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Table of contents

Lesson 3: Writing Ruby Scripts

Introduction

Objectives

In this tutorial we are going to look at:

  • getting user input
  • doing things multiple times using loops
  • building simple programs
  • using require to access other Ruby modules
  • using Ruby’s Date type
  • using File to read files in Ruby

Goal

By the end of this tutorial you will have used what you learnt in lesson 2 to build a couple of small Ruby programs, including one like this times tables game.

Running a file with Ruby

We’ve been having fun with irb in Lesson 2 - it’s a great tool for exploring and learning about Ruby. Ideally though we’d like to be able to save our programs for later, and run them whenever we want.

To do this, we can write Ruby script in a text editor (like you did with HTML in lesson 1), then run a the script from the command line with ruby name-of-your-script.rb.

Ruby files end with the extension .rb.

Task 1: My first Ruby script

  • Exit irb by running the exit command
  • Create a file in your text editor (we recommended Atom in lesson 1) called hello.rb
  • Confirm you can see the file in the command line by running ls
  • From the command line, use Ruby to run your file by typing ruby hello.rb and pressing enter
  • You should see nothing returned (because hello.rb is empty at the moment)

Interacting with the user

Now that we’re running our Ruby from a file, we can’t rely on irb to handle all of our user input and output. When we want to show something to the user we need to explicitly say so, and likewise, when we want some input from the user we have to explicitly ask for it.

Ruby provides a puts method (put string) and a gets method (get string).

  • puts - prints a string to the command line
  • gets - waits for the user to type one line of text into the command line, when they press enter, returns whatever the user typed (so you can store it in a variable)

Task 2: Hello world!

Add the following to your hello.rb file, save it, and run it with ruby hello.rb:

puts("What is your name?")
name = gets
puts("Hello #{name}")

We can use puts and gets to create a very simple text based user interface.

Getting things wrong in Ruby scripts

Just like irb, if you get something wrong in a Ruby script Ruby will tell you by printing an error message. Nothing to worry about - programmers make mistakes all the time.

Imagine a script called oops.rb containing:

puts("time for a deliberate error")
oops("this won't work")

When you run this with ruby oops.rb you’ll get an error like this:

time for a deliberate error
oops.rb:2:in `<main>': undefined method `oops' for main:Object (NoMethodError)

The oops.rb:2 bit at the start tells us what file the error was in, and what line (2 in this case) the error was on. There’s no method called oops so Ruby doesn’t know what to do.

Task 3: Create a script that reverses the input

Create a new script called reverse.rb in your text editor.

Use gets, .reverse and puts to get a string from the user, reverse it, and print it.

Answer

puts("Enter a string to reverse: ")
input = gets
puts("Your string reversed is: ")
puts(input.reverse)

Task 4: Create a script that multiplies two numbers

Create a new script called multiply.rb in your text editor.

Note that gets gets the user input as a string. In Ruby strings and numbers are not the same ("42" is not equal to 42).

When you need to ask the user for a number you must ask them for a string and then convert it into a number using .to_i like so:

user_input_as_string = gets
user_input_as_number = user_input_as_string.to_i

The i in .to_i stands for “integer” which is fancy way of saying “whole number”.

Use gets, .to_i and puts to get two strings from the user, converts them to numbers, multiplies them together and prints the result.

Answer

puts("Enter the first number:")
x_string = gets
x_number = x_string.to_i
puts("Enter the second number:")
y_string = gets
y_number = y_string.to_i
puts("#{x_number} * #{y_number} = #{x_number * y_number}")

Defining methods

The Ruby authors didn’t manage to write every method we’ll ever need into Ruby itself, so sometimes we need to define our own.

This means we can write abstractions that describe what we’re doing and we don’t have to repeat ourselves.

Earlier, we had this line of code:

age_dog_years = age_human_years * 7

This is okay, but it’s a little surprising to see that 7 in there - what’s it doing? It would be nicer if we could write:

age_dog_years = convert_to_dog_years(age_human_years)

and hide away the grubby implementation details inside a method.

In Ruby a method definition looks like this:

def method_name(argument)
  result = "some value"
  return result
end

You can name a method anything you could name a variable (they’re snake_cased by convention too). They can have as many lines of code inside them as you want.

Task 5: Create a script that converts human years to dog years

Create a script called dog-years.rb. Use puts, gets and .to_i to ask the user how old their dog is.

Assuming that dogs age faster than humans, and that there are seven “dog years” to one “human year”, define a method which:

  • takes an age in human years as a argument
  • multiplies the age in human years by seven
  • returns the result of the multiplication

Call the method in your script, and print the result.

