Boolean Operators Exercise

In case you get stuck anywhere, don't be afraid to ask the coaches! They are here to help and will gladly explain everything to you!

Take notes during the exercises. Even if you never look at them again, they will help you memorise things!

The following exercises might be a bit dry, but the concept you’ll be practising is quite important in the daily life of a programmer. So bear with it, there are more interesting exercises to come! ;)

  1. Imagine the condition is part of your code, like this:

    if condition
      puts "A"
      puts "B"

    Also, the following variables are given:

    list = [2, 3, 4]
    title = "Ruby Monstas"

    Analyse the following conditions and note their return value, like in the first row. Also write down what the code would execute in the if statement above.

    Fill this out without programming anything. Just think it through in your head. After that, you can check your answers with IRB.

    condition result (return value) puts …
    1 < 2 true A
    list.length == 3    
    "test" == 1    
    true || false    
    true && false    
    1 < 2 || 1 > 2    
    list.length > 3 && title.length == 12    
    !(list.length == 3)    
    !(list[1] == 3 || 10 != 12)    
    1 == 1 && (!("testing" == 1 || 1 == 0))    
    3 != 4 && !("A" != "a" || "Ruby" == "Ruby")    
  2. Consider this boolean expression:

    false && x

    You can tell what the result will be without knowing the value of x. Why?

  3. Write methods for the boolean operators ||, && and !. Here are the method signatures, just fill in the bodies:

    def my_not(a)
      # your code goes here
    def my_and(a, b)
      # your code goes here
    def my_or(a, b)
      # your code goes here

    You can test your implementation by running

    puts my_not(my_and(my_or(true, false), my_or(false, true)))

    Your program should output false as a result.

  4. The following is a so-called truth table. For more examples of truth tables, refer to this session’s cheat sheet.

    We have two variables called one and two. These are the input of our truth table. The third column contains the output of our truth table. E.g. for the inputs one = true and two = false, the expression results in true.

    Input one Input two Output
    false false false
    true false true
    false true true
    true true false
    1. How would you describe the output of this truth table with words? When is it true and when is it false?

    2. (BONUS) Find a formula that satisfies all 4 rules and outputs from the table above. You are only allowed to use these elements:

      • one
      • two
      • &&
      • ||
      • !
      • parentheses: ()

      For example: !one || two

      Hint: All listed elements are required to solve this puzzle!

      You can use the following snippet of code to test your formula:

      boolean_inputs = [
        [false, false],
        [true, false],
        [false, true],
        [true, true]
      expected_outputs = [
      def mystery_formula(one, two)
        # TODO: replace the following line with your formula!
            do |(one, two), expected_output|
        print "We expected mystery_formula(#{one}, #{two}) to return #{expected_output}, "
        actual_output = mystery_formula(one, two)
        if actual_output == expected_output
          puts 'and it dit! 🎉'
          puts "but it returned #{actual_output}. 😕"