List Comprehension

gistfile1.m

NSArray *a = @[@"Hello", @"Will This work?", @"can it work?", @"Yes!"];

// sure, this works...

NSMutableArray *b = [NSMutableArray arrayWithCapacity:[a count]];

[a enumerateObjectsUsingBlock: ^(id obj, NSUInteger idx, BOOL *stop) {
  [b insertObject: [obj uppercaseString] atIndex: idx];
}];

// but it assumes that the enumeration is always increasing
// you can't go through the array backwards (or concurrently)

[a enumerateObjectsWithOptions:NSEnumerationReverse usingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
  [b insertObject:[obj uppercaseString] atIndex:idx]; // crash!
}];

// why not just a simple for loop? It's also the fastest...

NSMutableArray * b = [NSMutableArray arrayWithCapacity:[a count]];

for (id item in a) {
  [b addObject:[item uppercaseString]];
}

// or even edit the array in-place?*

NSMutableArray *b = [NSMutableArray arrayWithArray:a];

for (__strong id item in b) { // __strong is for ARC
  item = [item uppercaseString];
}

// * only works for NSStrings, not NSNumbers. No idea why!