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 https://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?

`rand`

is a standard C function.`random`

is defined as part of the POSIX standard.`arc4random`

is provided on BSD and derived platforms.

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.