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In modular arithmetic, the modular multiplicative inverse of an integer a modulo m is an integer x such that

a\,x\equiv 1{\pmod {m}}.

Or in other words, such that:

\exists k\in \mathbb {Z} ,\qquad a\,x=1+k\,m

It can be shown that such an inverse exists if and only if a and m are coprime, but we will ignore this for this task.

Task

Either by implementing the algorithm, by using a dedicated library or by using a built-in function in your language, compute the modular inverse of 42 modulo 2017.

8th

<lang Forth> \ return "extended gcd" of a and b; The result satisfies the equation: \ a*x + b*y = gcd(a,b)

n:xgcd \ a b -- gcd x y

 dup 0 n:= if
   1 swap            \ -- a 1 0
 else
   tuck n:/mod
   -rot recurse
   tuck 4 roll
   n:* n:neg n:+
 then ;

\ Return modular inverse of n modulo mod, or null if it doesn't exist (n and mod \ not coprime):

n:invmod \ n mod -- invmod

 dup >r
 n:xgcd rot 1 n:= not if
   2drop null
 else
   drop dup 0 n:< if r@ n:+ then
 then
 rdrop ;

42 2017 n:invmod . cr bye </lang>

Output:

ALGOL 68

<lang algol68> BEGIN

  PROC modular inverse = (INT a, m) INT :
  BEGIN
     PROC extended gcd = (INT x, y) []INT :

CO

  Algol 68 allows us to return three INTs in several ways.  A [3]INT
  is used here but it could just as well be a STRUCT.

CO

     BEGIN

INT v := 1, a := 1, u := 0, b := 0, g := x, w := y; WHILE w>0 DO INT q := g % w, t := a - q * u; a := u; u := t; t := b - q * v; b := v; v := t; t := g - q * w; g := w; w := t OD; a PLUSAB (a < 0 | u | 0); (a, b, g)

     END;
     [] INT egcd = extended gcd (a, m);
     (egcd[3] > 1 | 0 | egcd[1] MOD m)      
  END;
  printf (($"42 ^ -1 (mod 2017) = ", g(0)$, modular inverse (42, 2017)))

CO

  Note that if ϕ(m) is known, then a^-1 = a^(ϕ(m)-1) mod m which
  allows an alternative implementation in terms of modular
  exponentiation but, in general, this requires the factorization of
  m.  If m is prime the factorization is trivial and ϕ(m) = m-1.
  2017 is prime which may, or may not, be ironic within the context
  of the Rosetta Code conditions.

CO END </lang>

Output:

42 ^ -1 (mod 2017) = 1969

AutoHotkey

Translation of C. <lang AutoHotkey>MsgBox, % ModInv(42, 2017)

ModInv(a, b) { if (b = 1) return 1 b0 := b, x0 := 0, x1 :=1 while (a > 1) { q := a // b , t := b , b := Mod(a, b) , a := t , t := x0 , x0 := x1 - q * x0 , x1 := t } if (x1 < 0) x1 += b0 return x1 }</lang>

Output:

AWK

syntax: GAWK -f MODULAR_INVERSE.AWK
converted from C

BEGIN {

   printf("%s\n",mod_inv(42,2017))
   exit(0)

} function mod_inv(a,b, b0,t,q,x0,x1) {

   b0 = b
   x0 = 0
   x1 = 1
   if (b == 1) {
     return(1)
   }
   while (a > 1) {
     q = int(a / b)
     t = b
     b = int(a % b)
     a = t
     t = x0
     x0 = x1 - q * x0
     x1 = t
   }
   if (x1 < 0) {
     x1 += b0
   }
   return(x1)

} </lang>

Output:

Batch File

Based from C's second implementation

Translation of: Perl

<lang dos>@echo off setlocal enabledelayedexpansion %== Calls the "function" ==% call :ModInv 42 2017 result echo !result! call :ModInv 40 1 result echo !result! call :ModInv 52 -217 result echo !result! call :ModInv -486 217 result echo !result! call :ModInv 40 2018 result echo !result! pause>nul exit /b 0

%== The "function" ==%

ModInv

set a=%1 set b=%2

if !b! lss 0 (set /a b=-b) if !a! lss 0 (set /a a=b - ^(-a %% b^))

set t=0&set nt=1&set r=!b!&set /a nr=a%%b

:while_loop if !nr! neq 0 ( set /a q=r/nr set /a tmp=nt set /a nt=t - ^(q*nt^) set /a t=tmp

set /a tmp=nr set /a nr=r - ^(q*nr^) set /a r=tmp goto while_loop )

if !r! gtr 1 (set %3=-1&goto :EOF) if !t! lss 0 set /a t+=b set %3=!t! goto :EOF</lang>

Output:

Bracmat

Translation of: Julia

<lang bracmat>( ( mod-inv

 =   a b b0 x0 x1 q
   .   !arg:(?a.?b)
     & ( !b:1
       |   (!b.0.1):(?b0.?x0.?x1)
         &   whl
           ' ( !a:>1
             & div$(!a.!b):?q
             & (!b.mod$(!a.!b)):(?a.?b)
             & (!x1+-1*!q*!x0.!x0):(?x0.?x1)
             )
         & (!x:>0|!x1+!b0)
       )
 )

& out$(mod-inv$(42.2017)) };</lang> Output

C

<lang c>#include <stdio.h>

int mul_inv(int a, int b) { int b0 = b, t, q; int x0 = 0, x1 = 1; if (b == 1) return 1; while (a > 1) { q = a / b; t = b, b = a % b, a = t; t = x0, x0 = x1 - q * x0, x1 = t; } if (x1 < 0) x1 += b0; return x1; }

int main(void) { printf("%d\n", mul_inv(42, 2017)); return 0; }</lang>

The above method has some problems. Most importantly, when given a pair (a,b) with no solution, it generates an FP exception. When given b=1, it returns 1 which is not a valid result mod 1. When given negative a or b the results are incorrect. The following generates results that should match Pari/GP for numbers in the int range.

