iOS -- mapping a method to an array

Is there a more concise way to do the following in iOS?

Suppose we have:

NSArray *existingArray;

and also our object self has a method someMethod which can take the objects in existingArray as input and returns objects of class Foo. We could create a new array *derivedArray by applying someMethod to the objects in existingArray as 开发者_如何学编程follows:

NSMutableArray *derivedArray = [[NSMutableArray alloc]init];
for (id obj in existingArray) {
  [derivedArray addObject: [self someMethod: obj]];
}

My question is, is there a more concise way to do that? Something like:

NSArray *derivedArray = [existingArray arrayByMappingMethod: someMethod withObject: self];  

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No. There is a makeObjectsPerformSelector: method (and a few other related ones), but none allow you to get a result from the calls. You could, of course, add your method as a category on NSArray, but Apple has not provided a method to do this.

In terms of performance improvements, change

[[NSMutableArray alloc] init];

to

[[NSMutableArray alloc] initWithCapacity:[existingArray count]];

This will allow the array to allocate memory for all of the objects at once, instead of reallocating every time you add an object.

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Inspire48's comment's probably the best idea. If you're targeting iOS 4+, Mike Ash wrote a number of collection utilities that are available on GitHub. It includes a map method.

It requires iOS 4 because it leverage blocks.

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Have a look at -[NSArray filteredArrayUsingPredicate:].

Edit: I may have assumed too much about what your -someMethod: method does. I guessed that it tests obj and returns either obj or nil, but just realized that it could easily return something completely different. Another possible answer, then, is -[NSArray makeObjectsPerformSelector:], but that would require that obj's class contain -someMethod rather than the object doing this filtering.

Edit 2: One more possibility is -valueForKey:, which when applied to an array will return an array of the results from sending -valueForKey: to the objects in the array. Again, though, the objects have to respond to -valueForKey: for the given key.