How to combine two Dictionary instances in Swift?

Question

How do I append one Dictionary to another Dictionary using Swift?

I am using the AlamoFire library to send JSON content to a REST server.

Dictionary 1

var dict1: [String: AnyObject] = [
    kFacebook: [
        kToken: token
    ]
]

Dictionary 2

var dict2: [String: AnyObject] = [
    kRequest: [
        kTargetUserId: userId
    ]
]

How do I combine the two dictionaries to make a new dictionary as shown below?

let parameters: [String: AnyObject] = [
    kFacebook: [
        kToken: token
    ],
    kRequest: [
        kTargetUserId: userId
    ]
]

I have tried dict1 += dict2, but I got a compile error:

Binary operator '+=' cannot be applied to two '[String : AnyObject]' operands

Answer 1

I love this approach:

dicFrom.forEach { (key, value) in dicTo[key] = value }

Swift 4 and 5

With Swift 4 Apple introduces a better approach to merge two dictionaries:

let dictionary = ["a": 1, "b": 2]
let newKeyValues = ["a": 3, "b": 4]

let keepingCurrent = dictionary.merging(newKeyValues) { (current, _) in current }
// ["b": 2, "a": 1]

let replacingCurrent = dictionary.merging(newKeyValues) { (_, new) in new }
// ["b": 4, "a": 3]

You have 2 options here (as with most functions operating on containers):

• merge mutates an existing dictionary
• merging returns a new dictionary

Answer 2

var d1 = ["a": "b"]
var d2 = ["c": "e"]

extension Dictionary {
    mutating func merge(dict: [Key: Value]){
        for (k, v) in dict {
            updateValue(v, forKey: k)
        }
    }
}

d1.merge(d2)

Refer to the awesome Dollar & Cent project https://github.com/ankurp/Cent/blob/master/Sources/Dictionary.swift

Answer 3

For Swift >= 2.2:
let parameters = dict1.reduce(dict2) { r, e in var r = r; r[e.0] = e.1; return r }

For Swift < 2.2:
let parameters = dict1.reduce(dict2) { (var r, e) in r[e.0] = e.1; return r }

Swift 4 has a new function: let parameters = dict1.reduce(into: dict2) { (r, e) in r[e.0] = e.1 }

It's really important to dig around the standard library: map, reduce, dropFirst, forEach etc. are staples of terse code. The functional bits are fun!