Translation of: Perl

<lang c>#include <stdio.h>

int mul_inv(int a, int b) {

       int t, nt, r, nr, q, tmp;
       if (b < 0) b = -b;
       if (a < 0) a = b - (-a % b);
       t = 0;  nt = 1;  r = b;  nr = a % b;
       while (nr != 0) {
         q = r/nr;
         tmp = nt;  nt = t - q*nt;  t = tmp;
         tmp = nr;  nr = r - q*nr;  r = tmp;
       }
       if (r > 1) return -1;  /* No inverse */
       if (t < 0) t += b;
       return t;

} int main(void) {

       printf("%d\n", mul_inv(42, 2017));
       printf("%d\n", mul_inv(40, 1));
       printf("%d\n", mul_inv(52, -217));  /* Pari semantics for negative modulus */
       printf("%d\n", mul_inv(-486, 217));
       printf("%d\n", mul_inv(40, 2018));
       return 0;

}</lang>

Output:

C++

Translation of: C

<lang cpp>#include <iostream>

using namespace std;

int mul_inv(int a, int b) { int b0 = b, t, q; int x0 = 0, x1 = 1; if (b == 1) return 1; while (a > 1) { q = a / b; t = b, b = a % b, a = t; t = x0, x0 = x1 - q * x0, x1 = t; } if (x1 < 0) x1 += b0; return x1; }

int main(void) { cout<<mul_inv(42, 2017)<<endl; return 0; } </lang>

Recursive implementation <lang cpp>#include <iostream>

short ObtainMultiplicativeInverse(int a, int b, int s0 = 1, int s1 = 0) {

   return b==0? s0: ObtainMultiplicativeInverse(b, a%b, s1, s0 - s1*(a/b));

}

int main(int argc, char* argv[]) {

   std::cout << ObtainMultiplicativeInverse(42, 2017) << std::endl;
   return 0;

} </lang>

C#

<lang csharp>public class Program {

   static void Main()
   {
       System.Console.WriteLine(42.ModInverse(2017));
   }

}

public static class IntExtensions {

   public static int ModInverse(this int a, int m)
   {
       if (m == 1) return 0;
       int m0 = m;
       (int x, int y) = (1, 0);

       while (a > 1) {
           int q = a / m;
           (a, m) = (m, a % m);
           (x, y) = (y, x - q * y);
       }
       return x < 0 ? x + m0 : x;
   }

}</lang>

Clojure

<lang lisp>(ns test-p.core

 (:require [clojure.math.numeric-tower :as math]))

(defn extended-gcd

 "The extended Euclidean algorithm--using Clojure code from RosettaCode for Extended Eucliean
 (see http://en.wikipedia.orwiki/Extended_Euclidean_algorithm)
 Returns a list containing the GCD and the Bézout coefficients
 corresponding to the inputs with the result: gcd followed by bezout coefficients "
 [a b]
 (cond (zero? a) [(math/abs b) 0 1]
       (zero? b) [(math/abs a) 1 0]
       :else (loop [s 0
                    s0 1
                    t 1
                    t0 0
                    r (math/abs b)
                    r0 (math/abs a)]
               (if (zero? r)
                 [r0 s0 t0]
                 (let [q (quot r0 r)]
                   (recur (- s0 (* q s)) s
                          (- t0 (* q t)) t
                          (- r0 (* q r)) r))))))

(defn mul_inv

 " Get inverse using extended gcd.  Extended GCD returns
   gcd followed by bezout coefficients. We want the 1st coefficients
  (i.e. second of extend-gcd result).  We compute mod base so result
   is between 0..(base-1) "
 [a b]
 (let [b (if (neg? b) (- b) b)
       a (if (neg? a) (- b (mod (- a) b)) a)
       egcd (extended-gcd a b)]
     (if (= (first egcd) 1)
       (mod (second egcd) b)
       (str "No inverse since gcd is: " (first egcd)))))

(println (mul_inv 42 2017)) (println (mul_inv 40 1)) (println (mul_inv 52 -217)) (println (mul_inv -486 217)) (println (mul_inv 40 2018))

</lang>

Output:

1969
0
96
121
No inverse since gcd is: 2

Common Lisp

Calculates the GCD of a and b based on the Extended Euclidean Algorithm. The function also returns the Bézout coefficients s and t, such that gcd(a, b) = as + bt. The algorithm is described on page http://en.wikipedia.org/wiki/Extended_Euclidean_algorithm#Iterative_method_2

(defun egcd (a b)

 (do ((r (cons b a) (cons (- (cdr r) (* (car r) q)) (car r))) ; (r+1 r) i.e. the latest is first.
      (s (cons 0 1) (cons (- (cdr s) (* (car s) q)) (car s))) ; (s+1 s)
      (u (cons 1 0) (cons (- (cdr u) (* (car u) q)) (car u))) ; (t+1 t)
      (q nil))
     ((zerop (car r)) (values (cdr r) (cdr s) (cdr u)))       ; exit when r+1 = 0 and return r s t
   (setq q (floor (/ (cdr r) (car r))))))                     ; inside loop; calculate the q

Calculates the inverse module for a = 1 (mod m). Note The inverse is only defined when a and m are coprimes, i.e. gcd(a, m) = 1.”

(defun invmod (a m)

 (multiple-value-bind (r s k) (egcd a m)
   (unless (= 1 r) (error "invmod: Values ~a and ~a are not coprimes." a m))  
    s))

</lang>

Output:

* (invmod 42 2017)

-48
* (mod -48 2017)

1969

D

Translation of: C

<lang d>T modInverse(T)(T a, T b) pure nothrow {

   if (b == 1)
       return 1;
   T b0 = b,
     x0 = 0,
     x1 = 1;

   while (a > 1) {
       immutable q = a / b;
       auto t = b;
       b = a % b;
       a = t;
       t = x0;
       x0 = x1 - q * x0;
       x1 = t;
   }
   return (x1 < 0) ? (x1 + b0) : x1;

}

void main() {

   import std.stdio;
   writeln(modInverse(42, 2017));

}</lang>

Output:

EchoLisp

<lang scheme> (lib 'math) ;; for egcd = extended gcd

(define (mod-inv x m)

   (define-values (g inv q) (egcd x m))
   (unless (= 1 g) (error 'not-coprimes (list x m) ))
   (if (< inv 0) (+ m inv) inv))

(mod-inv 42 2017) → 1969 (mod-inv 42 666) 🔴 error: not-coprimes (42 666) </lang>

Elixir

Translation of: Ruby

<lang elixir>defmodule Modular do

 def extended_gcd(a, b) do
   {last_remainder, last_x} = extended_gcd(abs(a), abs(b), 1, 0, 0, 1)
   {last_remainder, last_x * (if a < 0, do: -1, else: 1)}
 end
 
 defp extended_gcd(last_remainder, 0, last_x, _, _, _), do: {last_remainder, last_x}
 defp extended_gcd(last_remainder, remainder, last_x, x, last_y, y) do
   quotient   = div(last_remainder, remainder)
   remainder2 = rem(last_remainder, remainder)
   extended_gcd(remainder, remainder2, x, last_x - quotient*x, y, last_y - quotient*y)
 end
 
 def inverse(e, et) do
     {g, x} = extended_gcd(e, et)
     if g != 1, do: raise "The maths are broken!"
     rem(x+et, et)
   end
 end

