iPhone: random() function gives me the same random

Question

I am using function random()%x for the generation of a random number, but every time I start the application I see that it creates or generates the same number.

Like I am placing some images randomly based on that random number and I see all the images are placed at the same place no matter how many times I run the application.

Answer 1

arc4random will be better solution than rand() or random(). See this.

Answer 2

Do use srandom (or the equivalent for your random number function of choice), but also use conditionals around it, so that if you are debugging, things always happen the same way. I also tend to put NSLog warnings when doing things like that, so I don't ship brian-dead code.

#if DEBUG==0
srandom(time(NULL));
#else
NSLog(@"Debug build: Random numbers are not random!");
#endif

or

if(!debuggingBuild)
    srandom(time(NULL));
else
    NSLog(@"Debug build: Random numbers are not random!");

Answer 3

You'll likely have better luck with arc4random(), you don't need to explicitly seed it and it seems to be a "better" random.

Answer 4

Obligatory XKCD comic:

Answer 5

Call srandomdev() first.

srandomdev();
long my_rand = random();

Answer 6

In your application delegate:

- (void) applicationDidFinishLaunching:(UIApplication *)application 
{
    srandom(time(NULL));

    // ...

    for (int i = 0; i < 100; i++) {
      NSLog(@"%d", random());
    }
}

The reason this works is because pseudorandom number generators require a starting, or seed value. By using the time, you are more likely to get different sequences of "random" numbers upon each execution.

If you do not specify a seed value, the same seed is used on each execution, which yields the same sequence. This is usually undesired behavior, but in some cases it is useful to be able to generate the same sequence, for example, for testing algorithms.

In most cases, you will want to specify a seed value that will change between runs, which is where the current time comes in handy.