,3,4,4,5,6,6,7,7,8,9,9,10,11,11,12,12,13,14,14,15,16,16,17,17,18,19} {1,2,2,3,3,4,5,5,6,6,7,8,8,9,9,10,11,11,12,13,13,14,14,15,16,16,17,17,18,19}  ## MATLAB female.m: function Fn = female(n) if n == 0 Fn = 1; return end Fn = n - male(female(n-1)); end  male.m: function Mn = male(n) if n == 0 Mn = 0; return end Mn = n - female(male(n-1)); end  Output: >> n = (0:10); >> arrayfun(@female,n) ans = 1 1 2 2 3 3 4 5 5 6 6 >> arrayfun(@male,n) ans = 0 0 1 2 2 3 4 4 5 6 6  ## Maxima f[0]: 1$
m[0]: 0$f[n] := n - m[f[n - 1]]$
m[n] := n - f[m[n - 1]]$makelist(f[i], i, 0, 10); [1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6] makelist(m[i], i, 0, 10); [0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6] remarray(m, f)$

f(n) := if n = 0 then 1 else n - m(f(n - 1))$m(n) := if n = 0 then 0 else n - f(m(n - 1))$

makelist(f(i), i, 0, 10);
[1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6]

makelist(m(i), i, 0, 10);
[0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6]

