How can I reverse a NSArray in Objective-C?

I need to reverse my NSArray.

As an example:

[1,2,3,4,5] must become: [5,4,3,2,1]

What is the best way to achieve this?

There is a much easier solution, if you take advantage of the built-in reverseObjectEnumerator method on NSArray, and the allObjects method of NSEnumerator:

NSArray* reversedArray = [[startArray reverseObjectEnumerator] allObjects];

allObjects is documented as returning an array with the objects that have not yet been traversed with nextObject, in order:

This array contains all the remaining objects of the enumerator in enumerated order.

For obtaining a reversed copy of an array, look at danielpunkass' solution using reverseObjectEnumerator.

For reversing a mutable array, you can add the following category to your code:

@implementation NSMutableArray (Reverse)

- (void)reverse {
    if ([self count] <= 1)
        return;
    NSUInteger i = 0;
    NSUInteger j = [self count] - 1;
    while (i < j) {
        [self exchangeObjectAtIndex:i
                  withObjectAtIndex:j];

        i++;
        j--;
    }
}

@end

Some benchmarks

1. reverseObjectEnumerator allObjects

This is the fastest method:

NSArray *anArray = @[@"aa", @"ab", @"ac", @"ad", @"ae", @"af", @"ag",
        @"ah", @"ai", @"aj", @"ak", @"al", @"am", @"an", @"ao", @"ap", @"aq", @"ar", @"as", @"at",
        @"au", @"av", @"aw", @"ax", @"ay", @"az", @"ba", @"bb", @"bc", @"bd", @"bf", @"bg", @"bh",
        @"bi", @"bj", @"bk", @"bl", @"bm", @"bn", @"bo", @"bp", @"bq", @"br", @"bs", @"bt", @"bu",
        @"bv", @"bw", @"bx", @"by", @"bz", @"ca", @"cb", @"cc", @"cd", @"ce", @"cf", @"cg", @"ch",
        @"ci", @"cj", @"ck", @"cl", @"cm", @"cn", @"co", @"cp", @"cq", @"cr", @"cs", @"ct", @"cu",
        @"cv", @"cw", @"cx", @"cy", @"cz"];

NSDate *methodStart = [NSDate date];

NSArray *reversed = [[anArray reverseObjectEnumerator] allObjects];

NSDate *methodFinish = [NSDate date];
NSTimeInterval executionTime = [methodFinish timeIntervalSinceDate:methodStart];
NSLog(@"executionTime = %f", executionTime);

Result: executionTime = 0.000026

2. Iterating over an reverseObjectEnumerator

This is between 1.5x and 2.5x slower:

NSDate *methodStart = [NSDate date];
NSMutableArray *array = [NSMutableArray arrayWithCapacity:[anArray count]];
NSEnumerator *enumerator = [anArray reverseObjectEnumerator];
for (id element in enumerator) {
    [array addObject:element];
}
NSDate *methodFinish = [NSDate date];
NSTimeInterval executionTime = [methodFinish timeIntervalSinceDate:methodStart];
NSLog(@"executionTime = %f", executionTime);

Result: executionTime = 0.000071

3. sortedArrayUsingComparator

This is between 30x and 40x slower (no surprises here):

NSDate *methodStart = [NSDate date];
NSArray *reversed = [anArray sortedArrayUsingComparator: ^(id obj1, id obj2) {
    return [anArray indexOfObject:obj1] < [anArray indexOfObject:obj2] ? NSOrderedDescending : NSOrderedAscending;
}];

NSDate *methodFinish = [NSDate date];
NSTimeInterval executionTime = [methodFinish timeIntervalSinceDate:methodStart];
NSLog(@"executionTime = %f", executionTime);

Result: executionTime = 0.001100

So [[anArray reverseObjectEnumerator] allObjects] is the clear winner when it comes to speed and ease.

DasBoot has the right approach, but there are a few mistakes in his code. Here's a completely generic code snippet that will reverse any NSMutableArray in place:

/* Algorithm: swap the object N elements from the top with the object N 
 * elements from the bottom. Integer division will wrap down, leaving 
 * the middle element untouched if count is odd.
 */
for(int i = 0; i < [array count] / 2; i++) {
    int j = [array count] - i - 1;

    [array exchangeObjectAtIndex:i withObjectAtIndex:j];
}

You can wrap that in a C function, or for bonus points, use categories to add it to NSMutableArray. (In that case, 'array' would become 'self'.) You can also optimize it by assigning [array count] to a variable before the loop and using that variable, if you desire.

If you only have a regular NSArray, there's no way to reverse it in place, because NSArrays cannot be modified. But you can make a reversed copy:

NSMutableArray * copy = [NSMutableArray arrayWithCapacity:[array count]];

for(int i = 0; i < [array count]; i++) {
    [copy addObject:[array objectAtIndex:[array count] - i - 1]];
}

Or use this little trick to do it in one line:

NSArray * copy = [[array reverseObjectEnumerator] allObjects];

If you just want to loop over an array backwards, you can use a for/in loop with [array reverseObjectEnumerator], but it's likely a bit more efficient to use -enumerateObjectsWithOptions:usingBlock::

[array enumerateObjectsWithOptions:NSEnumerationReverse
                        usingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
    // This is your loop body. Use the object in obj here. 
    // If you need the index, it's in idx.
    // (This is the best feature of this method, IMHO.)
    // Instead of using 'continue', use 'return'.
    // Instead of using 'break', set '*stop = YES' and then 'return'.
    // Making the surrounding method/block return is tricky and probably
    // requires a '__block' variable.
    // (This is the worst feature of this method, IMHO.)
}];

(Note: Substantially updated in 2014 with five more years of Foundation experience, a new Objective-C feature or two, and a couple tips from the comments.)

After reviewing the other's answers above and finding Matt Gallagher's discussion here

I propose this:

NSMutableArray * reverseArray = [NSMutableArray arrayWithCapacity:[myArray count]]; 

for (id element in [myArray reverseObjectEnumerator]) {
    [reverseArray addObject:element];
}

As Matt observes:

In the above case, you may wonder if -[NSArray reverseObjectEnumerator] would be run on every iteration of the loop — potentially slowing down the code. <...>

Shortly thereafter, he answers thus:

<...> The "collection" expression is only evaluated once, when the for loop begins. This is the best case, since you can safely put an expensive function in the "collection" expression without impacting upon the per-iteration performance of the loop.