Answer

puts("How old is your dog in human years?")
age_human_years_string = gets
age_human_years_number = age_human_years_string.to_i

def convert_to_dog_years(age)
  return age * 7
end

age_dog_years = convert_to_dog_years(age_human_years_number)
puts("Your dog is #{age_dog_years} in dog years")

Repeating things

Meme where a cat looks at a bowl of fruit loops, and says 'brøether, may i have the lööps'

So far, each line of code we’ve written has run exactly once (each time a script runs). Ruby goes through each line in the script, runs it, then moves on to the next line. When it reaches the end of the file, the script exits.

One of the things that computers are really good at is repeating themselves. In Ruby we can use methods like loop and .times to make the computer repeat things.

You might want to try some of the following out in irb to get a feel for how they work. (If you get stuck in a loop in irb you can escape it by pressing ctrl + c):

5.times do
  puts("Odelay!")
end
10.times do |i|
  puts("#{10 - i} green bottles, hanging on the wall,")
  puts("#{10 - i} green bottles, hanging on the wall,")
  puts("And if one green bottle should accidentally fall,")
  puts("There'll be #{9 - i} green bottles hanging on the wall.")
  puts
end
loop do
  puts("What's your name?")
  name = gets
  puts("Hello #{name} (to exit this infinite loop, press ctrl+c)")
end

The bit between the do and the end is called a block, which is run by the method repeatedly. Blocks can take parameters after the do, which are wrapped in vertical pipes like |i|.

Task 6: Update one of your scripts to work in a loop

Earlier, you wrote a few scripts that ask the user for some input, do something with the input then print the result and exit. Change one of these scripts (e.g. multiply.rb) so they do this in an infinite loop (using loop).

If you get stuck in an infinite loop (a loop which never ends) in the command line you can break out by pressing ctrl + c.

Answer

loop do
  puts("Enter the first number:")
  x_string = gets
  x_number = x_string.to_i
  puts("Enter the second number:")
  y_string = gets
  y_number = y_string.to_i
  puts("#{x_number} * #{y_number} = #{x_number * y_number}")
end

Conditionals

One last thing to learn is how to get the computer to do something based on some condition. To do this, we can use the if statement, like so:

puts("What is your name?")
name = gets
if name.include?("r")
  puts("Hi #{name} - you sound like a pretty cool person")
else
  puts("Hi nerd")
end

An if statement takes a “condition” (something that evaluates to true or false). You can use things like == (equals), < (less than), or methods like .include? (like we did in the example).

Note that we use == instead of = to compare two values. This is because == tests if two values are equal, whereas = assigns the value on the right to the variable on the left.

If the condition is true Ruby will run the lines of code between the if and the else, otherwise Ruby will run the lines of code between the else and the end.

Tying it all together

Phew! That was a lot to learn. To recap, we’ve learned:

  • how to use irb
  • how to use numbers
  • what “strings” are and how to use them
  • how to use variables to name things
  • how to “interpolate” strings
  • how to call methods
  • how to run Ruby from a file
  • how to interact with a user
  • how to define methods
  • how to repeat things
  • how to do things conditionally

If you’ve understood all of that, well done! You now know the basics of programming. There’s a lot more to learn, but it all builds on these fundamentals.

To tie everything you’ve learned together, here’s one final “hard” task.

Task 7: Build a times tables game

Right at the beginning, we said you’d build this times tables game.

To do this you’ll need to use everything you’ve just learned. It’s a pretty difficult problem for a beginner, so if you get stuck on something don’t be afraid to ask for help.

Here’s a rough plan of how to implement this:

  • create a score variable to keep track of the users score
  • in a loop that happens 4 times
    • generate two numbers between 1 and 12 using rand and assign them to variables
    • print a message to the user asking them to multiply the numbers together
    • get the user’s answer and covert it to a number (using .to_i)
    • compare the user’s answer to the answer Ruby thinks is correct
    • show the user a message saying whether they’re correct (and add one to their score)
  • if the user scored 4 then show them a message saying they got them all right
  • otherwise, show the user a message commiserating them for their failure

For bonus points, consider whether you could define a method to do some of the work to make the loop easier to read.

