Dictionary Update

dict_update.swift

extension Dictionary {
    mutating func update(key: Key, initial: Value, update:Value->Value) {
        if let x = self[key] {
            self[key] = update(x)
        }
        else {
            self[key] = initial
        }
    }
}

var foo = ["a":1, "b":2, "c":3]
foo.update("b", initial: 1) { x in x + 1 }
foo.update("d", initial: 1) { x in x + 1 }
foo