rand() function in C is not random even when seeded

Question

This is most likely a machine dependent issue but I can\'t figure out what could be wrong.

#include 
#include 
#include 

        

Answer 1

1. rand() is quite bad, avoid it if possible. In any good RNG the first values will be indistinguishable from random even when the seed is close (hamming distance). In rand this is not the case.
2. If you must use rand then seed it, preferably with something higher entropy than time, and call rand() multiple times instead of reseeding-calling-reseeding.

For example of 2, consider:

#include 
#include 
#include 

int main(int argc, char** argv) {
  int t=time(NULL);
  srand(t);

  for(int i=0; i < 10; i++) {
    float r = (float)rand()/(float)(RAND_MAX);
    printf("%f\n", r);
  }
}

With the result:

0.460600
0.310486
0.339473
0.519799
0.258825
0.072276
0.749423
0.552250
0.665374
0.939103

It's still a bad RNG but at least the range is better when you allow it to use the internal state instead of giving it another similar seed.