Objective-C For-In Loop Get Index

Question

Consider the following statement:

for (NSString *string in anArray) {

    NSLog(@"%@", string);
}

How can I get the index of string in anArray without using a traditional for loop and without checking the value of string with every object in anArray?

Solution

Arrays are guaranteed to iterate in object order. So:

NSUInteger index = 0;
for(NSString *string in anArray)
{
    NSLog(@"%@ is at index %d", string, index);

    index++;
}

Alternatively, use the block enumerator:

[anArray
    enumerateObjectsUsingBlock:
       ^(NSString *string, NSUInteger index, BOOL *stop)
       {
           NSLog(@"%@ is at index %d", string, index);
       }];

Comments

[Joe Binney]

- indexOfObject is an O(n) operation, so this algorithm is O(n^2). It's strictly worse than incrementing an index on each iteration or using - enumerateObjectsUsingBlock:, both of which are still O(n).

[Pang]

This answer is wrong. If anArray contains duplicates, the index for the duplicated elements would be incorrect.