Iteration and Each

What is iteration?

Iterating is doing something several times. In programming, iteration is used to describe taking a collection of items (like an array) and doing something to each of the items in that collection.

For example, let’s say that we have a collection of names:

names = ["Megan", "Brian", "Sal"]

What if we wanted to print out all the names in the collection; we may do something like this:

print names[0]
print names[1]
print names[2]

And that works, right?

But what are some of the problems inherent to this approach? It wasn’t too terrible to do with just three students in this array, but what if we had ten students? A hundred? A thousand? A million?

When we have a solution that works for a small number of items, but it doesn’t work for a large number of items, we say that it doesn’t scale. We want to design solution that are dynamic, meaning they can work for various inputs.

Each

The scalability issue described above can be solved by using the method each. each is a method that iterates over a collection. This means that each allows us to do something for every element of an array. An Iteration is a single pass over an element. We can use each to print all the names in our collection like this:

names = ["Megan", "Brian", "Sal"]
names.each do |name|
  puts name
end

Let’s break this down. names is our collection. It is an Array of three strings. .each is a method that we call on names.

Everything between the do and end is the Block. The Block is what runs for each element in the Array. Since we have three elements, this block will run a total of three times.

name is the Block Variable. For each iteration, this variable will contain the current element we are iterating over. So for the first iteration, name holds the value "Megan", the second time it holds the value "Brian", and the third time it holds the value "Sal".

Syntax

In general, the format for using .each looks like this.

collection.each do |block_variable|
  # Code here runs for each element
end

Single-Line Syntax

You can replace a do/end with {/}. This allows you to write each on a single line. Our example from before could also be written as:

names = ["Megan", "Brian", "Sal"]
names.each {|name| puts name }

Generally, we avoid using single-line syntax.

Exercises

Use your debugger to work through the following exercises.

• If you had an array of numbers, e.g. [1,2,3,4], how do you print out the doubles of each number? Triples?
• If you had the same array, how would you only print out the even numbers? What about the odd numbers?
• How could you create a new array which contains each number multipled by 2?
• Given an array of first and last names, e.g. [“Alice Smith”, “Bob Evans”, “Roy Rogers”], how would you print out the full names line by line?
  • How would you print out only the first name?
  • How would you print out only the last name?
  • How could you print out only the initials?
  • How can you print out the last name and how many characters are in it?
  • How can you create an integer which represents the total number of characters in all the names?