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copied!<p>See the <code>NSMutableArray</code> method <a href="http://developer.apple.com/DOCUMENTATION/Cocoa/Reference/Foundation/Classes/NSMutableArray_Class/Reference/Reference.html#//apple_ref/doc/uid/20000138-BABCEEJD" rel="noreferrer"><code>sortUsingFunction:context:</code></a> </p> <p>You will need to set up a <em>compare</em> function which takes two objects (of type <code>Person</code>, since you are comparing two <code>Person</code> objects) and a <em>context</em> parameter.</p> <p>The two objects are just instances of <code>Person</code>. The third object is a string, e.g. @"birthDate".</p> <p>This function returns an <code>NSComparisonResult</code>: It returns <code>NSOrderedAscending</code> if <code>PersonA.birthDate</code> < <code>PersonB.birthDate</code>. It will return <code>NSOrderedDescending</code> if <code>PersonA.birthDate</code> > <code>PersonB.birthDate</code>. Finally, it will return <code>NSOrderedSame</code> if <code>PersonA.birthDate</code> == <code>PersonB.birthDate</code>.</p> <p>This is rough pseudocode; you will need to flesh out what it means for one date to be "less", "more" or "equal" to another date (such as comparing seconds-since-epoch etc.):</p> <pre><code>NSComparisonResult compare(Person *firstPerson, Person *secondPerson, void *context) { if ([firstPerson birthDate] < [secondPerson birthDate]) return NSOrderedAscending; else if ([firstPerson birthDate] > [secondPerson birthDate]) return NSOrderedDescending; else return NSOrderedSame; } </code></pre> <p>If you want something more compact, you can use ternary operators:</p> <pre><code>NSComparisonResult compare(Person *firstPerson, Person *secondPerson, void *context) { return ([firstPerson birthDate] < [secondPerson birthDate]) ? NSOrderedAscending : ([firstPerson birthDate] > [secondPerson birthDate]) ? NSOrderedDescending : NSOrderedSame; } </code></pre> <p>Inlining could perhaps speed this up a little, if you do this a lot.</p>

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