Loops

Loops repeat an action in code based on one or more conditions.

Introduction

Not to be confused with Fruit Loops, the addictive cereal that causes symptoms similar to ADHD in kids, loops in Ruby are simply blocks of code that are continually repeated until a certain condition is met.

Like me, you've probably experienced real-life loops when you were given detention in school and forced to repeatedly write the same line about not drawing small phallic shapes on your desk. Writing the same thing over and over and over is not only boring but also potentially error prone. You might have made a spelling mistake on one line and forgotten to dot an "i" on another line. It's the same with programming: the less code you have to write, the less chance you have of introducing bugs that can cause your program to crash and burn.

If you find yourself needing to repeat an action more than once in your code, you probably need loops in your life.

For all of the examples in this lesson, you should code along in irb or repl.it (an online REPL environment) to get a better feel for how they work.

Learning outcomes

By the end of this lesson, you should be able to do the following:

• Explain what a loop is and why it's useful.

• Explain what a loop loop is and how to use it.

• Explain what a while loop is and how to use it.

• Explain what a for loop is and how to use it.

• Explain what a #times loop is and how to use it.

• Explain what an until loop is and how to use it.

• Explain what #upto and #downto loops are and how to use them.

Loop

The loop loop (say what????) is Ruby's loop that just won't quit. It's an infinite loop that will keep going unless you specifically request for it to stop, using the break command. Most commonly, break is used with a condition, as illustrated in the example below.

i = 0
loop do
  puts "i is #{i}"
  i += 1
  break if i == 10
end

You won't see this loop used much in Ruby. If you find yourself using loop, know that there is probably a better loop for you out there, like one of the more specific loops below.

While loop

A while loop is similar to the loop loop except that you declare the condition that will break out of the loop up front.

i = 0
while i < 10
  puts "i is #{i}"
  i += 1
end

This is an example of using a while loop with a count. Because you declare the condition that breaks the loop up front, the intention of your code is much clearer, making this code easier to read than our loop loop above.

You can also use while loops to repeatedly ask a question of the user until they give the desired response:

puts 'Will you go to prom with me?' while gets.chomp != 'yes'

This example shows the flexibility advantage of a while loop: it will run until its break condition is met, which could be for a variable number of loops or a number of loops that is initially unknown. Who knows if your prospective prom date will say "yes" the first, fourth, or seventy-ninth time you ask? Of course, in real life, you should really just take "no" for an answer the first time.

Until loop

The until loop is the opposite of the while loop. A while loop continues for as long as the condition is true, whereas an until loop continues for as long as the condition is false. These two loops can therefore be used pretty much interchangeably. Ultimately, what your break condition is will determine which one is more readable.

As much as possible, you should avoid negating your logical expressions using ! (not). First, it can be difficult to actually notice the exclamation point in your code. Second, using negation makes the logic more difficult to reason through and therefore makes your code more difficult to understand. These situations are where until shines.

We can re-write our while loop examples using until.

i = 0
until i >= 10
  puts "i is #{i}"
  i += 1
end

You can see here that using until means that the loop will continue running until the condition i >= 10 is true.

The next example shows how you can use until to avoid the negation ! that the above while loop had to use.

puts 'Will you go to prom with me?' until gets.chomp == 'yes'

Much more readable! This code is guaranteed to get you a "yes".

Ranges

What if we know exactly how many times we want our loop to run? Ruby lets us use something called a range to define an interval. All we need to do is give Ruby the starting value, the ending value, and whether we want the range to be inclusive or exclusive.

(1..5) # inclusive range: 1, 2, 3, 4, 5
(1...5) # exclusive range: 1, 2, 3, 4

# We can make ranges of letters, too!
('a'..'d') # a, b, c, d

For loop

A for loop is used to iterate through a collection of information such as an array or range. These loops are useful if you need to do something a given number of times while also using an iterator.

for i in 0..5
  puts "#{i} zombies incoming!"
end

That's really all there is to it.

Times loop

If you need to run a loop for a specified number of times, then look no further than the trusty #times loop. It works by iterating through a loop a specified number of times and even throws in the bonus of accessing the number it's currently iterating through.

5.times { puts 'Hello, world!' }

We're sure you can guess what that code does. Ruby is easily readable that way!

5.times { |number| puts "Alternative fact number #{number}" }

Remember, loops will start counting from a zero index unless specified otherwise, so the first loop iteration will output Alternative fact number 0.

Upto and Downto loops

The Ruby methods #upto and #downto do exactly what you'd think they do from their names. You can use these methods to iterate from a starting number either up to or down to another number, respectively.

5.upto(10) { |num| print "#{num} " } #=> 5 6 7 8 9 10

10.downto(5) { |num| print "#{num} " } #=> 10 9 8 7 6 5

If you need to step through a series of numbers (or even letters) within a specific range, then these are the loops for you.

Assignment

1. Read the Loops & Iterators chapter of LaunchSchool's Introduction to Programming With Ruby.

2. Try re-writing the above examples using alternative loop methods to achieve the same results.

Additional resources

This section contains helpful links to other content. It isn't required, so consider it supplemental for if you need to dive deeper into something.

• If you'd like another overview of loops, read this Ruby Explained: Iteration article. (Don't worry about the #each method described here; we'll get to that in an upcoming lesson!)

• If you want yet another take, read Skork's article on loops. (Again, don't worry about the #each and #each_with_index methods here; they're coming up soon.)

Knowledge check

This section contains questions for you to check your understanding of this lesson. If you're having trouble answering the questions below on your own, review the material above to find the answer.

• What is a loop and why it is useful?

• What is a loop loop, and how would you use it?

• What is a while loop, and how would you use it?

• What is a for loop, and how would you use it?

• What is a times loop, and how would you use it?

• What is an until loop, and how would you use it?

• What are the upto and downto loops, and how would you use them?