Intro to Ruby

Class 2

Welcome Back!

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

IRB

What we will cover today

• Boolean Expressions and Conditionals
• Loops
• Collections

Booleans

A boolean is a 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

    # try some of these out in IRB
    first_num = 3
    second_num = 4
    first_num != second_num
    first_num <= 3
    first_num >= 4

    first_num = 5
    second_num = 5
    first_num == second_num

    # Combine comparison and assignment
    nums_are_equal = (first_num == second_num) 
    puts nums_are_equal
                

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 (||).

Ruby operators and boolean expressions have precendence rules!

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.

Can you tell what this code does?

    print "Guess my secret number: "
    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 
                

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 number == 5
        puts 'number 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 number == 5
        puts 'number is equal to 5'
    else
        puts 'number 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 or equal to 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 number > 10
        puts "number is greater than 10"
    elsif number <= 10 && number > 0
        puts "number is a number between 1 and 10"
    else
        puts "Wow, don't be so negative!"
    end
                

Let's Develop It!

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

    user_choice = gets.chomp

    if user_choice == '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 user_choice == '2'
        puts "You climb a tree. The dragon can't find you."
    else 
        puts "That's not a valid option."
    end 
                

    # run the file from your terminal
    ruby adventure.rb
                

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.

"gets.chomp" is the value of user input at the command line, with the trailing whitespace chomped off. To do math with it, convert it to an integer with the ".to_i" method

    user_input = gets.chomp.to_i 
                

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

    $ ruby program_name.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.

    number = 10

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

    puts 'Done!'
                

While Loops

    number = 10

    while number > 0
        puts "Loop number #{number}"
        number = number - 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 `number = number - 1` statement, to decrement the value of `number`, 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 |num|
        puts "Value of num is #{num}"
    end
                

Each loops

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

The loop has three parts:

The collection that will be looped through, in this case a range of "(0..5)"

The name to give each element when the loop begins again - "num" - 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.

Collections

There are three main types:

• Ranges
• Arrays
• Hashes (we will get to these next time)

    new_range = (1..10)

    new_array = [1, 3, 5]
                

Ranges

    inclusive_range = (1..3)  # contains 1, 2, 3
    exclusive_range = (1...3) # contains 1, 2
    letter_range = ('a'..'e') # contains 'a', 'b', 'c', 'd', 'e'
                

Ranges are the range of values between the given first and last elements.

Inclusive ranges have two dots, and include the last element.

Exclusive ranges have three dots, and do not include the last element.

Ranges must be defined from lowest value to highest.

Ranges

Try out these range methods in IRB.

    (1..99).max
    ('b'...'z').include?('j')
    (890..902).begin
    (890..902).first(4)
    (890..902).last(3)
    (890..902).end
    (22..28).min

    ('a'...'g').each do |letter|
        puts "#{letter} is a pretty good letter"
    end 

    (22..28).to_a
                

Arrays

Arrays have square brackets and can be filled with any type of object: integers, floats, strings, even other arrays or hashes.

    num_array = [1, 3, 5, 89, 212, 7, -100]
    string_array = ["wow", "woooo", "zowie"]
    new_array = Array.new # will have no elements inside it initially
    varied_array = ["one", 2, "THREE", 0.4, ["five", 6]]

    # methods to get information about an array
    num_array.length
    string_array.include?("yee ha")
                

Arrays are a great way to keep track of information that changes frequently.

Accessing Elements in an Arrays

Arrays are ordered and are integer-indexed, starting at 0.

Elements can be accessed by their position.

    num_array = [1, 3, 5, 89, 212, 7, -100]
    string_array = ["wow", "woooo", "zowie"]

    num_array[0]            # returns the zeroth element 
    string_array[2]         # returns the third element 
    string_array[-1]        # returns the last element
    num_array.last          # returns the last element 
    num_array[1..2]         # returns the second and third elements
    string_array.first      # returns the first element 
                

Adding & Deleting From Arrays

Adding and removing items to an array can be done in a variety of ways. These are the most common.

    string_array = ["wow", "woooo", "zowie"]

    # add
    string_array.push("hot diggity") # adds argument as last element
    string_array << "yikes"          # adds argument as last element

    # remove
    string_array.delete("wow")       # deletes the element that matches argument
    string_array.pop                 # removes and returns the last element
    string_array.shift               # removes and returns the first element
                

More Array Methods

Arrays are used a lot in Ruby. There are a lot of cool methods available for them.

    arr = ["dog", "cat", "turtle", "parakeet", "ferret"]

    arr.index("dog") # returns the index of the element that matches argument
    arr.join         # returns a string made up of all the elements
    arr.reverse      # returns new array with same elements, reversed
    arr.shuffle      # returns new array with same elements, shuffled
    arr.uniq         # returns a new array with only unique elements     
    arr.size         # returns the number of elements in the array
    arr.empty?       # returns a boolean 
    arr.clear        # removes all elements from the array
                

Learn more about arrays here.