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

Written by Mattt Thompson on August 12^th, 2013

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

Objective-C

NSUInteger r = arc4random_uniform(N);

Random int between 1 and N

Objective-C

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 rand48family of functions, including drand48(3).

Objective-C

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.

Objective-C

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

How Do I Randomly Order an NSArray?

Objective-C

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+0061 — U+007A), you can do a simple conversion from a char:

Objective-C

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:

Objective-C

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?

• rand is a standard C function.
• random is defined as part of the POSIX standard.
• arc4random is provided on BSD and derived platforms.