remfunction(f, m)$ ## Mercury :- module mutual_recursion. :- interface. :- import_module io. :- pred main(io::di, io::uo) is det. :- implementation. :- import_module int, list. main(!IO) :- io.write(list.map(f, 0..19), !IO), io.nl(!IO), io.write(list.map(m, 0..19), !IO), io.nl(!IO). :- func f(int) = int. f(N) = ( if N = 0 then 1 else N - m(f(N - 1)) ). :- func m(int) = int. m(N) = ( if N = 0 then 0 else N - f(m(N - 1)) ).  ## MiniScript f = function(n) if n > 0 then return n - m(f(n - 1)) return 1 end function m = function(n) if n > 0 then return n - f(m(n - 1)) return 0 end function print f(12) print m(12)  Output: 8 7 ## MiniZinc function var int: F(var int:n) = if n == 0 then 1 else n - M(F(n - 1)) endif; function var int: M(var int:n) = if (n == 0) then 0 else n - F(M(n - 1)) endif; ## MMIX  LOC Data_Segment GREG @ NL BYTE #a,0 GREG @ buf OCTA 0,0 t IS$128
Ja	IS	$127 LOC #1000 GREG @ // print 2 digits integer with trailing space to StdOut // reg$3 contains int to be printed
bp	IS	$71 0H GREG #0000000000203020 prtInt STO 0B,buf % initialize buffer LDA bp,buf+7 % points after LSD % REPEAT 1H SUB bp,bp,1 % move buffer pointer DIV$3,$3,10 % divmod (x,10) GET t,rR % get remainder INCL t,'0' % make char digit STB t,bp % store digit PBNZ$3,1B		% UNTIL no more digits
LDA	$255,bp TRAP 0,Fputs,StdOut % print integer GO Ja,Ja,0 % 'return' // Female function F GET$1,rJ		% save return addr
PBNZ	$0,1F % if N != 0 then F N INCL$0,1		% F 0 = 1
PUT	rJ,$1 % restore return addr POP 1,0 % return 1 1H SUBU$3,$0,1 % N1 = N - 1 PUSHJ$2,F		% do F (N - 1)
ADDU	$3,$2,0		% place result in arg. reg.
PUSHJ	$2,M % do M F ( N - 1) PUT rJ,$1		% restore ret addr
SUBU	$0,$0,$2 POP 1,0 % return N - M F ( N - 1 ) // Male function M GET$1,rJ
PBNZ	$0,1F PUT rJ,$1
POP	1,0		% return M 0 = 0
1H	SUBU	$3,$0,1
PUSHJ	$2,M ADDU$3,$2,0 PUSHJ$2,F
PUT	rJ,$1 SUBU$0,$0,$2
POP	1,0		$return N - F M ( N - 1 ) // do a female run Main SET$1,0		% for (i=0; i<25; i++){
1H	ADDU	$4,$1,0		%
PUSHJ	$3,F % F (i) GO Ja,prtInt % print F (i) INCL$1,1
CMP	t,$1,25 PBNZ t,1B % } LDA$255,NL
TRAP	0,Fputs,StdOut
// do a male run
SET	$1,0 % for (i=0; i<25; i++){ 1H ADDU$4,$1,0 % PUSHJ$3,M		%  M (i)
GO	Ja,prtInt	%  print M (i)
INCL	$1,1 CMP t,$1,25
PBNZ	t,1B		% }
LDA	$255,NL TRAP 0,Fputs,StdOut TRAP 0,Halt,0  Output: ~/MIX/MMIX/Rosetta> mmix mutualrecurs1 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 13 13 14 14 15 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 12 13 14 14 15  ## Modula-2 MODULE MutualRecursion; FROM InOut IMPORT WriteCard, WriteString, WriteLn; TYPE Fn = PROCEDURE(CARDINAL): CARDINAL; PROCEDURE F(n: CARDINAL): CARDINAL; BEGIN IF n=0 THEN RETURN 1; ELSE RETURN n-M(F(n-1)); END; END F; PROCEDURE M(n: CARDINAL): CARDINAL; BEGIN IF n=0 THEN RETURN 0; ELSE RETURN n-F(M(n-1)); END; END M; (* Print the first few values of one of the functions *) PROCEDURE Show(name: ARRAY OF CHAR; fn: Fn); CONST Max = 15; VAR i: CARDINAL; BEGIN WriteString(name); WriteString(": "); FOR i := 0 TO Max DO WriteCard(fn(i), 0); WriteString(" "); END; WriteLn; END Show; (* Show the first values of both F and M *) BEGIN Show("F", F); Show("M", M); END MutualRecursion.  Output: F: 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 M: 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9  ## Nemerle using System; using System.Console; module Hofstadter { F(n : int) : int { |0 => 1 |_ => n - M(F(n - 1)) } M(n : int) : int { |0 => 0 |_ => n - F(M(n - 1)) } Main() : void { foreach (n in [0 .. 20]) Write("{0} ", F(n)); WriteLine(); foreach (n in [0 .. 20]) Write("{0} ", M(n)); } }  ## Nim proc m(n: int): int proc f(n: int): int = if n == 0: 1 else: n - m(f(n-1)) proc m(n: int): int = if n == 0: 0 else: n - f(m(n-1)) for i in 1 .. 10: echo f(i) echo m(i)  ## Objeck Translation of: C class MutualRecursion { function : Main(args : String[]) ~ Nil { for(i := 0; i < 20; i+=1;) { f(i)->PrintLine(); }; "---"->PrintLine(); for (i := 0; i < 20; i+=1;) { m(i)->PrintLine(); }; } function : f(n : Int) ~ Int { return n = 0 ? 1 : n - m(f(n - 1)); } function : m(n : Int) ~ Int { return n = 0 ? 