Picking a Random Object in an NSArray

Question

Say I have an array with objects, 1, 2, 3 and 4. How would I pick a random object from this array?

Answer 1

@Darryl's answer is correct, but could use some minor tweaks:

NSUInteger randomIndex = arc4random() % theArray.count; 

Modifications:

• Using arc4random() over rand() and random() is simpler because it does not require seeding (calling srand() or srandom()).
• The modulo operator (%) makes the overall statement shorter, while also making it semantically clearer.

Answer 2

This is the simplest solution I could come up with:

id object = array.count == 0 ? nil : array[arc4random_uniform(array.count)]; 

It's necessary to check count because a non-nil but empty NSArray will return 0 for count, and arc4random_uniform(0) returns 0. So without the check, you'll go out of bounds on the array.

This solution is tempting but is wrong because it will cause a crash with an empty array:

id object = array[arc4random_uniform(array.count)]; 

For reference, here's the documentation:

u_int32_t arc4random_uniform(u_int32_t upper_bound);  arc4random_uniform() will return a uniformly distributed random number less than upper_bound. 

The man page doesn't mention that arc4random_uniform returns 0 when 0 is passed as upper_bound.

Also, arc4random_uniform is defined in <stdlib.h>, but adding the #import wasn't necessary in my iOS test program.