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In mathematics, Thiele's interpolation formula is an interpolation formula for a function f(•) of a single variable, due to the Danish mathematician Thorvald Nicolai Thiele. It is expressed as a continued fraction, where ρ represents the reciprocal difference:

f(x)=f(x_{1})+{\cfrac {x-x_{1}}{\rho (x_{1},x_{2})+{\cfrac {x-x_{2}}{\rho _{2}(x_{1},x_{2},x_{3})-f(x_{1})+{\cfrac {x-x_{3}}{\rho _{3}(x_{1},x_{2},x_{3},x_{4})-\rho (x_{1},x_{2})+\cdots }}}}}}

Demonstrate Thiele's interpolation function by:

Building a 32 row trig table of values of the trig functions sin, cos and tan. e.g. for x from 0 by 0.05 to 1.55...
Using columns from this table define an inverse - using Thiele's interpolation - for each trig function;
Finally: demonstrate the following well known trigonometric identities:
- 6 × sin^-1 ½ = π
- 3 × cos^-1 ½ = π
- 4 × tan^-1 1 = π

ALGOL 68

Works with: ALGOL 68 version Revision 1 - except the Currying (aka partial parametrisation) in test block is a proposal for ALGOL 68 Rev2

Works with: ALGOL 68G version Any - tested with release 1.18.0-9h.tiny - Currying is supported.

<lang algol68>PROC raise exception = ([]STRING msg)VOID: ( putf(stand error,("Exception:", $" "g$, msg, $l$)); stop );

The MODE of lx and ly here should really be a UNION of "something REAL",

"something COMPLex", and "something SYMBOLIC" ... #

PROC thiele=([]REAL lx,ly, REAL x) REAL: BEGIN

 []REAL xx=lx[@1],yy=ly[@1];
 INT n=UPB xx;
 IF UPB yy=n THEN

Assuming that the values of xx are distinct ... #

   [0:n-1,1:n]REAL p;
   p[0,]:=yy[];
   FOR i TO n-1 DO p[1,i]:=(xx[i]-xx[1+i])/(p[0,i]-p[0,1+i]) OD;
   FOR i FROM 2 TO n-1 DO
     FOR j TO n-i DO
       p[i,j]:=(xx[j]-xx[j+i])/(p[i-1,j]-p[i-1,j+1])+p[i-2,j+1]
     OD
   OD;
   REAL a:=0;
   FOR i FROM n-1 BY -1 TO 2 DO a:=(x-xx[i])/(p[i,1]-p[i-2,1]+a) OD;
   yy[1]+(x-xx[1])/(p[1,1]+a)
 ELSE
   raise exception(("Unequal length arrays supplied: ",whole(UPB xx,0)," NE ",whole(UPB yy,0))); SKIP
 FI

END;

test:(

 FORMAT real fmt = $g(0,real width-2)$;

 REAL lwb x=0, upb x=1.55, delta x = 0.05;

 [0:ENTIER ((upb x-lwb x)/delta x)]STRUCT(REAL x, sin x, cos x, tan x) trig table;

 PROC init trig table = VOID: 
   FOR i FROM LWB trig table TO UPB trig table DO 
     REAL x = lwb x+i*delta x; 
     trig table[i]:=(x, sin(x), cos(x), tan(x))
   OD;

 init trig table;

Curry the thiele function to create matching inverse trigonometric functions #

 PROC (REAL)REAL inv sin = thiele(sin x OF trig table, x OF trig table,),
                 inv cos = thiele(cos x OF trig table, x OF trig table,),
                 inv tan = thiele(tan x OF trig table, x OF trig table,);

 printf(($"pi estimate using "g" interpolation: "f(real fmt)l$, 
   "sin", 6*inv sin(1/2),
   "cos", 3*inv cos(1/2),
   "tan", 4*inv tan(1)
 ))

)</lang> Output:

pi estimate using sin interpolation: 3.1415926535898
pi estimate using cos interpolation: 3.1415926535898
pi estimate using tan interpolation: 3.1415926535898

D

<lang d>import std.stdio ; import std.math ;

U[] myMap(U,V) (U delegate(V) f, V[] a) {

   V[] r = new V[](a.length) ;
   foreach(i, v ; a)
       r[i] = f(v) ;
   return r ;

}

T[] myRng(T)(T start, T end, T step) {

   T[] r ;
   for(; start < end ; start += step)
       r ~= start ;
   return r ;

}

alias real delegate(real) RealFun ; const real End = 1.55L , Step = 0.05L , Start = 0.0L ;

class Thiele {

   const real[] F, X, RhoY, RhoX ;
   int NN ;
   
   this(real[] f, real[] x) {
       F   = f.dup ;
       X   = x.dup ;
       NN  = X.length ;
       assert(NN > 3, "at leat 4 values") ;
       assert(X.length == F.length, "input arrays not of same size") ;
       RhoY = rhoN(F, X) ;
       RhoX = rhoN(X, F) ;
   }

   this(RealFun fun, real start = Start, real end = End, real step = Step) {
       this(myMap(fun, myRng(start, end, step)), 
            myRng(start, end, step)) ;
   }

