Issue in understanding compactMap & flatMap

Question

I have gone through from multiple tutorials that flatMap/compactMap is used to flatten an array of array but in my case it's not working or I am not understanding it properly.

let myArray = [["Raja","Kumar", nil,"Waqas"],["UAE","SINGAPORE","dUBAI","HONGKONG"]]
let final = myArray.compactMap{ $0 }

print("Result:\(final)")

OutPut:

Result:[[Optional("Raja"), Optional("Kumar"), nil, Optional("Waqas")], [Optional("UAE"), Optional("SINGAPORE"), Optional("dUBAI"), Optional("HONGKONG")]]

I tried removing nil from the above array but still it's not flattening my array.

Any help would be much appreciated.

Here in short: `compactMap` filters all `nil` values from an array, `flatMap` turns an array of arrays into a single array by pulling all values from the nested arrays into a single one. — LinusGeffarth, Apr 03 '19 at 11:19

score 5 · Answer 1 · edited Jun 20 '20 at 09:12

`.compactMap`

...is used to produce a list without optional objects, you need to use compactMap on the inner array where you got nils, like so:

let result = myArray.map { $0.compactMap { $0 } }

Result: [["Raja", "Kumar", "Waqas"], ["UAE", "SINGAPORE", "dUBAI", "HONGKONG"]]

`.flatmap`

...is used to flatten a collection of collections, for example

let result = myArray.flatMap { $0.compactMap { $0 } }

Result: ["Raja", "Kumar", "Waqas", "UAE", "SINGAPORE", "dUBAI", "HONGKONG"]

score 5 · Accepted Answer · answered Apr 03 '19 at 11:11

5

compactMap should be used to filter out nil elements from an array of Optionals, while flatMap can be used to flatten out a multi-dimensional array. However, you need to do both.

let final = myArray.flatMap{$0.compactMap{$0}}

print("Result:\(final)")

answered Apr 03 '19 at 11:11

Dávid Pásztor

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score 3 · Answer 3 · answered Apr 03 '19 at 11:22

Please read the documentation

map(_:)

Returns an array containing the results of mapping the given closure over the sequence’s elements.

compactMap(_:)

Returns an array containing the non-nil results of calling the given transformation with each element of this sequence.

flatMap(_:)

Returns an array containing the concatenated results of calling the given transformation with each element of this sequence.

Each Discussion section contains an example.

Issue in understanding compactMap & flatMap

3 Answers3

.compactMap

.flatmap

`.compactMap`

`.flatmap`