rand(3) / random(3) / arc4random(3) / et al.

Written by Mattt Thompson on

What passes for randomness is merely a hidden chain of causality.

In a mechanical universe of material interactions expressed through mathematical equations, it is unclear whether nature encodes an element of chance, or if it's a uniquely human way to reconcile uncertainty.

We can be sure of one thing, however: in the closed, digital universe of CPU cycles, processes, and threads, there is no true randomness, only pseudorandomness.

Pseudorandomness, is often implemented in a way very similar to a cryptographic hash, as a deterministic function that returns a value based on the current time (salted, of course, by some initial seed value). Also like hash functions, there are a number of PRNG, or pseudorandom number generators, each of which are optimized for particular performance characteristics: uniformity, periodicity, and computational complexity.

Of course, for app developers, all of this is an academic exercise. And rather than bore you with any more high-minded, long-winded treatises on the philosophical nature of randomness, we're going to tackle this one FAQ-style.

Our goal this week: to clear up all of the lingering questions and misunderstandings about doing random things in Objective-C. Let's dive in!


How Do I Generate a Random Number in Objective-C?

tl;dr: Use arc4random() and its related functions.

Specifically, to generate a random number between 0 and N - 1, use arc4random_uniform(), which avoids modulo bias.

Random int between 0 and N - 1

NSUInteger r = arc4random_uniform(N);

Random int between 1 and N

NSUInteger r = arc4random_uniform(N) + 1;

Random double between 0 and 1

If you are generating a random double or float, another good option is the more obscure rand48 family of functions, including drand48(3).

srand48(time(0));
double r = drand48();

rand48 functions, unlike arc4random functions, require an initial value to be seeded before generating random numbers. This seed function, srand48(3), should only be run once.

How Do I Pick a Random Element from an NSArray?

Use arc4random_uniform(3) to generate a random number in the range of a non-empty array.

if ([array count] > 0) {
  id obj = array[arc4random_uniform([array count])];
}

How Do I Randomly Order an NSArray?

NSMutableArray *mutableArray = [NSMutableArray arrayWithArray:array];
NSUInteger count = [mutableArray count];
// See http://en.wikipedia.org/wiki/Fisher–Yates_shuffle
if (count > 1) {
  for (NSUInteger i = count - 1; i > 0; --i) {
      [mutableArray exchangeObjectAtIndex:i withObjectAtIndex:arc4random_uniform((int32_t)(i + 1))];
  }
}

NSArray *randomArray = [NSArray arrayWithArray:mutableArray];

This code is borrowed from TTTRandomizedEnumerator, which also provides randomized enumerators for NSSet, NSOrderedSet, and NSDictionary.

How Do I Generate a Random String?

If you're looking to generate "lorem ipsum"-style sentences, try constructing a Markov Chain from a corpus.

Otherwise, if you're looking to just get random letters, try one of the following methods:

Generate a Random Lowercase NSString

If you are operating on a known, contiguous range of Unicode characters, such as the lowercase letters (U+0061U+007A), you can do a simple conversion from a char:

NSString *letter = [NSString stringWithFormat:@"%c", arc4random_uniform(26) + 'a'];

Pick a Random Character From an NSString

Otherwise, a simple way to pick random letters from a set of your choosing is to simply create a string containing all of the possible letters:

NSString *vowels = @"aeiouy";
NSString *letter = [vowels substringWithRange:NSMakeRange(arc4random_uniform([vowels length]), 1)];

Why Should I Use arc4random(3) instead of rand(3) or random(3)?

C functions are typically denoted with a number 3 inside of parentheses, following the organizational convention of man pages.

  • arc4random does not require an initial seed (with srand or srandom), making it that much easier to use.
  • arc4random has a range up to 0x100000000 (4294967296), whereas rand and random top out at RAND_MAX = 0x7fffffff (2147483647).
  • rand has often been implemented in a way that regularly cycles low bits, making it more predictable.

What are rand(3), random(3), and arc4random(3), and Where Do They Come From?


If you have any additional questions about randomness on Objective-C, feel free to tweet @NSHipster. As always, corrections are welcome in the form of a pull request.