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Why does Javascript Math.pow () seem to return a rounded value compared to the same in C?

I understand that both C doubleand Javascript numbers are 64-bit IEEE 754 floating-point numbers. I am trying to port some code from C to Nodejs, and I am facing what seems to be a rounding error in Javascript Math.pow().

Here is my C program:

#include "stdio.h"
#include "math.h"

int main(int argc, char *argv[]) {
  double shared = 729;
  double exponent = 15;
  double prime = 1500450271;
  double power = pow(shared, exponent);
  double mod = fmod(power, prime);
  int intResult = (int) mod;
  printf("shared: %lf, exponent: %lf, prime: %lf, pow: %lf, mod: %lf, result: %i\n",
      shared, exponent, prime, power, mod, intResult); 
}

The output of which:

shared: 729.000000, exponent: 15.000000, prime: 1500450271.000000, pow: 8727963568087711970669458465954849888403456.000000, mod: 1488486878.000000, result: 1488486878

I wonder when I try to perform this calculation in Javascript (tested in node, Chrome, Firefox)

I get the following result:

> Math.pow( 729, 15)
8.727963568087713e+42
> Math.pow( 729, 15 ) % 1500450271;
93655693

Also, when I save the power result from C in a Javascript number:

> 8727963568087711970669458465954849888403456.0
8.727963568087712e+42

Note the difference 8.727963568087713e+42 !== 8.727963568087712e+42-> so it’s clear that the modulo operation does not give the same result.

I looked at the source for the v8 implementation of Math.pow, but the masm macros look like Greek atm to me.

.

+4
1

ECMAScript ( "JavaScript" ) pow. ECMA-262 :

x y.

C. F C99 IEC 60559 ( , IEEE 754), pow , .

, Node.js, , C, . . fmod .

:. - , , 80- x87. :

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

int main(int argc, char *argv[]) {
    int base = atoi(argv[1]);
    int exp  = atoi(argv[2]);
    printf("pow: %.15e\n", pow(base, exp));
    return 0;
}

gcc :

$ gcc -Wall -O2 pow.c -lm -o pow
$ ./pow 729 15
pow: 8.727963568087712e+42

-mpc64:

$ gcc -mpc64 -Wall -O2 pow.c -lm -o pow
$ ./pow 729 15
pow: 8.727963568087713e+42
+1

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