IO.puts Modular.inverse(42,2017)</lang>

Output:

ERRE

<lang ERRE>PROGRAM MOD_INV

!$INTEGER

PROCEDURE MUL_INV(A,B->T)

 LOCAL NT,R,NR,Q,TMP
 IF B<0 THEN B=-B
 IF A<0 THEN A=B-(-A MOD B)
 T=0  NT=1  R=B  NR=A MOD B
 WHILE NR<>0 DO
     Q=R DIV NR
     TMP=NT  NT=T-Q*NT  T=TMP
     TMP=NR  NR=R-Q*NR  R=TMP
 END WHILE
 IF (R>1) THEN T=-1 EXIT PROCEDURE  ! NO INVERSE
 IF (T<0) THEN T+=B

END PROCEDURE

BEGIN

    MUL_INV(42,2017->T) PRINT(T)
    MUL_INV(40,1->T) PRINT(T)
    MUL_INV(52,-217->T) PRINT(T)    ! pari semantics for negative modulus
    MUL_INV(-486,217->T)  PRINT(T)
    MUL_INV(40,2018->T) PRINT(T)

END PROGRAM </lang>

Output:

F#

<lang fsharp> //Calculate the Modular Inverse: Nigel Galloway: April 3rd., 2018 let MI n g =

 let rec fN n i g e l a =
   match e with
   | 0 -> g
   | _ -> let o = n/e
          fN e l a (n-o*e) (i-o*l) (g-o*a) 
 (n+(fN n 1 0 g 0 1))%n

</lang>

Output:

MI 2017 42 -> 1969

Factor

<lang>USE: math.functions 42 2017 mod-inv</lang>

Output:

Forth

ANS Forth with double-number word set <lang forth>

invmod { a m | v b c -- inv }

 m to v  
 1 to c  
 0 to b
 begin a
 while v a / >r
    c b s>d c s>d r@ 1 m*/ d- d>s to c to b
    a v s>d a s>d r> 1 m*/ d- d>s to a to v
 repeat b m mod dup to b 0<
 if m b + else b then ;

</lang> ANS Forth version without locals <lang forth>

modinv ( a m - inv)

 dup 1-              \ a m (m != 1)?
 if                  \ a m
   tuck 1 0          \ m0 a m 1 0
   begin             \ m0 a m inv x0
     2>r over 1 >    \ m0 a m (a > 1)?       R: inv x0
   while             \ m0 a m
     tuck /mod       \ m0 m (a mod m) (a/m)
     r> tuck *       \ m0 a' m' x0 (a/m)*x0  R: inv
     r> swap -       \ m0 a' m' x0 (inv-q)
   repeat            \ m0 a' m' inv' x0'
   2drop 2r> drop    \ m0 inv                R:
   dup 0<            \ m0 inv (inv < 0)?
   if over + then    \ m0 (inv'+ m0)
 then                \ x inv'
 nip                 \ inv

</lang>

42 2017 invmod . 1969
42 2017 modinv . 1969

FreeBASIC

<lang freebasic>' version 10-07-2018 ' compile with: fbc -s console

Type ext_euclid

   Dim As Integer a, b

End Type

' "Table method" aka "The Magic Box" Function magic_box(x As Integer, y As Integer) As ext_euclid

   Dim As Integer a(1 To 128), b(1 To 128), d(1 To 128), k(1 To 128)

   a(1) = 1 : b(1) = 0 : d(1) = x
   a(2) = 0 : b(2) = 1 : d(2) = y : k(2) = x \ y

   Dim As Integer i = 2

   While Abs(d(i)) <> 1
       i += 1
       a(i) = a(i -2) - k(i -1) * a(i -1)
       b(i) = b(i -2) - k(i -1) * b(i -1)
       d(i) = d(i -2) Mod d(i -1)
       k(i) = d(i -1) \ d(i)
       'Print a(i),b(i),d(i),k(i)
       If d(i -1) Mod d(i) = 0 Then Exit While
   Wend
   
   If d(i) = -1 Then '  -1 * (ab + by) = -1 * -1 ==> -ab -by = 1 
       a(i) = -a(i)
       b(i) = -b(i)
   End If

   Function = Type( a(i), b(i) )

End Function ' ------=< MAIN >=------

Dim As Integer x, y, gcd Dim As ext_euclid result

Do

   Read x, y
   If x = 0 AndAlso y = 0 Then Exit Do
   result = magic_box(x, y)
   With result
       gcd = .a * x + .b * y
       Print "a * "; Str(x); " + b * "; Str(y);  
       Print " = GCD("; Str(x); ", "; Str(y); ") ="; gcd
       If gcd > 1 Then 
           Print "No solution, numbers are not coprime"
       Else 
           Print "a = "; .a; ", b = ";.b
           Print "The Modular inverse of "; x; " modulo "; y; " = ";
           While .a < 0 : .a += IIf(y > 0, y, -y) : Wend 
           Print .a
           'Print "The Modular inverse of "; y; " modulo "; x; " = ";
           'While .b < 0 : .b += IIf(x > 0, x, -x) : Wend 
           'Print .b
       End if
   End With
   Print

Loop

Data 42, 2017 Data 40, 1 Data 52, -217 Data -486, 217 Data 40, 2018 Data 0, 0

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

Output:

a * 42 + b * 2017 = GCD(42, 2017) = 1
a = -48, b =  1
The Modular inverse of  42 modulo  2017 =  1969

a * 40 + b * 1 = GCD(40, 1) = 1
a =  0, b =  1
The Modular inverse of  40 modulo  1 =  0

a * 52 + b * -217 = GCD(52, -217) = 1
a =  96, b =  23
The Modular inverse of  52 modulo -217 =  96

a * -486 + b * 217 = GCD(-486, 217) = 1
a = -96, b = -215
The Modular inverse of -486 modulo  217 =  121

a * 40 + b * 2018 = GCD(40, 2018) = 2
No solution, numbers are not coprime

FunL

<lang funl>import integers.egcd

def modinv( a, m ) =

   val (g, x, _) = egcd( a, m )

   if g != 1 then error( a + ' and ' + m + ' not coprime' )
       
   val res = x % m

   if res < 0 then res + m else res

println( modinv(42, 2017) )</lang>

Output:

