Each vs. Map

Well, Day 1 is officially winding down and I already feel like I’m on more solid ground with the basics of Ruby than I was when I was working alone in Tulsa.

So, with that in mind, I wanted to share a few bits of knowledge that I gathered today.

One of the big aha moments for me today was the difference in end results that you get when using Array#each versus Array#map.

Let’s take a look at a bit of code…In this particular example, I’ve created an array containing the names of my beloved cats.

array_of_cats = ["amigo", "sly", "sway", "aj", "lil bit", "opals"]

Okay, my goal is to iterate through each element of my array and capitalize the first letter… (FYI: iterate is the fancy way of saying loop) So, what to use? Well that all depends on what you want the end result to look like.

array_of_cats.each { |cat| puts "#{cat.capitalize}" }

The above code will print a list of the cats with the first letter capitalized:

Amigo
Sly
Sway
Aj
Lil bit
Opals

But what happens to the actual array_of_cats?

Well, not much really. When you run array_of_cats again, the output hasn’t changed.

["amigo", "sly", "sway", "aj", "lil bit", "opals"]

Okay, so now what? What does that mean? It printed out the words capitalized but it’s like the code didn’t stick to the actual array or something strange (at least that’s what I thought at first). Well, it turns out that .each executes the block of code but then returns the original array.

So we need something else. Take a look at the below code.

array_of_cats.map { |cat| cat.capitalize }

["Amigo", "Sly", "Sway", "Aj", "Lil bit", "Opals"]

Do you see the difference? The .map method executes the block of code and literally changes the original array. Map is used to iterate but also to map the elements to something else. Pretty cool, huh?

Is that the only solution? As is generally the case with Ruby, the answer is no. Take a look at the code below.

array_of_cats.each { |cat| cat.capitalize! }

["Amigo", "Sly", "Sway", "Aj", "Lil bit", "Opals"]

Hey! What happened there? In this case, .each actually did change the original array…What did we do different? Suprisingly, it’s the exclamation point that makes all the difference. Check it out…

cat = "meow"
capitalized_cat = cat.capitalize
puts cat #prints unchanged cat or "meow"
puts capitalized_cat #prints newly created capitalized_cat or "Meow"

kitty = "meh"
capitalized_kitty = kitty.capitalize!
puts kitty #prints "Meh"
puts capitalized_kitty #prints "Meh"

Interesting, right? The capitalized_cat is actually a copy of the string in the original cat. The original string doesn’t change. However, when you use the exclamation point, it changes the original kitty as well as creates the capitalized_kitty.

Too cool. The key is knowing your end goal and then finding the method that will get you there. I originally thought that .each and .map could be used interchangeably and while sometimes they can be…they can actually have two different end results.

Okay, that’s all I got for today. More soon!