map()

One would think that Objective-C’s ability to pass functions around as objects in the form of selectors would make writing a map() method easy. Observe, however, the crucial differences of the NSArray equivalent to map(). (For those unfamilar with it, map(), when given an array and a method/function taking one argument, returns the result of mapping the function onto each item of the array.)

From the NSArray documentation:

makeObjectsPerformSelector:

Sends the aSelector message to each object in the array, starting with the first object and continuing through the array to the last object.

– (void)makeObjectsPerformSelector:(SEL)aSelector
Discussion

The aSelector method must not take any arguments. It shouldn’t have the side effect of modifying the receiving array.

This is different on no fewer than two levels. Firstly, even in the NSMutableArray subclass, this method is not allowed to have side effects. Frankly, I can think of few situations in which I would need to map an idempotent function onto an array; the point of map() is to be able to apply a function quickly to every element of an array and get back the changes! Secondly, an unaware or hurried programmer would think that this function was implemented so that one could write code like this:

- (void)printAnObject:(id)obj
{
NSLog([obj description]);
}

and then do this:
[anArray makeObjectsPerformSelector: @selector(printAnObject:)];

This is not the case – the above code would just make each element call printAnObject:, not call printAnObject with each element. I’m sure that to some it seems obvious, but I, for one, found this to be an insidiously tricky wart.

However, there is a (limited) workaround.

NSArray’s valueForKey: method (somewhat counter-intuitively) returns the result of invoking valueForKey on each of the array’s elements. As such, one can map KVC functions onto arrays. For example:

NSArray *arr = [NSArray arrayWithObjects: @"Get", @"CenterStage", @"0.6.2", @"because", @"it", @"rocks", nil];
[arr objectForKey: @"length"]; // returns [3, 10, 6, 7, 2, 5]

This can be used in many helpful ways; sadly, it only works on KVC-compliant properties/methods.

Anyway, moving on…

filter()

With OS X 10.4, Apple introduced the NSArray filteredArrayUsingPredicate: method, which allows one to filter an array based on criteria established by an NSPredicate. Observe:

NSArray *arr = [NSArray arrayWithObjects: @"This", @"is", @"the", @"first", @"CenterStage", @"release", @"I", @"helped", nil];
arr = [arr filteredArrayWithPredicate: [NSPredicate predicateWithFormat: @"SELF.length > 5"]]; // arr is now [@"CenterStage", "@"release", @"helped"]

Verbose, but useful.

reduce()

Frankly, Apple don’t give us any way to do this in pure Cocoa. The best way I’ve found (if you really need this, which is less often than one needs map() and filter()) is to use the F-Script framework and apply the \ operator to an array. Unfortunately, this takes a bit of overhead.

In conclusion, Cocoa and Objective-C almost bring us the joys of functional programming. We can only hope that Leopard and Obj-C 2.0 improve on these in some way.

List comprehensions, anyone?