Iteration

Goals

Use Ruby syntax to iterate over arrays
Filter through an array, given specific criteria

Iteration

Iteration is defined as the repetition of a process. We see iteration in the physical world all the time. Let’s think about the process of grading papers:

You have a collection of papers
For each paper…
- We read through it, mark score on top, record in gradebook
Then, repeat with next paper. Continue until all papers are graded

In programming, we follow a similar cadence. Let’s think about all those promotional emails we wake up to every morning:

The app has a long list of email addresses in it’s database
For each email address…
- The app sends an email off to that address at 5:30am
Then, repeat with next email address. Continue until all email addresses have been used

Syntax for Iteration

To write code that iterates over an array, we will follow the cadence we saw in the examples above:

Start with a collection (Array)
Identify each element
Do something with/to it
Repeat until all elements have been worked with

Let’s walk through what each piece of the code below is doing:

numbers = [2, 5, 9, 5]

numbers.each do |number|
  puts number * 2
end

Here’s an example with strings:

courses_offered = ["Math", "Science", "ELA", "Theater"]

courses_offered.each do |course|
  puts "Welcome to #{course}!"
end

print "courses_offered array is unchanged: #{courses_offered}"

Using each in the way we are above, we will not mutate, or change, the original array!

Try It: Iteration

Using the each method, iterate over your array of student names and print out a personalized message for each student.

A More Advanced Method

Sometimes, we don’t want to perform the exact same action on every element in an array. Let’s think about the favoriting feature on Kahoot:

A user can “favorite” a kahoot
A user can view all kahoots
A user can click a button to view favorited kahoots only

How did the program sift through all the elements in the kahoots array and know to only show some? Again, since Ruby is such a friendly language, it gives us a method called select that does the heavy lifting for us.

Let’s take a look at the syntax:

numbers = [2, 5, 9, 5]

small_numbers = numbers.select do |number|
  number < 7
end

puts small_numbers

Try It: Select

Using the select method, iterate over your array of student names select only the names that are longer than 4 characters.

students = ["Allie", "Ruby", "Lisa", "Maile", "Kai"]

long_names = students.select do |student|
  student.length > 4
end

puts long_names

Click here for an Early Finisher Challenge!

Using the same methods and syntax as the initial challenge, write code that will create an array of all the names from your original array that start with the letter "A".

How Useful Is This?

While we’ve gotten into some really interesting and intermediate programming concepts, it still may be hard to see the utility in this. Names alone aren’t enough. As educators, you may want to quickly determine the list of names of students who won’t have their permission slip signed, or who scored below a 70 on last Friday’s quiz, etc. You probably need the name of a student who meet some other criteria. That is what we dive into in the next section!

Next: Hashes