Can rand() really be this bad? [closed]

Question

Above is an example image generated by using rand() to get random coordinates and add a constant to the pixel value at those coordinates. This is how it looks from a few thousand iterations. I'm using the rand() from stdlib.h in Mac OS X Lion, giving it time(NULL) as the seed.

You can clearly see the vertical lines, as if the ones with an odd x-coordinate have higher values than those with even x-coordinates.

How would I implement a better algorithm, or where can I find one that doesn't have many dependencies? (I would prefer a header-only file).

Here's the code (sorry it took me so long):

void generate(int iterations = 1) {
 for (unsigned int x = 0;x < (area * 4);++x) {
  map[rand() % area] += 1;
 }
 number a = min();
 number b = max();
 for (int i = 0;i < area;++i) {
  map[i] -= a;
  map[i] /= b;
 }
}

Map contains double-floats and is later turned into RGB values.

Answer 1

C++11 provides pretty useful random number functionality in random. See here and here for more details.

Answer 2

Have you tried arc4random()? It provides much stronger and more uniform random values than rand().

You could also try SecRandomCopyBytes(), which is part of Security.framework. This basically just reads from /dev/random.

Answer 3

I think it would be most important to see how you use rand() to extract the coordinates. What is really surprising is, that you only see a pattern in the x coordinates, but not in the y ones. If there really was a deficiency in rand() it should appear in both.

That said, I could attempt a guess where these patterns come from: rand() is well known to produce more randomness in the high bits than in the low ones. Hence one should never use modulo to extract a smaller range, because this way one will only get the low bit parts with less randomness.

From this knowledge I would guess that you were extracting the low bits to produce the x values and the high bits to extract the y values. This would give you a much more random pattern along the y axis than along the x axis.

If this is now what you are doing, there still must be some artifact in your code, which causes larger randomness across the y axis than along the x axis. So without seeing your code it is hard to tell if this implementation is flawed.

Answer 4

On the mac you can just use random instead of rand. The man page for random notes the following:

BUGS

About 2/3 the speed of rand(3).

The historical implementation used to have a very weak seeding; the random sequence did not vary much with the seed. The current implementation employs a better pseudo-random number generator for the initial state calculation.

Applications requiring cryptographic quality randomness should use arc4random(3).