0 : n - f(m(n - 1)); } } ## Objective-C Objective-C has prior declaration rules similar to those stated above for C, for C-like types. In this example we show the use of a two class method; this works since we need an interface block that is like declaration of functions in C code. #import <Foundation/Foundation.h> @interface Hofstadter : NSObject + (int)M: (int)n; + (int)F: (int)n; @end @implementation Hofstadter + (int)M: (int)n { if ( n == 0 ) return 0; return n - [self F: [self M: (n-1)]]; } + (int)F: (int)n { if ( n == 0 ) return 1; return n - [self M: [self F: (n-1)]]; } @end int main() { int i; for(i=0; i < 20; i++) { printf("%3d ", [Hofstadter F: i]); } printf("\n"); for(i=0; i < 20; i++) { printf("%3d ", [Hofstadter M: i]); } printf("\n"); return 0; }  ## OCaml let rec f = function | 0 -> 1 | n -> n - m(f(n-1)) and m = function | 0 -> 0 | n -> n - f(m(n-1)) ;;  The let rec f ... and m ... construct indicates that the functions call themselves (rec) and each other (and). ## Octave We don't need to pre-declare or specify in some other way a function that will be defined later; but both must be declared before their use. (The code is written to handle vectors, as the testing part shows) function r = F(n) for i = 1:length(n) if (n(i) == 0) r(i) = 1; else r(i) = n(i) - M(F(n(i)-1)); endif endfor endfunction function r = M(n) for i = 1:length(n) if (n(i) == 0) r(i) = 0; else r(i) = n(i) - F(M(n(i)-1)); endif endfor endfunction  # testing ra = F([0:19]); rb = M([0:19]); disp(ra); disp(rb);  ## Oforth Oforth can declare methods objects without any implementation. This allows to implement mutual recursion. This does not work with functions (declaration and implementation must be together). Method new: M Integer method: F self 0 == ifTrue: [ 1 return ] self self 1 - F M - ; Integer method: M self 0 == ifTrue: [ 0 return ] self self 1 - M F - ; 0 20 seqFrom map(#F) println 0 20 seqFrom map(#M) println Output: [1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7, 8, 8, 9, 9, 10, 11, 11, 12, 13] [0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6, 7, 7, 8, 9, 9, 10, 11, 11, 12, 12]  ## Ol The letrec indicates that the definitions can be recursive, and fact that we placed these two in the same letrec block makes them mutually recursive. (letrec ((F (lambda (n) (if (= n 0) 1 (- n (M (F (- n 1))))))) (M (lambda (n) (if (= n 0) 0 (- n (F (M (- n 1)))))))) (print (F 19))) ; produces 12  ## Order Since Order is powered by the C preprocessor, definitions follow the same rule as CPP macros: they can appear in any order relative to each other as long as all are defined before the ORDER_PP block that calls them. #include <order/interpreter.h> #define ORDER_PP_DEF_8f \ ORDER_PP_FN(8fn(8N, \ 8if(8is_0(8N), \ 1, \ 8sub(8N, 8m(8f(8dec(8N))))))) #define ORDER_PP_DEF_8m \ ORDER_PP_FN(8fn(8N, \ 8if(8is_0(8N), \ 0, \ 8sub(8N, 8f(8m(8dec(8N))))))) //Test ORDER_PP(8for_each_in_range(8fn(8N, 8print(8f(8N))), 0, 19)) ORDER_PP(8for_each_in_range(8fn(8N, 8print(8m(8N))), 0, 19))  ## Oz declare fun {F N} if N == 0 then 1 elseif N > 0 then N - {M {F N-1}} end end fun {M N} if N == 0 then 0 elseif N > 0 then N - {F {M N-1}} end end in {Show {Map {List.number 0 9 1} F}} {Show {Map {List.number 0 9 1} M}} ## PARI/GP F(n)=if(n,n-M(F(n-1)),1) M(n)=if(n,n-F(M(n-1)),0) ## Pascal In Pascal we need to pre-declare functions/procedures; to do so, the forward statement is used. Program MutualRecursion; {M definition comes after F which uses it} function M(n : Integer) : Integer; forward; function F(n : Integer) : Integer; begin if n = 0 then F := 1 else F := n - M(F(n-1)); end; function M(n : Integer) : Integer; begin if n = 0 then M := 0 else M := n - F(M(n-1)); end; var i : Integer; begin for i := 0 to 19 do begin write(F(i) : 4) end; writeln; for i := 0 to 19 do begin write(M(i) : 4) end; writeln; end.  ## Perl sub F { my$n = shift; $n ?$n - M(F($n-1)) : 1 } sub M { my$n = shift; $n ?$n - F(M($n-1)) : 0 } # Usage: foreach my$sequence (\&F, \&M) {
print join(' ', map $sequence->($_), 0 .. 19), "\n";
}

