Aaron Aaron -4 years ago 95
Swift Question

Clean way to filter an array of dictionaries by unique dictionary values

Say I have an object:

struct Foo {
let id: Int
let bar: Int
}


Now I have 5 of these objects in an array:

let foo1 = Foo(id: 1, bar: 1)
let foo2 = Foo(id: 2, bar: 1)
let foo3 = Foo(id: 3, bar: 2)
let foo4 = Foo(id: 4, bar: 3)
let foo5 = Foo(id: 5, bar: 3)

let fooArray = [foo1, foo2, foo3, foo4, foo5]


What would be a clean way of filtering
foo
objects that have unique
bar
values?

// Desired output
let filteredArray = [foo1, foo3, foo4]


Assume there's anywhere from a few hundred to a few thousand objects to iterate upon.

Answer Source

One possible approach is to use a Set which keeps track of which bar values have already been seen:

var seenBarValues = Set<Int>()
let filteredArray = fooArray.filter { foo in
    if seenBarValues.contains(foo.bar) {
        // We already had a `Foo` with this `bar` value: skip.
        return false 
    } else {
        // First `Foo` with this `bar` value: remember and include.
        seenBarValues.insert(foo.bar)
        return true
    }
}

As @Hamish correctly pointed out, this can be shortened to

var seenBarValues = Set<Int>()
let filteredArray = fooArray.filter { 
    seenBarValues.insert($0.bar).inserted
}

using the fact that

public mutating func insert(_ newMember: Element) -> (inserted: Bool, memberAfterInsert: Element)

returns a tuple whose first member indicates if an element equal to the newly inserted one was already present in the set.

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