Go

The standard library function uses the extended Euclidean algorithm internally. <lang go>package main

import ( "fmt" "math/big" )

func main() { a := big.NewInt(42) m := big.NewInt(2017) k := new(big.Int).ModInverse(a, m) fmt.Println(k) }</lang>

Output:

GW-BASIC

Translation of: Pascal

Works with: PC-BASIC version any

<lang qbasic> 10 ' Modular inverse 20 LET E% = 42 30 LET T% = 2017 40 GOSUB 1000 50 PRINT MODINV% 60 END

990 ' increments e stp (step) times until bal is greater than t 992 ' repeats until bal = 1 (mod = 1) and returns count 994 ' bal will not be greater than t + e 1000 LET D% = 0 1010 IF E% >= T% THEN GOTO 1140 1020 LET BAL% = E% 1025 ' At least one iteration is necessary 1030 LET STP% = ((T% - BAL%) \ E%) + 1 1040 LET BAL% = BAL% + STP% * E% 1050 LET COUNT% = 1 + STP% 1060 LET BAL% = BAL% - T% 1070 WHILE BAL% <> 1 1080 LET STP% = ((T% - BAL%) \ E%) + 1 1090 LET BAL% = BAL% + STP% * E% 1100 LET COUNT% = COUNT% + STP% 1110 LET BAL% = BAL% - T% 1120 WEND 1130 LET D% = COUNT% 1140 LET MODINV% = D% 1150 RETURN </lang>

Output:

Haskell

<lang haskell>-- Given a and m, return Just x such that ax = 1 mod m. -- If there is no such x return Nothing. modInv :: Int -> Int -> Maybe Int modInv a m

 | 1 == g = Just (mkPos i)
 | otherwise = Nothing
 where
   (i, _, g) = gcdExt a m
   mkPos x
     | x < 0 = x + m
     | otherwise = x

-- Extended Euclidean algorithm. -- Given non-negative a and b, return x, y and g -- such that ax + by = g, where g = gcd(a,b). -- Note that x or y may be negative. gcdExt :: Int -> Int -> (Int, Int, Int) gcdExt a 0 = (1, 0, a) gcdExt a b =

 let (q, r) = a `quotRem` b
     (s, t, g) = gcdExt b r
 in (t, s - q * t, g)

main :: IO () main = mapM_ print [2 `modInv` 4, 42 `modInv` 2017]</lang>

Output:

Nothing
Just 1969

Icon and Unicon

Translation of: C

<lang unicon>procedure main(args)

   a := integer(args[1]) | 42
   b := integer(args[2]) | 2017
   write(mul_inv(a,b))

end

procedure mul_inv(a,b)

   if b == 1 then return 1
   (b0 := b, x0 := 0, x1 := 1)
   while a > 1 do {
       q := a/b
       (t := b, b := a%b, a := t)
       (t := x0, x0 := x1-q*x0, x1 := t)
       }
   return if (x1 > 0) then x1 else x1+b0

end</lang>

Output:

->mi
1969
->

Adding a coprime test:

<lang unicon>link numbers

procedure main(args)

   a := integer(args[1]) | 42
   b := integer(args[2]) | 2017
   write(mul_inv(a,b))

end

procedure mul_inv(a,b)

   if b == 1 then return 1
   if gcd(a,b) ~= 1 then return "not coprime"
   (b0 := b, x0 := 0, x1 := 1)
   while a > 1 do {
       q := a/b
       (t := b, b := a%b, a := t)
       (t := x0, x0 := x1-q*x0, x1 := t)
       }
   return if (x1 > 0) then x1 else x1+b0

end</lang>

IS-BASIC

<lang IS-BASIC>100 PRINT MODINV(42,2017) 120 DEF MODINV(A,B) 130 LET B=ABS(B) 140 IF A<0 THEN LET A=B-MOD(-A,B) 150 LET T=0:LET NT=1:LET R=B:LET NR=MOD(A,B) 160 DO WHILE NR<>0 170 LET Q=INT(R/NR) 180 LET TMP=NT:LET NT=T-Q*NT:LET T=TMP 190 LET TMP=NR:LET NR=R-Q*NR:LET R=TMP 200 LOOP 210 IF R>1 THEN 220 LET MODINV=-1 230 ELSE IF T<0 THEN 240 LET MODINV=T+B 250 ELSE 260 LET MODINV=T 270 END IF 280 END DEF</lang>

J

Solution:<lang j> modInv =: dyad def 'x y&|@^ <: 5 p: y'"0</lang> Example:<lang j> 42 modInv 2017 1969</lang> Notes:

Calculates the modular inverse as a^( totient(m) - 1 ) mod m.

5 p: y is Euler's totient function of y.

J has a fast implementation of modular exponentiation (which avoids the exponentiation altogether), invoked with the form m&|@^ (hence, we use explicitly-named arguments for this entry, as opposed to the "variable free" tacit style: the m&| construct must freeze the value before it can be used but we want to use different values in that expression at different times...).

Java

The BigInteger library has a method for this: <lang java>System.out.println(BigInteger.valueOf(42).modInverse(BigInteger.valueOf(2017)));</lang>

Output:

JavaScript

Using brute force. <lang javascript>var modInverse = function(a, b) {

   a %= b;
   for (var x = 1; x < b; x++) {
       if ((a*x)%b == 1) {
           return x;
       }
   }

}</lang>

Julia

Built-in

Julia includes a built-in function for this: <lang julia>invmod(a, b)</lang>

C translation

Translation of: C

The following code works on any integer type. To maximize performance, we ensure (via a promotion rule) that the operands are the same type (and use built-ins zero(T) and one(T) for initialization of temporary variables to ensure that they remain of the same type throughout execution). <lang julia>function modinv{T<:Integer}(a::T, b::T)

   b0 = b
   x0, x1 = zero(T), one(T)
   while a > 1
       q = div(a, b)
       a, b = b, a % b
       x0, x1 = x1 - q * x0, x0
   end
   x1 < 0 ? x1 + b0 : x1

end modinv(a::Integer, b::Integer) = modinv(promote(a,b)...)</lang>

Output:

julia> invmod(42, 2017)
1969

julia> modinv(42, 2017)
1969

Kotlin

<lang scala>// version 1.0.6

import java.math.BigInteger

fun main(args: Array<String>) {

   val a = BigInteger.valueOf(42)
   val m = BigInteger.valueOf(2017)
   println(a.modInverse(m))

}</lang>

Output:

Maple

Output:

Mathematica

The built-in function FindInstance works well for this <lang Mathematica>modInv[a_, m_] :=