Output:
1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12
0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12


## Phix

You should normally explicitly declare forward routines since it often makes things easier to understand (strictly only necessary when using optional or named parameters). There would be no point pre-declaring F, since it is not called before it is defined anyway.

with javascript_semantics
forward function M(integer n)

function F(integer n)
return iff(n?n-M(F(n-1)):1)
end function

function M(integer n)
return iff(n?n-F(M(n-1)):0)
end function

for i=0 to 20 do
printf(1," %d",F(i))
end for
printf(1,"\n")
for i=0 to 20 do
printf(1," %d",M(i))
end for

Output:
 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 13
0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 12


## PHP

<?php
function F($n) { if ($n == 0 ) return 1;
return $n - M(F($n-1));
}

function M($n) { if ($n == 0) return 0;
return $n - F(M($n-1));
}

$ra = array();$rb = array();
for($i=0;$i < 20; $i++) { array_push($ra, F($i)); array_push($rb, M($i)); } echo implode(" ",$ra) . "\n";
echo implode(" ", $rb) . "\n"; ?>  ## Picat Here are two approaches, both using tabling. For small values (say N < 50) tabling is not really needed. ### Tabled functions table f(0) = 1. f(N) = N - m(f(N-1)), N > 0 => true. table m(0) = 0. m(N) = N - f(m(N-1)), N > 0 => true. ### Tabled predicates Translation of: Prolog table female(0,1). female(N,F) :- N>0, N1 = N-1, female(N1,R), male(R, R1), F = N-R1. table male(0,0). male(N,F) :- N>0, N1 = N-1, male(N1,R), female(R, R1), F = N-R1. ### Test go => N = 30, println(func), test_func(N), println(pred), test_pred(N), nl. nl. % Testing the function based approach test_func(N) => println([M : I in 0..N, male(I,M)]), println([F : I in 0..N, female(I,F)]), nl. % Testing the predicate approach test_pred(N) => println([M : I in 0..N, male(I,M)]), println([F : I in 0..N, female(I,F)]), nl. Output: func [0,0,1,2,2,3,4,4,5,6,6,7,7,8,9,9,10,11,11,12,12,13,14,14,15,16,16,17,17,18,19] [1,1,2,2,3,3,4,5,5,6,6,7,8,8,9,9,10,11,11,12,13,13,14,14,15,16,16,17,17,18,19] pred [0,0,1,2,2,3,4,4,5,6,6,7,7,8,9,9,10,11,11,12,12,13,14,14,15,16,16,17,17,18,19] [1,1,2,2,3,3,4,5,5,6,6,7,8,8,9,9,10,11,11,12,13,13,14,14,15,16,16,17,17,18,19] ### Larger values For larger values, tabling is essential and then one can discern that the predicate based approach is a little faster. Here are the times for testing N=1 000 000: • func: 1.829s • pred: 1.407s ## PicoLisp (de f (N) (if (=0 N) 1 (- N (m (f (dec N)))) ) ) (de m (N) (if (=0 N) 0 (- N (f (m (dec N)))) ) ) ## PL/I test: procedure options (main); M: procedure (n) returns (fixed) recursive; /* 8/1/2010 */ declare n fixed; if n <= 0 then return (0); else return ( n - F(M(n-1)) ); end M; F: procedure (n) returns (fixed) recursive; declare n fixed; if n <= 0 then return (1); else return ( n - M(F(n-1)) ); end F; declare i fixed; do i = 1 to 15; put skip list ( F(i), M(i) ); end; end test; ## PostScript /female{ /n exch def n 0 eq {1} { n n 1 sub female male sub }ifelse }def /male{ /n exch def n 0 eq {0} { n n 1 sub male female sub }ifelse }def  Library: initlib /F { { {0 eq} {pop 1} is? {0 gt} {dup 1 sub F M sub} is? } cond }. /M { { {0 eq} {pop 0} is? {0 gt} {dup 1 sub M F sub} is? } cond }.  ## PowerShell function F($n) {
if ($n -eq 0) { return 1 } return$n - (M (F ($n - 1))) } function M($n) {
M: procedure expose $m.$f.; parse arg n; if $m.n==. then$m.n= n-F(M(n-1));  return $m.n  output when using the default input of: 99 Js= 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 Fs= 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 13 13 14 14 15 16 16 17 17 18 19 19 20 21 21 22 22 23 24 24 25 25 26 27 27 28 29 29 30 30 31 32 32 33 34 34 35 35 36 37 37 38 38 39 40 40 41 42 42 43 43 44 45 45 46 46 47 48 48 49 50 50 51 51 52 53 53 54 55 55 56 56 57 58 58 59 59 60 61 61 Ms= 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 12 13 14 14 15 16 16 17 17 18 19 19 20 20 21 22 22 23 24 24 25 25 26 27 27 28 29 29 30 30 31 32 32 33 33 34 35 35 36 37 37 38 38 39 40 40 41 42 42 43 43 44 45 45 46 46 47 48 48 49 50 50 51 51 52 53 53 54 54 55 56 56 57 58 58 59 59 60 61 61  ### with memoization, specific entry This version is identical in function to the previous example, but it also can compute and display a specific request (indicated by a negative number for the argument). /*REXX program shows mutual recursion (via the Hofstadter Male and Female sequences). */ /*───────────────── If LIM is negative, a single result is shown for the abs(lim) entry.*/ parse arg lim .; if lim=='' then lim= 99; aLim= abs(lim) w= length(aLim);$m.=.;      $m.0= 0;$f.=.;     $f.0= 1; Js=; Fs=; Ms= do j=0 for aLim+1; call F(J); call M(j) if lim<0 then iterate Js= Js right(j, w); Fs= Fs right($f.j, w);      Ms= Ms right($m.j, w) end /*j*/ if lim>0 then say 'Js=' Js; else say 'J('aLim")=" right( aLim, w) if lim>0 then say 'Fs=' Fs; else say 'F('aLim")=" right($f.aLim, w)
if lim>0  then  say 'Ms='   Ms;        else  say  'M('aLim")="     right($m.aLIM, w) exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/ F: procedure expose$m. $f.; parse arg n; if$f.n==.  then $f.n= n-M(F(n-1)); return$f.n
M: procedure expose $m.$f.; parse arg n; if $m.n==. then$m.n= n-F(M(n-1));  return $m.n  output when using the input of: -70000 J(70000)= 70000 F(70000)= 43262 M(70000)= 43262  output when using the input of a negative ¼ million: -250000 J(250000)= 250000 F(250000)= 154509 M(250000)= 154509  ## Ring see "F sequence : " for i = 0 to 20 see "" + f(i) + " " next see nl see "M sequence : " for i = 0 to 20 see "" + m(i) + " " next func f n fr = 1 if n != 0 fr = n - m(f(n - 1)) ok return fr func m n mr = 0 if n != 0 mr = n - f(m(n - 1)) ok return mr ## Ruby def F(n) n == 0 ? 1 : n - M(F(n-1)) end def M(n) n == 0 ? 0 : n - F(M(n-1)) end p (Array.new(20) {|n| F(n) }) p (Array.new(20) {|n| M(n) })  Output: [1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7, 8, 8, 9, 9, 10, 11, 11, 12] [0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6, 7, 7, 8, 9, 9, 10, 11, 11, 12] In ruby there is no need to pre-declare M for it to be used in the definition of F. (However M must be defined before F calls it). ## Run BASIC print "F sequence:"; for i = 0 to 20 print f(i);" "; next i print :print "M sequence:"; for i = 0 to 20 print m(i);" "; next i end function f(n) f = 1 if n <> 0 then f = n - m(f(n - 1)) end function function m(n) m = 0 if n <> 0 then m = n - f(m(n - 1)) end function Output: F sequence:1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 13 M sequence:0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 12 ## Rust fn f(n: u32) -> u32 { match n { 0 => 1, _ => n - m(f(n - 1)) } } fn m(n: u32) -> u32 { match n { 0 => 0, _ => n - f(m(n - 1)) } } fn main() { for i in (0..20).map(f) { print!("{} ", i); } println!(""); for i in (0..20).map(m) { print!("{} ", i); } println!("") }  Output: 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 ## S-lang % Forward definitions: [also deletes any existing definition] define f(); define m(); define f(n) { if (n == 0) return 1; else if (n < 0) error("oops"); return n - m(f(n - 1)); } define m(n) { if (n == 0) return 0; else if (n < 0) error("oops"); return n - f(m(n - 1)); } foreach$1 ([0:19])
() = printf("%d  ", f($1)); () = printf("\n"); foreach$1 ([0:19])
() = printf("%d  ", m($1)); () = printf("\n"); Output: 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 ## Sather class MAIN is f(n:INT):INT pre n >= 0 is if n = 0 then return 1; end; return n - m(f(n-1)); end; m(n:INT):INT pre n >= 0 is if n = 0 then return 0; end; return n - f(m(n-1)); end; main is loop i ::= 0.upto!(19); #OUT + #FMT("%2d ", f(i)); end; #OUT + "\n"; loop i ::= 0.upto!(19); #OUT + #FMT("%2d ", m(i)); end; end; end; There's no need to pre-declare F or M. ## Scala def F(n:Int):Int = if (n == 0) 1 else n - M(F(n-1)) def M(n:Int):Int = if (n == 0) 0 else n - F(M(n-1)) println((0 until 20).map(F).mkString(", ")) println((0 until 20).map(M).mkString(", "))  Output: 1, 1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7, 8, 8, 9, 9, 10, 11, 11, 12 0, 0, 1, 2, 2, 3, 4, 4, 5, 6, 6, 7, 7, 8, 9, 9, 10, 11, 11, 12 ## Scheme define declarations are automatically mutually recursive: (define (F n) (if (= n 0) 1 (- n (M (F (- n 1)))))) (define (M n) (if (= n 0) 0 (- n (F (M (- n 1))))))  If you wanted to use a let-like construct to create local bindings, you would do the following. The define construct above is just a syntactic sugar for the following where the entire rest of the scope is used as the body. (letrec ((F (lambda (n) (if (= n 0) 1 (- n (M (F (- n 1))))))) (M (lambda (n) (if (= n 0) 0 (- n (F (M (- n 1)))))))) (F 19)) # evaluates to 12  The letrec indicates that the definitions can be recursive, and fact that we placed these two in the same letrec block makes them mutually recursive. ## Seed7 $ include "seed7_05.s7i";

