when i was writing my first solution i used ceil and log functions for calculating how many computers get copies with available cables (without using the same number of cables as one copy before)... so i calculated "int init = (int)ceil(log((double)k) / log(2.0));" and used that in my program but i got WA#6... then i wrote my own function that does exactly the same:
int getPow(int x)
{
    int c = 0;
    int n = 1;
    while(n < x){
        c++;
        n *= 2;
    }
    return c;
}

and then i got AC... can someone explain in what case the ceil and log functions would give wrong results.

THANK YOU

I also didn't got that, but this is fact: I changed my ceils and logs to self-written function and got AC. It is still challenge for me to explain, why

I've do the same....
Maybe the problem is due to the compiler...

Same here....and I compared my log2(x)) to (int)(log((double)x) / log(2.0)) for all x from 1 to 0x7FFFFFFF on my system and they're all identical.

You needn't use function log() or something else,You can use a const array like (2,4,8,16,32,64,128,256,512,1024....).