Block[{x,k}, x /. FindInstance[a x == 1 + k m, {x, k}, Integers]]</lang>

Another way by using the built-in function PowerMod : <lang Mathematica>PowerMod[a,-1,m]</lang> For example :

modInv[42, 2017]

{1969}

PowerMod[42, -1, 2017]

1969

МК-61/52

Modula-2

Translation of: C

<lang Modula-2>MODULE ModularInverse;

 FROM InOut IMPORT WriteString, WriteInt, WriteLn;

 TYPE Data = RECORD x : INTEGER;
                    y : INTEGER
             END;

 VAR c  : INTEGER;
     ab : ARRAY [1..5] OF Data;

PROCEDURE mi(VAR a, b : INTEGER): INTEGER;

 VAR t, nt, r, nr, q, tmp : INTEGER;

BEGIN

 b := ABS(b);
 IF a < 0 THEN a := b - (-a MOD b) END;
 t := 0; nt := 1; r := b; nr := a MOD b;
 WHILE (nr # 0) DO
   q := r / nr;
   tmp := nt; nt := t - q * nt; t := tmp;
   tmp := nr; nr := r - q * nr; r := tmp;
 END;
 IF (r > 1) THEN RETURN -1 END;
 IF (t < 0) THEN RETURN t + b END;
 RETURN t;

END mi;

BEGIN

 ab[1].x := 42;   ab[1].y := 2017;
 ab[2].x := 40;   ab[2].y := 1;
 ab[3].x := 52;   ab[3].y := -217;
 ab[4].x := -486; ab[4].y := 217;
 ab[5].x := 40;   ab[5].y := 2018;
 WriteLn;
 WriteString("Modular inverse");
 WriteLn;
 FOR c := 1 TO 5 DO
   WriteInt(ab[c].x, 6); WriteString(", ");
   WriteInt(ab[c].y, 6); WriteString(" = ");
   WriteInt(mi(ab[c].x, ab[c].y),6);
   WriteLn;
 END;

END ModularInverse.</lang>

Output:

Modular inverse
    42,   2017 =   1969
    40,      1 =      0
    52,   -217 =     96
  -486,    217 =    121
    40,   2018 =     -1

newLISP

<lang NewLisp> (define (modular-multiplicative-inverse a n)

   (if (< n 0)
       (setf n (abs n)))
       
   (if (< a 0)
       (setf a (- n (% (- 0 a) n))))
   
   (setf t 0)
   (setf nt 1)
   (setf r n)
   (setf nr (mod a n))
   
   (while (not (zero? nr))
       (setf q (int (div r nr)))
       (setf tmp nt)
       (setf nt (sub t (mul q nt)))
       (setf t tmp)
       (setf tmp nr)
       (setf nr (sub r (mul q nr)))
       (setf r tmp))
   
   (if (> r 1)
       (setf retvalue nil))
   
   (if (< t 0)
       (setf retvalue (add t n))
       (setf retvalue t))
   retvalue)

(println (modular-multiplicative-inverse 42 2017)) </lang>

Output:

Nim

Translation of: C

<lang nim>proc mulInv(a0, b0): int =

 var (a, b, x0) = (a0, b0, 0)
 result = 1
 if b == 1: return
 while a > 1:
   let q = a div b
   a = a mod b
   swap a, b
   result = result - q * x0
   swap x0, result
 if result < 0: result += b0

echo mulInv(42, 2017)</lang>

OCaml

Translation of: C

<lang ocaml>let mul_inv a = function 1 -> 1 | b ->

 let rec aux a b x0 x1 =
   if a <= 1 then x1 else
   if b = 0 then failwith "mul_inv" else
   aux b (a mod b) (x1 - (a / b) * x0) x0
 in
 let x = aux a b 0 1 in
 if x < 0 then x + b else x</lang>

Testing:

# mul_inv 42 2017 ;;
- : int = 1969

Translation of: Haskell

<lang ocaml>let rec gcd_ext a = function

 | 0 -> (1, 0, a)
 | b ->
     let s, t, g = gcd_ext b (a mod b) in
     (t, s - (a / b) * t, g)

let mod_inv a m =

 let mk_pos x = if x < 0 then x + m else x in
 match gcd_ext a m with
 | i, _, 1 -> mk_pos i
 | _ -> failwith "mod_inv"</lang>

Testing:

# mod_inv 42 2017 ;;
- : int = 1969

Oforth

Usage : a modulus invmod

<lang Oforth>// euclid ( a b -- u v r ) // Return r = gcd(a, b) and (u, v) / r = au + bv

euclid(a, b)

| q u u1 v v1 |

  b 0 < ifTrue: [ b neg ->b ]
  a 0 < ifTrue: [ b a neg b mod - ->a ]

  1 dup ->u ->v1
  0 dup ->v ->u1

  while(b) [
     b a b /mod ->q ->b ->a
     u1 u u1 q * - ->u1 ->u
     v1 v v1 q * - ->v1 ->v
     ]
  u v a ;

invmod(a, modulus)

  a modulus euclid 1 == ifFalse: [ drop drop null return ]
  drop dup 0 < ifTrue: [ modulus + ] ;</lang>

Output:

42 2017 invmod println
1969

PARI/GP

Pascal

<lang Pascal> // increments e step times until bal is greater than t // repeats until bal = 1 (mod = 1) and returns count // bal will not be greater than t + e

function modInv(e, t : integer) : integer;

 var
   d : integer;
   bal, count, step : integer;
 begin
   d := 0;
   if e < t then
     begin
       count := 1;
       bal := e;
       repeat
         step := ((t-bal) DIV e)+1;
         bal := bal + step * e;
         count := count + step;
         bal := bal - t;
       until bal = 1;
       d := count;
     end;
   modInv := d;
 end;</lang>

Testing:

    Writeln(modInv(42,2017));

Output:

Perl

Various CPAN modules can do this, such as: <lang perl>use bigint; say 42->bmodinv(2017);

or

use Math::ModInt qw/mod/; say mod(42, 2017)->inverse->residue;

or

use Math::Pari qw/PARI lift/; say lift PARI "Mod(1/42,2017)";

or

use Math::GMP qw/:constant/; say 42->bmodinv(2017);

or

use ntheory qw/invmod/; say invmod(42, 2017);</lang> or we can write our own: <lang perl>sub invmod {

 my($a,$n) = @_;
 my($t,$nt,$r,$nr) = (0, 1, $n, $a % $n);
 while ($nr != 0) {
   # Use this instead of int($r/$nr) to get exact unsigned integer answers
   my $quot = int( ($r - ($r % $nr)) / $nr );
   ($nt,$t) = ($t-$quot*$nt,$nt);
   ($nr,$r) = ($r-$quot*$nr,$nr);
 }
 return if $r > 1;
 $t += $n if $t < 0;
 $t;

}

say invmod(42,2017);</lang> Notes: Special cases to watch out for include (1) where the inverse doesn't exist, such as invmod(14,28474), which should return undef or raise an exception, not return a wrong value. (2) the high bit of a or n is set, e.g. invmod(11,2**63), (3) negative first arguments, e.g. invmod(-11,23). The modules and code above handle these cases, but some other language implementations for this task do not.