const func integer: m (in integer: n) is forward;

const func integer: f (in integer: n) is func
result
var integer: res is 0;
begin
if n = 0 then
res := 1;
else
res := n - m(f(n - 1));
end if;
end func;

const func integer: m (in integer: n) is func
result
var integer: res is 0;
begin
if n = 0 then
res := 0;
else
res := n - f(m(n - 1));
end if;
end func;

const proc: main is func
local
var integer: i is 0;
begin
for i range 0 to 19 do
end for;
writeln;
for i range 0 to 19 do
end for;
writeln;
end func;
Output:
  1  1  2  2  3  3  4  5  5  6  6  7  8  8  9  9 10 11 11 12
0  0  1  2  2  3  4  4  5  6  6  7  7  8  9  9 10 11 11 12


## Sidef

func F(){}
func M(){}

F = func(n) { n > 0 ? (n - M(F(n-1))) : 1 }
M = func(n) { n > 0 ? (n - F(M(n-1))) : 0 }

[F, M].each { |seq|
{|i| seq.call(i)}.map(^20).join(' ').say
}

Output:
1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12
0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12

## Smalltalk

Using block closure.

|F M ra rb|

F := [ :n |
(n == 0)
ifTrue: [ 1 ]
ifFalse: [ n - (M value: (F value: (n-1))) ]
].

