Inside random() function - How is it implemented?

Question

We commonly use random() function to create random numbers in C++.

The use of void srand (unsigned int seed) would improve the results since it would generate random numbers depending on the value of seed.

Can anyone tell me how the random function is actually implemented?

Answer 1

Here is a detailed description of the algorithm used in the GNU C library's random() function.

Basically:

#include <stdio.h>

#define MAX 1000
#define seed 1

main() {
  int r[MAX];
  int i;

  r[0] = seed;
  for (i=1; i<31; i++) {
    r[i] = (16807LL * r[i-1]) % 2147483647;
    if (r[i] < 0) {
      r[i] += 2147483647;
    }
  }
  for (i=31; i<34; i++) {
    r[i] = r[i-31];
  }
  for (i=34; i<344; i++) {
    r[i] = r[i-31] + r[i-3];
  }
  for (i=344; i<MAX; i++) {
    r[i] = r[i-31] + r[i-3];
    printf("%d\n", ((unsigned int)r[i]) >> 1);
  }
}