NSArray Equivalent of Map

Question

Given an NSArray of NSDictionary objects (containing similar objects and keys) is it possible to write perform a map to an array of specified key? For example, in Ruby it can be done with:

array.map(&:name) 

Answer 1

It only saves a couple lines, but I use a category on NSArray. You need to ensure your block never returns nil, but other than that it's a time saver for cases where -[NSArray valueForKey:] won't work.

@interface NSArray (Map)  - (NSArray *)mapObjectsUsingBlock:(id (^)(id obj, NSUInteger idx))block;  @end  @implementation NSArray (Map)  - (NSArray *)mapObjectsUsingBlock:(id (^)(id obj, NSUInteger idx))block {     NSMutableArray *result = [NSMutableArray arrayWithCapacity:[self count]];     [self enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {         [result addObject:block(obj, idx)];     }];     return result; }  @end 

Usage is much like -[NSArray enumerateObjectsWithBlock:]:

NSArray *people = @[                      @{ @"name": @"Bob", @"city": @"Boston" },                      @{ @"name": @"Rob", @"city": @"Cambridge" },                      @{ @"name": @"Robert", @"city": @"Somerville" }                   ]; // per the original question NSArray *names = [people mapObjectsUsingBlock:^(id obj, NSUInteger idx) {     return obj[@"name"]; }]; // (Bob, Rob, Robert)  // you can do just about anything in a block NSArray *fancyNames = [people mapObjectsUsingBlock:^(id obj, NSUInteger idx) {     return [NSString stringWithFormat:@"%@ of %@", obj[@"name"], obj[@"city"]]; }]; // (Bob of Boston, Rob of Cambridge, Robert of Somerville) 

Answer 2

I've no idea what that bit of Ruby does but I think you are looking for NSArray's implementation of -valueForKey:. This sends -valueForKey: to every element of the array and returns an array of the results. If the elements in the receiving array are NSDictionaries, -valueForKey: is nearly the same as -objectForKey:. It will work as long as the key doesn't start with an @