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今天讲Objective-C的可变数组，之前讲的NSArray是不可变的，immutable，今天讲NSMutableArray，它是可变的。

Mutability

Although the NSArray class itself is immutable, this has no bearing on any collected objects. If you add a mutable string to an immutable array, for example, like this:

NSMutableString *mutableString = [NSMutableString stringWithString:@"Hello"];NSArray *immutableArray = @[mutableString];

there’s nothing to stop you from mutating the string:

if ([immutableArray count] > 0) {id string = immutableArray[0];if ([string isKindOfClass:[NSMutableString class]]) {[string appendString:@" World!"];}}

If you need to be able to add or remove objects from an array after initial creation, you’ll need to use NSMutableArray, which adds a variety of methods to add , remove or replace one or more objects:

NSMutableArray *mutableArray = [NSMutableArray array];[mutableArray addObject:@"gamma"];[mutableArray addObject:@"alpha"];[mutableArray addObject:@"beta"];[mutableArray replaceObjectAtIndex:0 withObject:@"epsilon"];

This example creates an array that ends up with the objects @"epsilon", @"alpha", @"beta".

It’s also possible to sort a mutable array in place, without creating a secondary array:

[mutableArray sortUsingSelector:@selector(caseInsensitiveCompare:)];

In this case the contained items will be sorted into the ascending, case insensitive order of @"alpha", @"beta", @"epsilon".