a pretty flat map

Higher order FUNctions in Swift

map vs. flatMap

Did you ever wonder about the difference between map & flatMap? Well, in this post I want to show you some use cases on when to call which function. It’s actually pretty easy to understand. 😊

The example

In my example we declared the following two types:

Nothing new so far and pretty lame code. Now let’s continue with the fun part:

Challenge #01

Retrieve an array containing all firstNames from the people array.

This one is actually pretty easy. We just have to call map on people and return the firstName property like so:

let firstNames = people.map { $0.firstName }

The result is an array of Strings —how cool is that?! :)

Challenge #02

Retrieve an array containing all names from the products array of every person in people.

Let’s try our luck by using map again. First we would try to get an array containing all products. Maybe we try something like this:

let allProducts = people.map { $0.products }

Now we have a little problem: allProducts is an array of arrays of products. This makes totally sense as we map over each persons products array and append that array to our resulting array allProducts.
To join those arrays we could call joined() on allProducts and use map one last time to get an array of all product names:

let allProducts = people.map { $0.products }
let joinedProducts = allProducts.joined()
let productNames = joinedProducts.map { $0.name }

This works as expected but is not so nice to read. The fact that we have to manually create 3 arrays looks very bad.

flatMap

Of course there is a much better solution just waiting to be used: flatMap looks very similar too map. But there is one crucial difference.

map transforms an array and appends it to a resulting array. This works fine if the resulting array isn’t another array by itself.

flatMap transforms an array and appends it to a resulting array just like map. But after that it joins all arrays in the resulting arrays.

Seems like this is the perfect fit in our case, so let’s try it out:

Very cool, hm? flatMap returns an array containing all products. On this array we then call map to get the name of each product.

I hope all those arrays didn’t confuse you too much and I think it’s the best to write the example by yourself in a Playground and experiment a little bit with it.

Good luck & happy coding 🎄

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Yasmin, thank you very much for correcting my grammar once more 😇
Bob, thank you for mentioning me in your recent posts ✌🏻