Question

I am building a program which will multiply by 2, and reach long accurate numbers.
Here I build a program in which each digit is stored in a different variable.
When I compile the program, and I type 2^63, It gives me the correct answer.
But when I type 2^64, I get a “Segmentation fault”.

What is it? What can I do?

#include <stdio.h>
int main(void)
{
  printf("2^n, enter n: ");
  int n, array = 1;

  scanf("%d", &n);
  int num[array];

  num[0] = 2;
  int remainder = 0;
  if (n > 1) {
    int counter = 0;
    int counter3 = 1;

    for (counter3 = 1; counter3 < n; counter3++) {
      remainder = 0;

      for (counter = 0; counter < array; counter++) {
        num[counter] *= 2;
        num[counter] += remainder;
        remainder = 0;

        if (num[counter] / 10 != 0) {
          remainder = num[counter] / 10;
          num[counter] -= remainder * 10;
          if (counter+1 >= array) {
            array++;
            num[counter+1] = remainder;
            remainder = 0;
            break;
          }
        }
      }
    }
  }

  int counter2;
  for (counter2 = array - 1; counter2 >= 0; counter2--)
    printf("%d", num[counter2]);

  putchar('\n');
  return 0;
}

Answer 1

I think your problem is

int num[array];

This is a static array, once its size is declared to be size of 1 (array=1 only a few lines above) then thats the size of the array.

Big number libraries are very complicated and best left up to a 3rd party library.

Try taking a look at GMP

If you must do it yourself, try using a dynamic array class like std::vector