Question

I’m trying to create an NSDictionary of unique keys with each value being the count of the unique keys.

i.e.

Given the array [apple, apple, banana, apple, pear, banana] I’d get the dictionary…

{
    apple : 3,
    banana : 2,
    pear : 1
}

I’m sure I’ve seen code before where you can do something like…

NSMutableArray *uniqueKeys = [NSMutableArray array];

for (NSString *string in myArray) {
    uniqueKeys[string]++;
}

But I can’t remember what the exact syntax is.

I know I could do it long hand by extracting the NSNumber and changing it and putting it back but I wanted to use this method if possible.

Answer 1

Seems like you may be able to use NSCountedSet.
If “apple” and “apple” are equal, you’ll be able to know both how many “fruits” are in your counted-set as well as how many “apple”s are in the set.

Apple’s Documentation:

The NSCountedSet class declares the programmatic interface to a
mutable, unordered collection of indistinct objects. A counted set is
also known as a bag.

Each distinct object inserted into an NSCountedSet object has a
counter associated with it. NSCountedSetkeeps track of the number of
times objects are inserted and requires that objects be removed the
same number of times. Thus, there is only one instance of an object in
an NSSet object even if the object has been added to the set multiple
times. The count method defined by the superclass NSSet has special
significance; it returns the number of distinct objects, not the total
number of times objects are represented in the set. The NSSet and
NSMutableSet classes are provided for static and dynamic sets
(respectively) whose elements are distinct.

Update

Here is an example:

NSCountedSet * fruits = [[NSCountedSet alloc] initWithArray:@[@"apple", @"apple", @"banana", @"apple", @"pear", @"banana"]];
NSLog(@"fruit count = %d", [fruits count]);
NSLog(@"num apples = %d", [fruits countForObject:@"apple"]);
NSLog(@"num bananas = %d", [fruits countForObject:@"banana"]);
NSLog(@"num pears = %d", [fruits countForObject:@"pear"]);
NSLog(@"num oranges = %d", [fruits countForObject:@"orange"]);

Which will output:

2013-02-02 13:07:59.058 test[46614:c07] fruit count = 3
2013-02-02 13:07:59.059 test[46614:c07] num apples = 3
2013-02-02 13:07:59.062 test[46614:c07] num bananas = 2
2013-02-02 13:07:59.062 test[46614:c07] num pears = 1
2013-02-02 13:07:59.063 test[46614:c07] num oranges = 0