Create an array of NSSet having values of distinct properties of an object present in another NSArray

Question

I have an array having objects with different properties. I want to create an array of sets which contain objects with same value of a single property of the object.

Suppose this is an array of object which has property a and b

1: {a:10, b:5}, 2: {a:2,b:5}, 3: {a:20,b:5}, 4: {a:5,b:5}, 5: {a:4,b:20}, 6: {a:51,b:20}

I want to create another array of NSSet of objects with distinct values of property b

so the result would be the following Array of 2 NSSet

1: {a:10, b:5}, {a:2,b:5}, {a:20,b:5}, {a:5,b:5}

2: {a:4,b:20}, {a:51,b:20}

How can this be done?

Answer 1

I'd do this by first creating a dictionary of sets where the keys of the dictionary are the unique values of "b".

Note: This is untested code. There could be typos here.

NSArray *objectArray = ... // The array of "SomeObject" with the "a" and "b" values;
NSMutableDictionary *data = [NSMutableDictionary dictionary];
for (SomeObject *object in objectArray) {
    id b = object.b;
    NSMutableSet *bSet = data[b];
    if (!bSet) {
        bSet = [NSMutableSet set];
        data[b] = bSet;
    }

    [bSet addObject:object];
}

NSArray *setArray = [data allValues];

setArray will contain your array of sets.

This codes also assumes you have a sane isEqual: and hash implementation on your SomeObject class.