Perl 6

<lang perl6>sub inverse($n, :$modulo) {

   my ($c, $d, $uc, $vc, $ud, $vd) = ($n % $modulo, $modulo, 1, 0, 0, 1);
   my $q;
   while $c != 0 {
       ($q, $c, $d) = ($d div $c, $d % $c, $c);
       ($uc, $vc, $ud, $vd) = ($ud - $q*$uc, $vd - $q*$vc, $uc, $vc);
   }
   return $ud % $modulo;

}

say inverse 42, :modulo(2017)</lang>

Phix

Translation of: C

<lang Phix>function mul_inv(integer a, n)

   if n<0 then n = -n end if
   if a<0 then a = n - mod(-a,n) end if
   integer t = 0,  nt = 1,
           r = n,  nr = a;   
   while nr!=0 do
       integer q = floor(r/nr)
       {t, nt} = {nt, t-q*nt}
       {r, nr} = {nr, r-q*nr}
   end while
   if r>1 then return "a is not invertible" end if
   if t<0 then t += n end if
   return t

end function

?mul_inv(42,2017) ?mul_inv(40, 1) ?mul_inv(52, -217) /* Pari semantics for negative modulus */ ?mul_inv(-486, 217) ?mul_inv(40, 2018)</lang>

Output:

1969
0
96
121
"a is not invertible"

PHP

Algorithm Implementation <lang php><?php function invmod($a,$n){

       if ($n < 0) $n = -$n;
       if ($a < 0) $a = $n - (-$a % $n);

$t = 0; $nt = 1; $r = $n; $nr = $a % $n; while ($nr != 0) { $quot= intval($r/$nr); $tmp = $nt; $nt = $t - $quot*$nt; $t = $tmp; $tmp = $nr; $nr = $r - $quot*$nr; $r = $tmp; } if ($r > 1) return -1; if ($t < 0) $t += $n; return $t; } printf("%d\n", invmod(42, 2017)); ?></lang>

Output:

PicoLisp

Translation of: C

<lang PicoLisp>(de modinv (A B)

  (let (B0 B  X0 0  X1 1  Q 0  T1 0)
     (while (< 1 A)
        (setq
           Q (/ A B)
           T1 B
           B (% A B)
           A T1
           T1 X0
           X0 (- X1 (* Q X0))
           X1 T1 ) )
     (if (lt0 X1) (+ X1 B0) X1) ) )

(println

  (modinv 42 2017) )

(bye)</lang>

PL/I

Translation of: REXX

<lang pli>*process source attributes xref or(!);

/*--------------------------------------------------------------------
* 13.07.2015 Walter Pachl
*-------------------------------------------------------------------*/
minv: Proc Options(main);
Dcl (x,y) Bin Fixed(31);
x=42;
y=2017;
Put Edit('modular inverse of',x,' by ',y,' ---> ',modinv(x,y))
        (Skip,3(a,f(4)));
modinv: Proc(a,b) Returns(Bin Fixed(31));
Dcl (a,b,ob,ox,d,t) Bin Fixed(31);
ob=b;
ox=0;
d=1;

If b=1 Then;
Else Do;
  Do While(a>1);
    q=a/b;
    r=mod(a,b);
    a=b;
    b=r;
    t=ox;
    ox=d-q*ox;
    d=t;
    End;
  End;
If d<0 Then
  d=d+ob;
Return(d);
End;
End;</lang>

Output:

modular inverse of  42 by 2017 ---> 1969

PowerShell

<lang powershell>function invmod($a,$n){

       if ([int]$n -lt 0) {$n = -$n}
       if ([int]$a -lt 0) {$a = $n - ((-$a) % $n)}

$t = 0 $nt = 1 $r = $n $nr = $a % $n while ($nr -ne 0) { $q = [Math]::truncate($r/$nr) $tmp = $nt $nt = $t - $q*$nt $t = $tmp $tmp = $nr $nr = $r - $q*$nr $r = $tmp } if ($r -gt 1) {return -1} if ($t -lt 0) {$t += $n} return $t }

invmod 42 2017</lang>

Output:

PS> .\INVMOD.PS1
1969
PS>

PureBasic

Using brute force. <lang PureBasic>EnableExplicit Declare main() Declare.i mi(a.i, b.i)

If OpenConsole("MODULAR-INVERSE")

 main() : Input() : End

EndIf

Macro ModularInverse(a, b)

 PrintN(~"\tMODULAR-INVERSE(" + RSet(Str(a),5) + "," + 
        RSet(Str(b),5)+") = " + 
        RSet(Str(mi(a, b)),5))

EndMacro

Procedure main()

 ModularInverse(42, 2017)  ; = 1969
 ModularInverse(40, 1)     ; = 0
 ModularInverse(52, -217)  ; = 96
 ModularInverse(-486, 217) ; = 121
 ModularInverse(40, 2018)  ; = -1

EndProcedure

Procedure.i mi(a.i, b.i)

 Define x.i = 1,
        y.i = Int(Abs(b)),
        r.i = 0  
 If y = 1 : ProcedureReturn 0 : EndIf  
 While x < y
   r = (a * x) % b   
   If r = 1 Or (y + r) = 1
     Break
   EndIf
   x + 1
 Wend
 If x > y - 1 : x = -1 : EndIf  
 ProcedureReturn x

EndProcedure</lang>

Output:

        MODULAR-INVERSE(   42, 2017) =  1969
        MODULAR-INVERSE(   40,    1) =     0
        MODULAR-INVERSE(   52, -217) =    96
        MODULAR-INVERSE( -486,  217) =   121
        MODULAR-INVERSE(   40, 2018) =    -1

Python

Iteration and error-handling

Implementation of this pseudocode with this. <lang python>>>> def extended_gcd(aa, bb):

   lastremainder, remainder = abs(aa), abs(bb)
   x, lastx, y, lasty = 0, 1, 1, 0
   while remainder:
       lastremainder, (quotient, remainder) = remainder, divmod(lastremainder, remainder)
       x, lastx = lastx - quotient*x, x
       y, lasty = lasty - quotient*y, y
   return lastremainder, lastx * (-1 if aa < 0 else 1), lasty * (-1 if bb < 0 else 1)

