[Solved] pow() giving wrong result

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Sorry that I won’t go through your example and your intermediary function; the issue you’re having occurs due to double being insufficient, not the long long. It is just that the number grows too large, causing it to require more and more precision towards the end, more than double can safely represent.

Here, try this really simple programme out, or just trust in the output I append to it to see what I mean:

#include <stdio.h>

int main( ){

    double a;
    long long b;

    a = 674293938766347782.0;
    b = a;

    printf( "%f\n", a );
    printf( "%lld", b );

    getchar( );
    return 0;
}

/*
    Output:
    674293938766347780.000000
    674293938766347776
*/

You see, the double may have 8 bytes, just as much as the long long has, but it is designed so that it would also be able to hold non-integral values, which makes it less precise than long long can get in some cases like this one.

I don’t know the exact specifics, but here, in MSDN it is said that its representation range is from -1.7e308 to +1.7e308 with (probably just on average) 15 digit precision.

So, if you are going to work with positive integers only, stick with your function. If you want to have an optimized version, check this one out: https://stackoverflow.com/a/101613/2736228

It makes use of the fact that, for example, while calculating x to the power 8, you can get away with 3 operations:

...
    result = x * x;             // x^2
    result = result * result;   // (x^2)^2 = x^4
    result = result * result;   // (x^4)^2 = x^8
...

Instead of dealing with 7 operations, multiplying them one by one.

solved pow() giving wrong result