Intro to Ruby

Class 2

Welcome!

Homework Discussion

How was last week's homework? Do you have any questions or concepts that you'd like to discuss?

Review

• Arithmetic and variables
• Data types

What we will cover today

• Methods
• Text editor, command line, and ruby shell
• Boolean Expressions and Conditionals
• Loops

Methods

• Methods define the behavior for an Object.
• String Objects can reverse, for example.
• Some methods are accessed using our friend 'dot' and some are standalone.
• Let's call some methods

            
irb> puts "hello"
hello
=> nil
irb> "2".to_i
=> 2
irb> 2.to_s
=> "2"
irb> "2" / 5
NoMethodError: undefined method `/' for "2":String
            
          

User Input

• To obtain user input, use `gets`
• To print out information, use `puts`
• Let's create our first program together

Put the code below in a file and save it as name.rb

puts 'Hello there, and what\'s your name?'
name = gets
puts 'Your name is ' + name.chomp! + '? What a lovely name!'
puts 'Pleased to meet you, ' + name + '. :)'
          

Run your program from the command line:

        ruby name.rb
          

Let's Develop It

• Write your own program using 'puts' and 'gets' to ask a user for their age and then tell them how old they are in dog years.
• reminder: 'gets' method returns a string. To do math on it, convert it to an integer with .to_i method.

#1 dog year = 7 human years
user_age = gets.to_i
            

Questions?

Booleans

A boolean is a basic data type. It can have only two values: true or false.

Boolean Expressions

Code that compares values and returns True or False is called a Boolean expression

• Test for equality by using `==`. We can't use `=` because that is used for assignment
• Test for greater than and less than using `>` and `<`

Boolean Expressions cheat sheet

Learn more about logical operators in Ruby here.

Boolean Expressions practice

a = 3
b = 4
puts a != b
puts a <= 3
puts a >= 4
a = 5
b = 5
puts a == b
c = a == b # Combine comparison and assignment
puts c
puts 3 < 5
            

Remember: Equals does not equal "equals equals"

Further reading on boolean expressions...

Boolean Expressions

A boolean expression evaluates to true or false. It can also have multiple parts, joined by AND (&&) or OR (||).

Further practice on boolean expressions...

Let's Develop It

Take a few minutes and experiment with boolean expressions in IRB. You can start with the examples below.

true && false
1 == 1 && 2 > 37
"boop" == "bip" || 7 == 8
false || true
89 > 88 || 89 < 90
true || not(1 == 1 || 2 == 65)
          

Putting Booleans to Work

So, what's the value of knowing if a statement is true or false? Often, you'll use that to control whether a piece of code will execute or not.

user_guess = gets.chomp.to_i
secret_number = 312

if user_guess < secret_number
  puts "Too low!"
elsif user_guess > secret_number
  puts "Too high!"
else
  puts "You guessed it. Wow maybe you're psychic...."
end
          

Can you tell what this code does?

Conditionals

When we want different code to execute depending on certain criteria, we use a conditional

We achieve this using if statements and boolean expressions.

if x == 5
  puts 'x is equal to 5'
end
          

Conditionals

We often want a different block to execute if the statement is false. This can be accomplished using else.

if x == 5
  puts 'x is equal to 5'
else
  puts 'x is not equal to 5'
end
          

Conditionals

The following shows some examples of conditionals with more complex boolean expressions:

# And
if x > 3 && y > 3
  puts 'Both values are greater than 3'
end

# Or
if x != 0 || y != 0
  puts 'The point x,y is not on the x or y axis'
end

# Not
if not(x > y)
  puts 'x is less than y'
end
          

Chained conditionals

Conditionals can also be chained.

Chained conditionals use elsif to test if additional statements are true. The single 'else' action will only happen if all preceding conditions are false.

For example:

if x > 10
  puts "x is greater than 10"
elsif x <= 10 && x > 0
  puts "x is a number between 1 and 10"
else
  puts "Wow, don't be so negative, dude"
end
          

Let's Develop It

Write a program in your text editor that uses conditionals and user input to allow the user to play an adventure game. For example, you can "puts" story and options to the command line, and user can input their choices.

Remember, to do math with "gets", convert it to an integer with the ".to_i" method

            input = gets.chomp.to_i
          

Run your program by calling it with Ruby from the command line.

            $ ruby program_name.rb
          

Let's Develop It Example

# adventure.rb
puts "A vicious dragon is chasing you!"
puts "Options:"
puts "1 - Hide in a cave"
puts "2 - Climb a tree"

input = gets.chomp

if input == '1'
  puts "You hide in a cave. The dragon finds you and asks if you'd like to play Scrabble. Maybe it's not so vicious after all!"
elsif input == '2'
  puts "You climb a tree. The dragon can't find you."
else
  puts "That's not a valid option."
end
          

#command line
ruby dragon.rb
          

Loops

It is often useful to perform a task and to repeat the process until a certain point is reached.

The repeated execution of a set of statements is called iteration, or, more commonly, a loop.

One way to achieve this, is with the while loop.

x = 10

while x > 0
  puts "Loop number #{x}"
  x = x - 1
end

puts 'Done'
            

While Loops

x = 10

while x > 0
  puts "Loop number #{x}"
  x = x - 1
end
          

The while statement takes a condition, and as long as it evaluates to true, the code block beneath it is repeated. This creates a loop.

Without the `x = x - 1` statement, to increment the value of x, this would be an infinite loop :( :( :(

While loops

Consider the following example that uses a while loop to sing you a song.

num_bottles = 99

while num_bottles > 0
  puts "#{num_bottles} bottles of beer on the wall,
  #{num_bottles} bottles of beer, take one down, pass it
  around, #{num_bottles - 1} bottles of beer on the wall!"

  num_bottles = num_bottles - 1
end
          

"#{num_bottles}" is an example of string interpolation

Let's Develop It

• Write a program that obtains user input like the last program
• This program should not exit until the user says it should (maybe by entering "quit"?)
• Use a loop!
• You can use the next slide as an example.

Let's Develop It: Example

# loopy.rb
loopy = true

while loopy == true
  puts "Are we having fun yet?"
  puts "Answer 'true' or 'false'."
  user_input = gets.chomp
  if user_input == 'false'
    loopy = false
  end
end
          

Learn more about loops in Ruby here.

Each loops

The most commonly used type of loop in Ruby is an each loop. It uses the .each method to iterate over a collection of elements, doing work to each one.

First, we need a collection. Let's use a range of numbers to loop over.

(0..5).each do |i|
  puts "Value of i is #{i}"
end
            

Each loops

(0..5).each do |i|
  puts "Value of i is #{i}"
end
            

The loop has three parts:

• The collection that will be looped through, in this case a range- "(0..5)"
• The name to give each element when the loop begins again - "i" - in the pipes "| |"
• The code to execute with the element - the puts statement

We will revisit the each loop when we have a better understanding of collections.

Questions?

Homework

Practice: Read Chapters 5 and 6 of Learn to Program.