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Iteration: Each Vs. Collect

Objectives

1. Identify the return values of the each and collect methods.
2. Implement the each and collect methods.

Overview

This lesson will give a deeper dive on how to use the each and collect methods.

For our examples we will be building a hamburger method that takes an array of toppings as an argument.

What we want our methods to do

• Take in an array of hamburger toppings.
• Iterate through each topping, one at a time.
• Manipulate that data (do something to it).
• Return the manipulated data.

Each

The most important thing to remember about each is that it does not change the return value. It implicitly returns the original array.

toppings = ["pickles", "mushrooms", "bacon"]

def hamburger(toppings)
  toppings.each do |topping|
    puts "I love #{topping} on my burgers!"
  end
end

This method will print:

I love pickles on my burgers!
I love mushrooms on my burgers!
I love bacon on my burgers!

But the return value is still:

["pickles", "mushrooms", "bacon"]

If we want a different return value, we have to explicitly tell it to do so.

In this version of our burger method we set an empty array called my_statements, which we will then explicitly return after we finish our loop. Inside our each statement loop, we manipulate each topping by interpolating it inside a string. We then push that string into our my_statements array. After we iterate over each topping in our array, we return the new my_statements array.

You'll notice that since the each doesn't return the thing we want, we have to add an extra line at the end that returns the my_statements array.

def hamburger(toppings)
  my_statements = []
  toppings.each do |topping|
    my_statements << "I love #{topping} on my burgers!"
  end
  my_statements
end

Our new return value:

["I love pickles on my burgers!",
 "I love mushrooms on my burgers!",
 "I love bacon on my burgers!"]

However, if we do want a different return value, there is a handy method called map, also known as collect. These methods are abstractions of our each method. An abstraction is the process of taking away or removing characteristics from something in order to reduce it to a set of essential characteristics. Let's take a look at a few examples.

Map & Collect

toppings = ["pickles", "mushrooms", "bacon"]

def hamburger(toppings)
  toppings.map do |topping|
    puts "I love #{topping} on my burgers!"
  end
end

Since map and collect are the same thing, this can be expressed exactly the same way with collect, like the following.

toppings = ["pickles", "mushrooms", "bacon"]

def hamburger(toppings)
  toppings.collect do |topping|
    puts "I love #{topping} on my burgers!"
  end
end

This method will print:

I love pickles on my burgers!
I love mushrooms on my burgers!
I love bacon on my burgers!

This method will return:

[nil, nil, nil]

Why does it return nil?

If you look inside our map loop, you will see that we are using puts, which always has a nil return value. What this is telling us is that our return value is indeed being changed by map. Let's look at another example.

Here we are no longer using puts, but instead implicitly returning what is inside our block. Again showing that map will give us a new return value based on the logic inside our block.

def burger(toppings)
  toppings.collect do |topping|
    "I love #{topping} on my burgers"
  end
end

Our new return value:

["I love pickles on my burgers",
 "I love mushrooms on my burgers",
 "I love bacon on my burgers"]

Takeaway:

• If you want the transformations to be reflected in the return value use map or collect.
• If you want to return the original return value use each.

Does this need an update?

Please open a GitHub issue or pull-request. Provide a detailed description that explains the issue you have found or the change you are proposing. Then "@" mention your instructor on the issue or pull-request, and send them a link via Connect.

PHRG Iteration: Each Vs. Collect