M := [ :n |
(n == 0)
ifTrue: [ 0 ]
ifFalse: [ n - (F value: (M value: (n-1))) ]
].

ra := OrderedCollection new.
rb := OrderedCollection new.
0 to: 19 do: [ :i |
].

ra displayNl.
rb displayNl.


## SNOBOL4

        define('f(n)') :(f_end)
f       f = eq(n,0) 1 :s(return)
f = n - m(f(n - 1)) :(return)
f_end

define('m(n)') :(m_end)
m       m = eq(n,0) 0 :s(return)
m = n - f(m(n - 1)) :(return)
m_end

*       # Test and display
L1      s1 = s1 m(i) ' ' ; s2 = s2 f(i) ' '
i = le(i,25) i + 1 :s(L1)
output = 'M: ' s1; output = 'F: ' s2
end
Output:
M: 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 12 13 14 14 15 16 16
F: 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 13 13 14 14 15 16 16

## SNUSP

The program shown calculates F(3) and demonstrates simple and mutual recursion.

       /======\
F==!/=!\?\+#  | />-<-\
|    \@\-@/@\===?/<#
|      |    |
$+++/======|====/ ! /=/ /+<<-\ | \!/======?\>>=?/<# dup | \<<+>+>-/ ! ! \======|====\ | | | | /===|==\ | M==!\=!\?\#| | | \@/-@/@/===?\<# ^ \>-<-/ | ^ ^ ^ ^ | | | | subtract from n | | | mutual recursion | | recursion | n-1 check for zero ## SPL f(n)= ? n=0, <= 1 <= n-m(f(n-1)) . m(n)= ? n=0, <= 0 <= n-f(m(n-1)) . > i, 0..20 fs += " "+f(i) ms += " "+m(i) < #.output("F:",fs) #.output("M:",ms) Output: F: 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 13 M: 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 12  ## Standard ML fun f 0 = 1 | f n = n - m (f (n-1)) and m 0 = 0 | m n = n - f (m (n-1)) ;  The fun construct creates recursive functions, and the and allows a group of functions to call each other. The above is just a shortcut for the following: val rec f = fn 0 => 1 | n => n - m (f (n-1)) and m = fn 0 => 0 | n => n - f (m (n-1)) ;  which indicates that the functions call themselves (rec) and each other (and). Output: > val terms = List.tabulate (10, fn x => x); val terms = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]: int list > map f terms; val it = [1, 1, 2, 2, 3, 3, 4, 5, 5, 6]: int list > map m terms; val it = [0, 0, 1, 2, 2, 3, 4, 4, 5, 6]: int list  ## Swift It just works. No special pre-declaration is necessary. func F(n: Int) -> Int { return n == 0 ? 1 : n - M(F(n-1)) } func M(n: Int) -> Int { return n == 0 ? 0 : n - F(M(n-1)) } for i in 0..20 { print("\(F(i)) ") } println() for i in 0..20 { print("\(M(i)) ") } println()  ## Symsyn F param Fn if Fn = 0 1 R else (Fn-1) nm1 save Fn call F nm1 result Fr save Fr call M Fr result Mr restore Fr restore Fn (Fn-Mr) R endif return R M param Mn if Mn = 0 0 R else (Mn-1) nm1 save Mn call M nm1 result Mr save Mr call F Mr result Fr restore Mr restore Mn (Mn-Fr) R endif return R start i if i <= 19 call F i result res "$s res ' '" $s + i goif endif$s []
$s i if i <= 19 call M i result res "$s res ' '" $s + i goif endif$s []

## Tailspin

templates male
when <=0> do 0 !
otherwise def n: $;$n - 1 -> male -> female -> $n -$ !
end male

templates female
when <=0> do 1 !
otherwise def n: $;$n - 1 -> female -> male -> $n -$ !
end female

0..10 -> 'M$;:$->male; F$;:$->female;
' -> !OUT::write
Output:
M0: 0 F0: 1
M1: 0 F1: 1
M2: 1 F2: 2
M3: 2 F3: 2
M4: 2 F4: 3
M5: 3 F5: 3
M6: 4 F6: 4
M7: 4 F7: 5
M8: 5 F8: 5
M9: 6 F9: 6
M10: 6 F10: 6


## Tcl

proc m {n} {
if { $n == 0 } { expr 0; } else { expr {$n - [f [m [expr {$n-1}] ]]}; } } proc f {n} { if {$n == 0 } { expr 1; } else {
expr {$n - [m [f [expr {$n-1}] ]]};
}
}

for {set i 0} {$i < 20} {incr i} { puts -nonewline [f$i];
puts -nonewline " ";
}
puts ""
for {set i 0} {$i < 20} {incr i} { puts -nonewline [m$i];
puts -nonewline " ";
}
puts ""


## TI-89 BASIC

Define F(n) = when(n=0, 1, n - M(F(n - 1)))
Define M(n) = when(n=0, 0, n - F(M(n - 1)))

