Control Flow: Loops

Learning Goals

• Write basic loops with the while construct
• Use the #times method to run a block of code a specified number of times

Introduction

In this lesson, we'll show how to use control flow to run the same line(s) of code multiple times in a loop. Make sure to follow along by running IRB and experimenting with the example code.

Basic Loops in Ruby

In JavaScript, there are a few common approaches for control flow that will allow us to run the same lines of code over and over again. The most basic tool is the while loop, which works like this in JavaScript:

let i = 0;
while (i < 5) {
  console.log("Looping!");
  i++;
}

Ruby has a while construct too, which works much the same way:

i = 0
while i < 5
  puts "Looping!"
  i += 1
end

Ruby also has an until loop, which acts like the opposite of a while loop, and will run a block of code until some condition is true:

counter = 0
until counter == 10
  puts "Counting!"
  counter += 1
end

Looping with #times

JavaScript has a for loop, which is commonly used to run some condition a set number of times:

for (let i = 0; i < 10; i++) {
  console.log(`Looping!`);
  console.log(`i is: ${i}`);
}

While Ruby does also have a for loop, you will see them used less often by Rubyists, since Ruby has a few other more expressive ways to run code in a loop. One such approach is to use the #times loop:

10.times do |i|
  puts "Looping!"
  puts "i is: #{i}"
end

The syntax here is quite different from JavaScript, so let's break it down.

#times is a method in the Integer class that can be called on a number. It takes a block of code as an argument, starting with do and ending with end, and will run the code inside that block in a loop. You can optionally provide a block parameter, which in the example above, is represented by the i variable within the pipes: |i|.

Between the do and the end, you write whatever code you want to run in the loop.

Each time Ruby runs the loop, it will assign a new value for the block parameter i, which will start at 0 and end at 9 (since we're running this loop 10 times):

10.times do |i|
  puts "i is: #{i}"
end
# i is: 0
# i is: 1
# i is: 2
# i is: 3
# i is: 4
# i is: 5
# i is: 6
# i is: 7
# i is: 8
# i is: 9

In addition to the do and end syntax, Ruby also allows you to use curly parentheses to define blocks. This syntax is commonly used for one-line blocks:

10.times { |i| puts "i is: #{i}" }

Ruby blocks are just about as important a feature of the Ruby language as callbacks are in JavaScript (and they serve similar purposes). They're an incredibly powerful tool, and you'll see them used in many different scenarios, including writing loops, so make sure to familiarize yourself with this syntax!

Looping with #each

The last looping construct we'll discuss in Ruby is the #each method. The #each method can be used with a few different data types, including arrays and objects (as we'll see later), but one common use case for the #each method is to use it with a Range:

(1..20).each do |num|
  puts num
end

This can be helpful for running a loop of a specified set of numbers, starting with any value you like! Compare this to equivalent code in JavaScript using a for loop:

for (let i = 1; i <= 20; i++) {
  console.log(i);
}

You can see how methods like #each help us write cleaner code that more clearly expresses our intent.

Instructions

Time to get some practice! Write your code in the looping.rb file. Run bin/rspec to check your work. Your goal is to practice using control flow in Ruby to familiarize yourself with the syntax. There is a JavaScript version of the solution for each of these deliverables in the js/index.js file you can look at (but if you want an extra challenge, try solving them in Ruby without looking at the JavaScript solution).

Write a method #happy_new_year using a while or until loop that outputs numbers starting at 10 and counting down to 1. After reaching 1, print out "Happy New Year!"

happy_new_year
# => 10
# => 9
# => 8
# => ...
# => 1
# => Happy New Year!

Write a method #reverse_string that takes one argument, a string, and reverses it. Don't use the built-in .reverse method. Instead, loop through the characters in the input string and reverse it.

reverse_string("hello")
# => "olleh"

Write a method #fizzbuzz_printer that prints the numbers from 1 to 100. For multiples of three, print "Fizz" instead of the number and for the multiples of five print "Buzz". For numbers which are multiples of both three and five, print "FizzBuzz".

fizzbuzz
# 1
# 2
# Fizz
# 4
# Buzz
# Fizz
# 7
# ...
# 14
# FizzBuzz
# 16
# ...
# 100

Resources

• Ruby Loops
• Mastering Ruby Blocks