Determining if Swift dictionary contains key and obtaining any of its values

Why not simply check for dict.keys.contains(key)? Checking for dict[key] != nil will not work in cases where the value is nil. As with a dictionary [String: String?] for example.

You don't need any special code to do this, because it is what a dictionary already does. When you fetch dict[key] you know whether the dictionary contains the key, because the Optional that you get back is not nil (and it contains the value).

So, if you just want to answer the question whether the dictionary contains the key, ask:

let keyExists = dict[key] != nil

If you want the value and you know the dictionary contains the key, say:

let val = dict[key]!

But if, as usually happens, you don't know it contains the key - you want to fetch it and use it, but only if it exists - then use something like if let:

if let val = dict[key] {
    // now val is not nil and the Optional has been unwrapped, so use it
}

The accepted answer let keyExists = dict[key] != nil will not work if the Dictionary contains the key but has a value of nil.

If you want to be sure the Dictionary does not contain the key at all use this (tested in Swift 4).

if dict.keys.contains(key) {
  // contains key
} else { 
  // does not contain key
}