## TXR

(defun f (n)
(if (>= 0 n)
1
(- n (m (f (- n 1))))))

(defun m (n)
(if (>= 0 n)
0
(- n (f (m (- n 1))))))

(each ((n (range 0 15)))
(format t "f(~s) = ~s; m(~s) = ~s\n" n (f n) n (m n)))
$txr mutual-recursion.txr f(0) = 1; m(0) = 0 f(1) = 1; m(1) = 0 f(2) = 2; m(2) = 1 f(3) = 2; m(3) = 2 f(4) = 3; m(4) = 2 f(5) = 3; m(5) = 3 f(6) = 4; m(6) = 4 f(7) = 5; m(7) = 4 f(8) = 5; m(8) = 5 f(9) = 6; m(9) = 6 f(10) = 6; m(10) = 6 f(11) = 7; m(11) = 7 f(12) = 8; m(12) = 7 f(13) = 8; m(13) = 8 f(14) = 9; m(14) = 9 f(15) = 9; m(15) = 9 ## uBasic/4tH Translation of: BBC BASIC uBasic/4tH supports mutual recursion. However, the underlying system can't support the stress this puts on the stack - at least not for the full sequence. This version uses memoization to alleviate the stress and speed up execution. LOCAL(1) ' main uses locals as well FOR a@ = 0 TO 200 ' set the array @(a@) = -1 NEXT PRINT "F sequence:" ' print the F-sequence FOR a@ = 0 TO 20 PRINT FUNC(_f(a@));" "; NEXT PRINT PRINT "M sequence:" ' print the M-sequence FOR a@ = 0 TO 20 PRINT FUNC(_m(a@));" "; NEXT PRINT END _f PARAM(1) ' F-function IF a@ = 0 THEN RETURN (1) ' memoize the solution IF @(a@) < 0 THEN @(a@) = a@ - FUNC(_m(FUNC(_f(a@ - 1)))) RETURN (@(a@)) ' return array element _m PARAM(1) ' M-function IF a@ = 0 THEN RETURN (0) ' memoize the solution IF @(a@+100) < 0 THEN @(a@+100) = a@ - FUNC(_f(FUNC(_m(a@ - 1)))) RETURN (@(a@+100)) ' return array element  Output: F sequence: 1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12 13 M sequence: 0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12 12 0 OK, 0:199 ## UNIX Shell Works with: Bourne Again SHell M() { local n n=$1
if [[ $n -eq 0 ]]; then echo -n 0 else echo -n$(( n - $(F$(M $((n-1)) ) ) )) fi } F() { local n n=$1
if [[ $n -eq 0 ]]; then echo -n 1 else echo -n$(( n - $(M$(F $((n-1)) ) ) )) fi } for((i=0; i < 20; i++)); do F$i
echo -n " "
done
echo
for((i=0; i < 20; i++)); do
M \$i
echo -n " "
done
echo


## Ursala

Forward declarations are not an issue in Ursala, which allows any definition to depend on any symbol declared within the same scope. However, cyclic dependences are not accepted unless the programmer explicitly accounts for their semantics. If the recurrence can be solved using a fixed point combinator, the compiler can be directed to use one by the #fix directive as shown, in this case with one of a family of functional fixed point combinators from a library. (There are easier ways to define these functions in Ursala than by mutual recursion, but fixed points are useful for other things as well.)

#import std
#import nat
#import sol

#fix general_function_fixer 0

F = ~&?\1! difference^/~& M+ F+ predecessor
M = ~&?\0! difference^/~& F+ M+ predecessor

This test program applies both functions to the first twenty natural numbers.

#cast %nLW

test = ^(F*,M*) iota 20
Output:
(
<1,1,2,2,3,3,4,5,5,6,6,7,8,8,9,9,10,11,11,12>,
<0,0,1,2,2,3,4,4,5,6,6,7,7,8,9,9,10,11,11,12>)

## Vala

int F(int n) {
if (n == 0) return 1;
return n - M(F(n - 1));
}

int M(int n) {
if (n == 0) return 0;
return n - F(M(n - 1));
}

void main() {
print("n : ");
for (int s = 0; s < 25; s++){
print("%2d ", s);
}
print("\n------------------------------------------------------------------------------\n");
print("F : ");
for (int s = 0; s < 25; s++){
print("%2d ", F(s));
}
print("\nM : ");
for (int s = 0; s < 25; s++){
print("%2d ", M(s));
}
}

Output:
n :  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
------------------------------------------------------------------------------
F :  1  1  2  2  3  3  4  5  5  6  6  7  8  8  9  9 10 11 11 12 13 13 14 14 15
M :  0  0  1  2  2  3  4  4  5  6  6  7  7  8  9  9 10 11 11 12 12 13 14 14 15


