Question

I need to use pow in my c++ program and if i call the pow() function this way:

long long test = pow(7, e);

Where

e is an integer value with the value of 23.

I always get 821077879 as a result. If i calculate it with the windows calculator i get 27368747340080916343.. Whats wrong here? ):

I tried to cast to different types but nothing helped here… What could be the reason for this? How i can use pow() correctly?

Thanks!

Answer 1

The result is doesn’t fit in long long.

If you want to deal with very big numbers then use a library like GMP

Or store it as a floating point (which won’t be as precise).

Applying modulo:

const unsigned int b = 5; // base
const unsigned int e = 27; // exponent
const unsigned int m = 7; // modulo

unsigned int r = 1; // remainder

for (int i = 0; i < e; ++i)
  r = (r * b) % m;

// r is now (pow(5,27) % 7)