Getting an NSArray of a single attribute from an NSArray

Question

I am facing a very regular scenario.

I have an NSArray which has object of a custom type, say Person. The Person class has the attributes: firstName, lastName and age.

How can I get an NSArray containing only one attribute from the NSArray having Person objects?

Something like:

NSArray *people;
NSArray *firstNames = [people getArrayOfAttribute:@"firstName" andType:Person.Class]

I have a solution of writing a for loop and fill in the firstNames array but I don't want to do that.

Answer 1

NSArray will handle this for you using KVC

NSArray *people ...;
NSArray *firstName = [people valueForKey:@"firstName"];

This will give you an array of the firstName values from each entry in the array

Answer 2

Check out the filterUsingPredicate: method in NSMutableArray, basically you create a NSPredicate object that will define how the array will be filtered.

http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/Predicates/Articles/pUsing.html#//apple_ref/doc/uid/TP40001794-CJBDBHCB

This guide will give you an overview, and has a section for dealing with arrays.

Answer 3

You can also use block based enumeration:

NSArray *people;  // assumably has a bunch of people
NSMutableArray *firstNames = [NSMutableArray array];

[people enumerateObjectsUsingBlock: 
 ^(id obj, NSUInteger idx, BOOL*flag){
     // filter however you want...
     [firstNames addObject:[Person firstName]];
 }];

The benefit is it is fast and efficient if you have a bunch of people...