>>> def modinv(a, m): g, x, y = extended_gcd(a, m) if g != 1: raise ValueError return x % m

>>> modinv(42, 2017) 1969 >>> </lang>

Recursion and an option type

Or, using functional composition as an alternative to iterative mutation, and wrapping the resulting value in an option type, to allow for the expression of computations which establish the absence of a modular inverse:

<lang python>from functools import (reduce) from itertools import (chain)

modInv :: Int -> Int -> Maybe Int

def modInv(a):

   return lambda m: (
       lambda ig=gcdExt(a)(m): (
           lambda i=ig[0]: (
               Just(i + m if 0 > i else i) if 1 == ig[2] else (
                   Nothing()
               )
           )
       )()
   )()

gcdExt :: Int -> Int -> (Int, Int, Int)

def gcdExt(x):

   def go(a, b):
       if 0 == b:
           return (1, 0, a)
       else:
           (q, r) = divmod(a, b)
           (s, t, g) = go(b, r)
       return (t, s - q * t, g)
   return lambda y: go(x, y)

TEST ---------------------------------------------------

Numbers between 2010 and 2015 which do yield modular inverses for 42:

main :: IO ()

def main():

   print (
       mapMaybe(
           lambda y: bindMay(modInv(42)(y))(
               lambda mInv: Just((y, mInv))
           )
       )(
           enumFromTo(2010)(2025)
       )
   )

-> [(2011, 814), (2015, 48), (2017, 1969), (2021, 1203)]

GENERIC ABSTRACTIONS ------------------------------------

enumFromTo :: Int -> Int -> [Int]

def enumFromTo(m):

   return lambda n: list(range(m, 1 + n))

bindMay (>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b

def bindMay(m):

   return lambda mf: (
       m if m.get('Nothing') else mf(m.get('Just'))
   )

Just :: a -> Maybe a

def Just(x):

   return {'type': 'Maybe', 'Nothing': False, 'Just': x}

mapMaybe :: (a -> Maybe b) -> [a] -> [b]

def mapMaybe(mf):

   return lambda xs: reduce(
       lambda a, x: maybe(a)(lambda j: a + [j])(mf(x)),
       xs,
       []
   )

maybe :: b -> (a -> b) -> Maybe a -> b

def maybe(v):

   return lambda f: lambda m: v if m.get('Nothing') else (
       f(m.get('Just'))
   )

Nothing :: Maybe a

def Nothing():

   return {'type': 'Maybe', 'Nothing': True}

MAIN ---

main()</lang>

Output:

[(2011, 814), (2015, 48), (2017, 1969), (2021, 1203)]

Racket

<lang racket> (require math) (modular-inverse 42 2017) </lang>

Output:

REXX

<lang rexx>/*REXX program calculates and displays the modular inverse of an integer X modulo Y.*/ parse arg x y . /*obtain two integers from the C.L. */ if x== | x=="," then x= 42 /*Not specified? Then use the default.*/ if y== | y=="," then y= 2017 /* " " " " " " */ say 'modular inverse of ' x " by " y ' ───► ' modInv(x,y) exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/ modInv: parse arg a,b 1 ob; z=0 /*B & OB are obtained from the 2nd arg.*/

       $=1
       if b\=1     then    do  while a>1
                           parse value  a/b  a//b  b  z       with      q  b  a  t
                           z=$ - q*z;              $=trunc(t)
                           end   /*while*/
       if $<0  then $=$+ob
       return $</lang>

output when using the default inputs of: 42 2017

modular inverse of  42  by  2017  ───►  1969

Ring

<lang ring> see "42 %! 2017 = " + multInv(42, 2017) + nl

func multInv a,b

    b0 = b  
    x0 = 0
    multInv = 1
    if b = 1 return 0 ok
    while a > 1
          q = floor(a / b)
          t = b  
          b = a % b
          a = t
          t = x0 
          x0 = multInv - q * x0
          multInv = t  
    end
    if multInv < 0 multInv = multInv + b0 ok
    return multInv

</lang> Output:

42 %! 2017 = 1969

Ruby

<lang ruby>#based on pseudo code from http://en.wikipedia.org/wiki/Extended_Euclidean_algorithm#Iterative_method_2 and from translating the python implementation. def extended_gcd(a, b)

 last_remainder, remainder = a.abs, b.abs
 x, last_x, y, last_y = 0, 1, 1, 0
 while remainder != 0
   last_remainder, (quotient, remainder) = remainder, last_remainder.divmod(remainder)
   x, last_x = last_x - quotient*x, x
   y, last_y = last_y - quotient*y, y
 end

 return last_remainder, last_x * (a < 0 ? -1 : 1)

end

def invmod(e, et)

 g, x = extended_gcd(e, et)
 if g != 1
   raise 'The maths are broken!'
 end
 x % et

end</lang>

?> invmod(42,2017)
=> 1969

Run BASIC

<lang runbasic>print multInv(42, 2017) end

function multInv(a,b) b0 = b multInv = 1 if b = 1 then goto [endFun] while a > 1 q = a / b t = b b = a mod b a = t t = x0 x0 = multInv - q * x0 multInv = int(t) wend if multInv < 0 then multInv = multInv + b0 [endFun] end function</lang>

Output:

Rust

<lang rust>fn mod_inv(a: isize, module: isize) -> isize {

 let mut mn = (module, a);
 let mut xy = (0, 1);
 
 while mn.1 != 0 {
   xy = (xy.1, xy.0 - (mn.0 / mn.1) * xy.1);
   mn = (mn.1, mn.0 % mn.1);
 }
 
 while xy.0 < 0 {
   xy.0 += module;
 }
 xy.0

}

fn main() {

 println!("{}", mod_inv(42, 2017))

}</lang>

Output:

Scala

Based on the Handbook of Applied Cryptography, Chapter 2. See http://cacr.uwaterloo.ca/hac/ . <lang scala> def gcdExt(u: Int, v: Int): (Int, Int, Int) = {

 @tailrec
 def aux(a: Int, b: Int, x: Int, y: Int, x1: Int, x2: Int, y1: Int, y2: Int): (Int, Int, Int) = {
   if(b == 0) (x, y, a) else {
     val (q, r) = (a / b, a % b)
     aux(b, r, x2 - q * x1, y2 - q * y1, x, x1, y, y1)
   }
 }
 aux(u, v, 1, 0, 0, 1, 1, 0)

}

def modInv(a: Int, m: Int): Option[Int] = {

 val (i, j, g) = gcdExt(a, m)
 if (g == 1) Option(if (i < 0) i + m else i) else Option.empty

}</lang>

Translated from C++ (on this page) <lang scala> def modInv(a: Int, m: Int, x:Int = 1, y:Int = 0) : Int = if (m == 0) x else modInv(m, a%m, y, x - y*(a/m)) </lang>

Output:

scala> modInv(2,4)
res1: Option[Int] = None

scala> modInv(42, 2017)
res2: Option[Int] = Some(1976)

Seed7

The library bigint.s7i defines the bigInteger function modInverse. It returns the modular multiplicative inverse of a modulo b when a and b are coprime (gcd(a, b) = 1). If a and b are not coprime (gcd(a, b) <> 1) the exception RANGE_ERROR is raised.

