Hash Iteration with Collect

Overview

We'll use #collect to iterate over a hash.

Objectives

1. Identify when #collect is most often used when iterating over hashes.

2. Give examples of using #collect to iterate over a hash.

Using #collect

Note: #collect is actually an alias for #map. That means the two methods can be used interchangeably, and effect the same behavior.

We use #collect to iterate over a collection of data, such as an array or a hash, and return a collection of the data therein.

We have seen it used with arrays to iterate over an array, operate on the data it contains, and return a collection with this altered data.

When working with hashes, we'll most often see #collect used to collect the values of the hash's keys and/or collect data that we've operated on over the course of an iteration.

Let's take a look at an example.

#collecting Hash Values

Let's use #collect to return all of the values of the keys in a given hash.

In this example, we are once again the managers at Chuck E. Cheese's. Chuck E. Cheese's is still a great place to have a birthday party, and, shockingly, the same three birthdays are still going on.

We will be operating on the following hash that tracks birthday kids and their associated ages:

birthday_kids = {
	"Timmy" => 9, 
	"Sarah" => 6, 
	"Amanda" => 27
}

Our managers have asked us to give them the list of ages of the birthday kids so they know how many candles to buy for the birthday cakes. Let's iterate over the birthday_kids hash and collect the ages.

birthday_kids = {
	"Timmy" => 9, 
	"Sarah" => 6, 
	"Amanda" => 27
}

birthday_kids.collect do |kids_name, age|
	age
end

# => [9, 6, 27]

Note that the return value is an array of the values we collected.

Advanced Example

You might have noticed that our previous example didn't operate on the data we were collecting. Let's step it up a level. In this example, we'll iterate over the birthday_kids hash using #collect and return the age of each child, in dog years:

birthday_kids.collect do |name, age|
	age * 7
end

In this case, we are multiplying the value of each age by 7 and collecting the return values of that multiplication into a new array. The above method call would return:

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