Answer

def multiplication_challenge
  x = rand(12) + 1
  y = rand(12) + 1

  print("What is #{x} multiplied by #{y} ? ")
  user_input = gets
  user_input_as_a_number = user_input.to_i

  correct_answer = x * y
  if user_input_as_a_number == correct_answer
    puts(":) correct!")
    return true
  else
    puts(":( oops!")
    puts("The answer was #{correct_answer}")
    return false
  end
end

number_of_rounds = 4
score = 0

puts("Times tables challenge")
puts("----------------------")
puts
puts("You will be asked #{number_of_rounds} questions.")
puts("Press enter to start...")
gets

number_of_rounds.times do |i|
  if multiplication_challenge
    score = score + 1
  end
end

if score == number_of_rounds
  puts("You scored #{score}/#{number_of_rounds}. Well done!")
else
  puts("You scored #{score}/#{number_of_rounds}. Better luck next time!")
end

Using require to access other Ruby modules

In lesson 2 we used a number of built-in Ruby methods, like rand (to get a random number), .reverse (to reverse a string), gets and puts (to read and write strings from the command line).

These methods are so useful that they live in the core of Ruby, so you can always call them.

Some bits of Ruby are used less often, and these are not included by default. To use these other bits you need to require them. For example, the "date" module lets you work with dates and times, but it isn’t included by default.

Running require("date") includes the date code so you can use it in your script. For example:

irb(main):001:0> require("date")
=> true
irb(main):002:0> Date.today
=> #<Date: 2019-05-30 ((2458633j,0s,0n),+0s,2299161j)>

Working with Dates in Ruby

Ruby’s date type makes doing things like adding days to a date much easier (if you didn’t have a date type you’d need to worry about how many days there are in a month and so on).

irb(main):003:0> one_thousand_days_in_the_future = Date.today + 1000
=> #<Date: 2022-02-23 ((2459633j,0s,0n),+0s,2299161j)>

The .today method gets today’s date, but you can also create dates for other days using .new:

irb(main):004:0> Date.new(2022, 2, 23)
=> #<Date: 2022-02-23 ((2459633j,0s,0n),+0s,2299161j)>

Because different cultures represent dates in different ways (e.g. Japan use 2019-05-30, the USA use 05/30/2019 and the UK use 30/05/2019), there’s a special method for formatting dates as strings called .strftime. It takes a “format” string as an argument specifying how to show the date:

irb(main):005:0> japan_format = Date.today.strftime("%Y-%m-%d")
=> "2019-05-30"
irb(main):006:0> usa_format = Date.today.strftime("%m/%d/%Y")
=> "05/30/2019"
irb(main):007:0> uk_format = Date.today.strftime("%d/%m/%Y")
=> "30/05/2019"

There are lots of placeholders you can use in your date string, including:

  • %Y - the year as a 4 digit number
  • %m - the month as a 2 digit number
  • %d - the day as a 2 digit number
  • %A - the day of the week as a word (e.g. Thursday)

Task 1: What day were you born on?

Using irb and the Date type, work out what day of the week you were born on.

Answer

irb(main):001:0> require("date")
=> true

irb(main):002:0> birthday = Date.new(1988, 1, 20)
=> #<Date: 1988-01-20 ((2447181j,0s,0n),+0s,2299161j)>

irb(main):003:0> birthday.strftime("%A")
=> "Wednesday"

Using File to read files in Ruby

Sometimes it’s nice to be able to store things in files, and read these files using Ruby so we can do something with the contents. Ruby has a File.read method that lets us do this.

File.read takes the path to the file as an argument, and returns the contents of the file as a string.

irb(main):004:0> File.read("/Users/your-username/Desktop/lesson-1-html-and-css/index.html")
=> "<!doctype html>\n<html>...

We can work with the contents using all the string methods (like .reverse) we’ve already seen.

There are lots of other useful methods like .size (which returns how many characters there are in the string), and .sub (which takes two arguments, and substitutes the first occurence of the first string with the second).

irb(main):005:0> "Cats are the best".sub("Cat", "Dog")
=> "Dogs are the best"
irb(main):006:0> "Dogs are the best".size
=> 17

Task 2: count the characters in a file

We created a file in lesson-1-html-and-css/index.html. Use File.read and .size to work out how many characters there are in the file.

In Finder you can copy the path to a directory by right clicking, holding down the option key (), and choosing Copy “Directory” as Pathname. Alternatively, you can drag the folder onto the terminal and it will type the path for you.

Answer

irb(main):006:0> file = File.read("/Users/your-username/Desktop/lesson-1-html-and-css/index.html")
=> "<!doctype html>\n<html>...
irb(main):007:0> file.size
1260

Further reading

If you learn best from weird books then _why’s (poignant) guide is excellent (see cartoon below). If you’d prefer something less silly, have a look at:

Which are all great.

A comic strip from _why's (poignant) guide showing Dr. Cham lost in some tunnels under a mysterious castle