avoid force unwrapping inside a filter closure in Swift 4

I am trying to filter array of [String?] with "abc".

// keysForAutoComplete: [String?]
let tempFilteredResult = keysForAutoComplete.filter ($0?.contains("abc"))!


How can I avoid force unwrapping within the closure ? I know about if let and guard let statements, but not quite sure how to apply them here?

keysForAutoComplete

2 Answers
2

You can avoid force unwrapping by directly comparing to true:

true

let tempFilteredResult = keysForAutoComplete.filter $0?.contains("abc") == true


or by using the nil coalescing operator ??:

??

let tempFilteredResult = keysForAutoComplete.filter $0?.contains("abc") ?? false


Explanation:

$0?.contains("abc") is using optional chaining and the result is a Bool? which is nil if the element is nil or Optional(true) or Optional(false) depending on whether the String contains "abc". You can compare a Bool? to a Bool, so comparing to true will return true only when there is a String that contains "abc".

$0?.contains("abc")

Bool?

nil

nil

Optional(true)

Optional(false)

String

"abc"

Bool?

Bool

true

true

String

"abc"

The nil coalescing operator unwraps the value if it is not nil, or replaces it with the supplied default value if it is nil. Since you want a nil item to return false for the comparison, the way to safely unwrap is to use ?? false.

nil

nil

nil

false

?? false

If you'd like the result to be [String], you can use compactMap along with the trinary operator ?: to generate an array of [String]:

[String]

compactMap

?:

[String]

let tempFilteredResult = keysForAutoComplete.compactMap $0?.contains("abc") == true ? $0 : nil


Explanation:

Here, ?: is used to return the original value if it contains "abc" or nil if it does not. compactMap then eliminates the nil values and unwraps the String? to return a [String].

?:

"abc"

nil

compactMap

nil

String?

[String]

true

Bool

?? false

You can use compactMap operator to remove nil values and left your array as [String] type, so every element will be a non-optional value. After that, you can use filter on the new array. This way, you don't need to be worried about nil values, then unwrapping is no needed.

compactMap

nil

[String]

filter

let keysForAutoComplete = ["abc", "ab", nil, "bca", "abcd"]
let tempFilteredResult = keysForAutoComplete.compactMap$0.filter$0.contains("abc")


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