flatMap is not removing nil objects from array

Question

I'm trying to remove the nil objects from array I'm getting when I'm generating objects:

 static func books(fromDictArray array: [[String: Any]]) -> [Books?] {

        return array.map(GettyTwo.init)
    }

Here I'm trying to only take the non nil objects:

 let inventoryBooks = [Books.books(fromDictArray: arrayOfBooks)].flatMap{$0} //this doesn't remove the nil objects

But if I do it this way:

let noNilElements = inventoryBooks.flatMap{$0}

This will remove the nil objects.

My question to you guys is why this is not working:

let inventoryBooks = [Books.books(fromDictArray: arrayOfBooks)].flatMap{$0}

What I'm doing wrong?

You're wrapping Books.books(fromDictArray: arrayOfBooks) in an array literal. You probably didn't mean to do that ;) — Hamish
– Hamish, Commented Apr 22, 2017 at 23:05
Square brackets are used to make an array literal – so when you say [Books.books(fromDictArray: arrayOfBooks)], you get a [[Books?]], not a [Books?]. flatMap(_:) can flatten nested arrays, so that's what it does – giving you back a [Books?]. Just get rid of the square brackets. — Hamish
– Hamish, Commented Apr 22, 2017 at 23:12

McCygnus · Accepted Answer · 2017-04-22 23:17:18Z

2

You shouldn't have it in the []. Instead try:

let inventoryBooks = Books.books(fromDictArray: arrayOfBooks).flatMap{$0}

Having it is the [] is making an array with one item in it, the result of the fromDictArray call. So it's structured like [[1, 2, 3]] instead of [1, 2, 3]. So when you call flatmap it checks that one item, which that isn't nil, and returns it.

edited Apr 22, 2017 at 23:17

answered Apr 22, 2017 at 23:14

McCygnus

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3 Comments

Hamish Over a year ago

You forgot to apply your own suggestion to the code ;) And it's the flattening version of flatMap(_:) that is called with the [[Books?]], not the optional unwrapping one – so you get back a [Books?].

McCygnus Over a year ago

What do you mean? It looks right to me now. ;) Went too quick with the copy and paste. Thanks.

Hamish Over a year ago

I mean saying "So when you call flatmap it checks that one item, which that isn't nil, and returns it." isn't correct, as it's the flattening version of flatMap(_:) being called, so there's no nil check going on – it's simply getting rid of one nesting layer, returning a [Books?]. If it were the optional unwrapping version of flatMap(_:) being called, you'd get back a [[Books?]].

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