## VBA

Private Function F(ByVal n As Integer) As Integer
If n = 0 Then
F = 1
Else
F = n - M(F(n - 1))
End If
End Function

Private Function M(ByVal n As Integer) As Integer
If n = 0 Then
M = 0
Else
M = n - F(M(n - 1))
End If
End Function

Public Sub MR()
Dim i As Integer
For i = 0 To 20
Debug.Print F(i);
Next i
Debug.Print
For i = 0 To 20
Debug.Print M(i);
Next i
End Sub

Output:
 1  1  2  2  3  3  4  5  5  6  6  7  8  8  9  9  10  11  11  12  13
0  0  1  2  2  3  4  4  5  6  6  7  7  8  9  9  10  11  11  12  12 

## Wren

var M  // forward declaration

var F = Fn.new { |n|
if (n == 0) return 1
return n - M.call(F.call(n-1))
}

M = Fn.new { |n|
if (n == 0) return 0
return n - F.call(M.call(n-1))
}

System.write("F(0..20): ")
(0..20).each { |i| System.write("%(F.call(i))  ") }
System.write("\nM(0..20): ")
(0..20).each { |i| System.write("%(M.call(i))  ") }
System.print()

Output:
F(0..20): 1  1  2  2  3  3  4  5  5  6  6  7  8  8  9  9  10  11  11  12  13
M(0..20): 0  0  1  2  2  3  4  4  5  6  6  7  7  8  9  9  10  11  11  12  12


## x86 Assembly

Works with: nasm

Since all "labels" (symbols), if not local, can be seen by the whole code in the same source unit, we don't need special care to let the subroutine func_f call func_m. If the function would have been in another source unit, we should have declared it extern (the linker will resolve the symbol), as done for printf.
(It must be linked with the C standard library libc or similar and a startup code; lazyly a gcc mutrec.o works, being mutrec.o produced by e.g. nasm -f elf mutrec.asm)

	global	main
extern	printf

section	.text

func_f
mov	eax, [esp+4]
cmp	eax, 0
jz	f_ret
dec	eax
push	eax
call	func_f
mov	[esp+0], eax
call	func_m
mov	ebx, [esp+4]
sub	ebx, eax
mov	eax, ebx
ret
f_ret
mov	eax, 1
ret

func_m
mov	eax, [esp+4]
cmp	eax, 0
jz	m_ret
dec	eax
push	eax
call	func_m
mov	[esp+0], eax
call	func_f
mov	ebx, [esp+4]
sub	ebx, eax
mov	eax, ebx
ret
m_ret
xor	eax, eax
ret

main
mov	edx, func_f
call	output_res
mov	edx, func_m
call	output_res
ret

output_res
xor	ecx, ecx
loop0
push	ecx
call	edx

push    edx

push	eax
push	form
call	printf

pop     edx
pop     ecx

inc	ecx
cmp	ecx, 20
jnz	loop0

push	newline
call	printf

ret

section	.rodata
form
db	'%d ',0
newline
db	10,0

end


## XPL0

code    ChOut=8, CrLf=9, IntOut=11;

ffunc M; \forward-referenced function declaration

func F(N);
int N;
return if N=0 then 1 else N - M(F(N-1));

func M(N);
int N;
return if N=0 then 0 else N - F(M(N-1));

int I;
[for I:= 0 to 19 do [IntOut(0, F(I));  ChOut(0, ^ )];
CrLf(0);
for I:= 0 to 19 do [IntOut(0, M(I));  ChOut(0, ^ )];
CrLf(0);
]
Output:
1 1 2 2 3 3 4 5 5 6 6 7 8 8 9 9 10 11 11 12
0 0 1 2 2 3 4 4 5 6 6 7 7 8 9 9 10 11 11 12


## Yabasic

Translation of: AWK
// User defined functions
sub F(n)
if n = 0 return 1
return n - M(F(n-1))
end sub

sub M(n)
if n = 0 return 0
return n - F(M(n-1))
end sub

for i = 0 to 20
print F(i) using "###";
next
print
for i = 0 to 20
print M(i) using "###";
next
print

## zkl

This works if the functions are in a file or on one line (in the REPL) as zkl doesn't like referencing undefined objects. You could also pass/close the other function.

fcn f(n){ if(n==0)return(1); n-m(f(n-1,m),f) }
fcn m(n){ if(n==0)return(0); n-f(m(n-1,f),m) }
[0..19].apply(f).println();  // or foreach n in ([0..19]){ print(f(n)," ") }
[0..19].apply(m).println();  // or foreach n in ([0..19]){ print(m(n)," ") }
Output:
L(1,1,2,2,3,3,4,5,5,6,6,7,8,8,9,9,10,11,11,12)
L(0,0,1,2,2,3,4,4,5,6,6,7,7,8,9,9,10,11,11,12)
`