Most of the time, when we use a for loop, it’s to iterate over some collection. Typically an array. We used to write things like this:

for (NSInteger i = 0; i < [array count]; i++) {
    id obj = array[i];
    // do something with obj
}

Then we came up with the foreach loop.

for (id obj in array) {
    // do something with obj
}

That worked well unless you wanted to know the index of the object that you’re using. We’ve all written code like this:

NSInteger i = 0;
for (id obj in array) {
    // do something with obj
    i++;
}

This one is particularly insidious since it creates a new local variable i outside of the lexical scope of the loop. Gross!

Please, stop using all of these. Instead, use enumerateObjectsUsingBlock:

[array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
    //do something with obj at idx
}];

This abstracts away how the computer is doing something and let’s you focus solely on what it is you’re trying to accomplish. You get the index of the iteration and can stop the enumeration at any time by setting *stop = YES.

We used to have to use arrays because Objective-C didn’t have blocks. We’ve left the dark ages and can now focus on what we’re trying to accomplish instead of being mired in the minutia of how the computer works.

EDIT: Several readers have pointed out the performance gains when using the different methods. Good point, and definitely worth a read.