GoogleJSON框架。按照（简单）说明进行安装。

Google "JSON Framework". Follow the (easy) instructions to install it.

然后去：

//let's say there's NSString *jsonString.

NSArray *userData = [jsonString JSONValue];

NSMutableArray *creditCards = [NSMutableArray array];
for (NSDictionary *user in userData) {
    [creditCards addObject:[user objectForKey:@"creditCardNumber"]];
}

你将使用NSMutableArray * creditCards充满NSString而放弃底部包含信用卡号的对象。

You'll drop out the bottom of that with NSMutableArray *creditCards full of NSString objects containing the credit card numbers.