NSMutableArray vs NSArray which is better

Question

This is a bit of a silly question, but if I want to add an object to an array I can do it with both NSMutableArray and NSArray, which should I use?

Answer 1

This test gives the best answer:

Method 1:

NSTimeInterval start = [NSDate timeIntervalSinceReferenceDate];
NSMutableArray *mutableItems = [[NSMutableArray alloc] initWithCapacity:1000];
for (int i = 0; i < 10000; i++) {
    [mutableItems addObject:[NSDate date]];
}
NSTimeInterval  end = [NSDate timeIntervalSinceReferenceDate];
NSLog(@"elapsed time = %g", (end - start) * 1000.0);

Method 2:

...
NSArray *items = [[[NSArray alloc] init] autorelease];
or (int i = 0; i < 10000; i++) {
    items = [items arrayByAddingObject:[NSDate date]];
}
...

Output:

Method 1: elapsed time = 0.011135 seconds.    
Method 2: elapsed time = 9.712520 seconds.