C Math pow(): unexpected results

Question

Consider the following code:

#include <math.h>
#include <stdio.h>
int main()
{
    printf("%f\n", pow(43,10));
}

This outputs: 21611482313284248.000000

See http://codepad.org/eSa4ASF2 for playground.

But if I run the operation with Windows Calculator (x^y function) I get this result: 21611482313284249

What's happening??

Why has been downvoted?? I understand it's duplicated (even if I've looked for duplicates before with no success...), but there's no need of downvoting it for this! — ABCplus, Jun 27 '14 at 10:07

score 4 · Accepted Answer · answered Jun 27 '14 at 09:19

In IEEE-754, double (binary-64) can represent all integers exactly up to 9007199254740992 (that is 2 power 53). After that not all integer numbers can be represented exactly in a double. Your number 21611482313284249 is greater than 9007199254740992 and cannot be represented exactly in a double.

score 3 · Answer 2 · answered Jun 27 '14 at 09:14

3

The result is so large, the significand of a double-precision float does not contain enough precision to represent every integer.

answered Jun 27 '14 at 09:14

kusma

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C Math pow(): unexpected results

2 Answers2