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

## Written by Mattt Thompson — August 12th, 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`

``````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+0061``U+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.

NSMutableHipster

Questions? Corrections? Issues and pull requests are always welcome — NSHipster is made better by readers like you.

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