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Johnny Reina

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# Intro to the map function

this post was originally published on my Github Pages site on September 13th, 2017

This might sound a bit outlandish or ridiculous, but I seldom write loops nowadays. What I have found is that just about every programming language includes a set of methods or applicable functions that can replace just about every loop that I was previously writing. These higher-order functions are called map, filter, and fold.

## Map

The `map` function takes a function you provide and iterates over each item in your list, applying the function you provide and putting the result in the same spot in the new array. Since `map` takes an array and returns an array, you can chain together your `map` calls and transform your data incrementally.

### Baby steps

We'll start off with some easy examples:

That didn't hurt too much, did it? In the first `map`, we essentially applied `f(x) = 2x` to each element in the sequence. Likewise, the second `map` applied `f(x) = xÂ²` to each element. Easy cheesy.

### Learning to crawl

Here's an example of mapping over an array of objects:

In the first `map` we are pulling out each person's name, their age in the second `map`, and their second hobby in the third `map`. If what is happening here isn't immediately apparent to you, here is the same thing in an imperative style:

Crazy, right? What we can do with a single expression with `map`, takes FOUR lines with imperative code. Oh, and did you notice the fact that we did not mutate the value of the original array in the imperative example? This is also true for `map`, which is important since we might need to do other things to that original array.

### Up and running!

Alright, "this is child's play", you say. Where are we REALLY gonna use `map`? Well, buckle up! Check out this real world example:

This example is straight outta polysvg, albeit cut down a bit for brevity. This `map` chain takes an array of six zeroes and performs the following steps:
1. Numbers each spot according to its index
2. Multiplies each position by 60 to get the angles of each vertex from the centroid of a hexagon
3. Converts each angle to radians
4. Converts from polar to Cartesian coordinates
5. Rounds these nasty floats
6. Applies an offset to each point equal to the radius

Without annotations, this is about nineteen lines of code.. Most importantly, it works and you can try it out yourself.

### When should I use `map`?

Since `map` has a 1:1 relationship between the number of things you put in and the number of things you get out, you should use `map` when you want to transform `x` amount of things into `x` amount of other things. If you need to turn `x` amount of things into `x - 5` amount of things, `map` might not be the ideal solution right away. You may need to segregate your things into subsets, and then `map` each subset separately.

### JavaScript is the worst! What other languages have `map`?

Like, all the good ones. Though the names might be a bit different. In an effort to avoid plagiarism and only write what I really know about, I'll list out a few equivalent methods/functions here.

Language Function/Method
JavaScript Array.prototype.map
C# IEnumerable.Select<T> (as part of System.Linq)
Python map
PHP array_map
MongoDB \$project (as part of an aggregation pipeline)

### Alright, I'm convinced. When do I start?

Right now! Go! `map` all the things!

The best way to get familiar with `map` is to just start using it.

Angelo Dias

Will you talk about Filter and Fold too?

Johnny Reina

Not sure if you caught it but filter and fold are published now!

Johnny Reina

Yes. I should have filter and fold out within the next few days.

Jason Walzak

Shouldn't it be person.hobbies[0]?

Map is really cool though. This is really helping with my understanding.

Johnny Reina

It depends on what you are after. In the given example, I wanted to pull out the second item in the hobbies array from each person.

I'm glad I could help. When I first bumped into map it freaked me out quite a bit.

Scala and Java have map too. But it's overrated.