   private static real[] rhoN(real[] f, real[] x) {
       int N = x.length;
       real[][] p = new real[][] (N, N) ;
       for(int i = 0; i < N ; i++)
           p[i][0] = f[i] ;
       for(int i = 0; i < N - 1 ; i++)
           p[i][1] = (x[i] - x[i+1]) / (p[i][0] - p[i+1][0]) ;
       for(int j = 2; j < N - 1; j++)
           for(int i = 0; i < N - j - 1 ; i++)
               p[i][j] = p[i+1][j-2] + (x[i] - x[i+j]) /  
                         (p[i][j-1] - p[i+1][j-1]) ;                           ;
       return p[1].dup ;    
   }

   real evalY(real x) {
       real a = 0.0L ;
       for(int i = NN - 3; i >= 2 ; i--)
           a = (x - X[i]) / (RhoY[i] - RhoY[i-2] + a) ;
       return F[1] + (x - X[1]) / (RhoY[1] + a) ;        
   }
   real evalX(real y) { // inverse
       real a = 0.0L ;
       for(int i = NN - 3; i >= 2 ; i--)
           a = (y - F[i]) / (RhoX[i] - RhoX[i-2] + a) ;
       return X[1] + (y - F[1]) / (RhoX[1] + a) ;        
   }

}

void main() {

   auto fun = (real x) { return std.math.sin(x) ; } ;
   auto t = new Thiele(fun) ;
   writefln(" %d interpolating points", t.NN) ;
   writefln("std.math.sin(0.5) : %20.18f", std.math.sin(0.5L)) ;
   writefln("  Thiele sin(0.5) : %20.18f", t.evalY(0.5L)) ;    
   writefln("*%20.18f library constant", std.math.PI) ;
   writefln(" %20.18f 6*sin`(0.5)", t.evalX(0.5L) * 6.0L) ;
   t = new Thiele((real x) {return std.math.cos(x) ; }) ;
   writefln(" %20.18f 3*cos`(0.5)", t.evalX(0.5L) * 3.0L) ;
   t = new Thiele((real x) {return std.math.tan(x) ; }) ;
   writefln(" %20.18f 4*tan`(0.5)", t.evalX(1.0L) * 4.0L) ;

}</lang>

output:

 32 interpolating points
std.math.sin(0.5) : 0.479425538604203000
  Thiele sin(0.5) : 0.479425538604203000
*3.141592653589793239 library constant
 3.141592653589793238 6*sin`(0.5)
 3.141592653589793238 3*cos`(0.5)
 3.141592653589793238 4*tan`(0.5)

Tcl

This example is incorrect. Please fix the code and remove this message.

Details: It produces the wrong values for output.

Works with: Tcl version 8.5

<lang tcl># Multi-list map; deeper voodoo than normal! proc map args {

   foreach {a b} [lrange $args 0 end-1] {

lappend parms [incr v] $b upvar 1 $a $v

   }
   foreach {*}$parms {lappend result [uplevel 1 [lindex $args end]]}
   return $result

}

proc thiele {X Y x} {

   # Sanity check
   if {[llength $X] != [llength $Y]} {

error "unequal length lists supplied: [llength $X] != [llength $Y]"

   #
   ### Compute the table of reciprocal differences
   #
   set n [expr {[llength $Y] - 1}]
   # Zero-th order
   set r [list $Y]
   # First order
   lappend r [map a [lrange $X 0 end-1] b [lrange $X 1 end] \

c [lrange $Y 0 end-1] d [lrange $Y 1 end] \ e [lrepeat $n 0] { expr {($a - $b) / ($c - $d + $e)}

   }]
   # Higher order
   for {set i 2} {$i <= $n} {incr i} {

set p [lindex $r end]; # previous row set pp [lindex $r end-1]; # doubly-previous row; longer than $p by 1 lappend r [map a [lrange $X 0 end-$i] b [lrange $X $i end] \ c [lrange $p 0 end-1] d [lrange $p 1 end] \ e [lrange $pp 1 end-1] { expr {($a - $b) / ($c - $d + $e)} }]

   #
   ### Actually evaluate Thiele's formula
   #
   set a 0.0
   foreach b [lreverse [lrange $X 2 end]] c [lreverse [lrange $r 2 end]] \

d [lreverse [lrange $r 0 end-2]] { set a [expr {($x - $b) / ([lindex $c 0] - [lindex $d 0] + $a)}]

   }
   expr {[lindex $Y 0] + ($x - [lindex $X 0]) / ([lindex $r 1 0] + $a)}

}</lang> Demonstration code: <lang tcl>proc initThieleTest {} {

   for {set i 0} {$i < 32} {incr i} {

lappend trigTable(x) [set x [expr {0.05 * $i}]] lappend trigTable(sin) [expr {sin($x)}] lappend trigTable(cos) [expr {cos($x)}] lappend trigTable(tan) [expr {tan($x)}]

   interp alias {} invSin {} thiele $trigTable(sin) $trigTable(x)
   interp alias {} invCos {} thiele $trigTable(cos) $trigTable(x)
   interp alias {} invTan {} thiele $trigTable(tan) $trigTable(x)

} initThieleTest puts "pi estimate using sin interpolation: [expr {6 * [invSin 0.5]}]" puts "pi estimate using cos interpolation: [expr {3 * [invCos 0.5]}]" puts "pi estimate using tan interpolation: [expr {4 * [invTan 1.0]}]"</lang> Output:

pi estimate using sin interpolation: 3.7607373341712798
pi estimate using cos interpolation: 1.3300617930864258
pi estimate using tan interpolation: 8.98541899822322