<lang seed7>const func bigInteger: modInverse (in var bigInteger: a,

   in var bigInteger: b) is func
 result
   var bigInteger: modularInverse is 0_;
 local
   var bigInteger: b_bak is 0_;
   var bigInteger: x is 0_;
   var bigInteger: y is 1_;
   var bigInteger: lastx is 1_;
   var bigInteger: lasty is 0_;
   var bigInteger: temp is 0_;
   var bigInteger: quotient is 0_;
 begin
   if b < 0_ then
     raise RANGE_ERROR;
   end if;
   if a < 0_ and b <> 0_ then
     a := a mod b;
   end if;
   b_bak := b;
   while b <> 0_ do
     temp := b;
     quotient := a div b;
     b := a rem b;
     a := temp;

     temp := x;
     x := lastx - quotient * x;
     lastx := temp;

     temp := y;
     y := lasty - quotient * y;
     lasty := temp;
   end while;
   if a = 1_ then
     modularInverse := lastx;
     if modularInverse < 0_ then
       modularInverse +:= b_bak;
     end if;
   else
     raise RANGE_ERROR;
   end if;
 end func;</lang>

Original source: [1]

Sidef

Built-in: <lang ruby>say 42.modinv(2017)</lang>

Algorithm implementation: <lang ruby>func invmod(a, n) {

 var (t, nt, r, nr) = (0, 1, n, a % n)
 while (nr != 0) {
   var quot = int((r - (r % nr)) / nr);
   (nt, t) = (t - quot*nt, nt);
   (nr, r) = (r - quot*nr, nr);
 }
 r > 1 && return()
 t < 0 && (t += n)
 t

}

say invmod(42, 2017)</lang>

Output:

Tcl

Translation of: Haskell

<lang tcl>proc gcdExt {a b} {

   if {$b == 0} {

return [list 1 0 $a]

   }
   set q [expr {$a / $b}]
   set r [expr {$a % $b}]
   lassign [gcdExt $b $r] s t g
   return [list $t [expr {$s - $q*$t}] $g]

} proc modInv {a m} {

   lassign [gcdExt $a $m] i -> g
   if {$g != 1} {

return -code error "no inverse exists of $a %! $m"

   }
   while {$i < 0} {incr i $m}
   return $i

}</lang> Demonstrating <lang tcl>puts "42 %! 2017 = [modInv 42 2017]" catch {

   puts "2 %! 4 = [modInv 2 4]"

} msg; puts $msg</lang>

Output:

42 %! 2017 = 1969
no inverse exists of 2 %! 4

tsql

<lang tsql>;WITH Iterate(N,A,B,X0,X1) AS ( SELECT 1 ,CASE WHEN @a < 0 THEN @b-(-@a % @b) ELSE @a END ,CASE WHEN @b < 0 THEN -@b ELSE @b END ,0 ,1 UNION ALL SELECT N+1 ,B ,A%B ,X1-((A/B)*X0) ,X0 FROM Iterate WHERE A != 1 AND B != 0 ), ModularInverse(Result) AS ( SELECT -1 FROM Iterate WHERE A != 1 AND B = 0 UNION ALL SELECT TOP(1) CASE WHEN X1 < 0 THEN X1+@b ELSE X1 END AS Result FROM Iterate WHERE (SELECT COUNT(*) FROM Iterate WHERE A != 1 AND B = 0) = 0 ORDER BY N DESC ) SELECT * FROM ModularInverse</lang>

uBasic/4tH

Translation of: C

<lang>Print FUNC(_MulInv(42, 2017)) End

_MulInv Param(2)

 Local(5)

 c@ = b@
 f@ = 0
 g@ = 1

 If b@ = 1 Then Return

 Do While a@ > 1
   e@ = a@ / b@
   d@ = b@
   b@ = a@ % b@
   a@ = d@

   d@ = f@
   f@ = g@ - e@ * f@
   g@ = d@
 Loop

 If g@ < 0 Then g@ = g@ + c@

Return (g@)</lang>

Translation of: Perl

<lang>Print FUNC(_mul_inv(42, 2017)) Print FUNC(_mul_inv(40, 1)) Print FUNC(_mul_inv(52, -217)) Print FUNC(_mul_inv(-486, 217)) Print FUNC(_mul_inv(40, 2018))

End

_mul_inv Param(2)

 Local(6)

 If (b@ < 0) b@ = -b@
 If (a@ < 0) a@ = b@ - (-a@ % b@)
 c@ = 0 : d@ = 1 :  e@ = b@ :  f@ = a@ % b@

 Do Until (f@ = 0)
   g@ = e@/f@
   h@ = d@ :  d@ = c@ - g@*d@ :  c@ = h@
   h@ = f@ :  f@ = e@ - g@*f@ :  e@ = h@
 Loop

 If (e@ > 1) Return (-1)  ' No inverse'
 If (c@ < 0) c@ = c@ + b@

Return (c@)</lang>

Output:

1969
0
96
121
-1

0 OK, 0:156

XPL0

<lang XPL0>code IntOut=11, Text=12; int X; def A=42, M=2017; [for X:= 2 to M-1 do

   if rem(A*X/M) = 1 then [IntOut(0, X);  exit];

Text(0, "Does not exist"); ]</lang>

Output:

zkl

Translation of: Haskell

<lang zkl>fcn gcdExt(a,b){

  if(b==0) return(1,0,a);
  q,r:=a.divr(b); s,t,g:=gcdExt(b,r); return(t,s-q*t,g);

} fcn modInv(a,m){i,_,g:=gcdExt(a,m); if(g==1) {if(i<0)i+m} else Void}</lang> divr(a,b) is [integer] (a/b,remainder)

Output:

modInv(2,4)  //-->Void
modInv(42,2017)  //-->1969