{:jason, "~> 1.4"},
  {:kino, "~> 0.9", override: true},
  {:youtube, github: "brooklinjazz/youtube"},
  {:hidden_cell, github: "brooklinjazz/hidden_cell"},
  {:visual, github: "brooklinjazz/visual"}


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Review Questions

Upon completing this lesson, a student should be able to answer the following questions.

  • What are booleans, and what information might they represent in a program?
  • What are the boolean operators, and what do they do?
  • What are strict vs non-strict boolean operators?
  • What are if statements, and how are they useful in our programs?


Booleans (BOO-LEE-ANS) are true and false values in your code. They are often useful for representing conditions and triggering actions in your program.


As part of programming, you’ll often create branching paths. Essentially there are multiple things that could happen depending on certain conditions. We often call this control flow.

Let’s take an example and say we have a program that controls a light. Either we can tell the light to power on or power off. In this case, on would be true and off would be false.

flowchart LR
  A[is_powered] --> true --> on
  A[is_powered] --> false --> off

Your Turn

In the Elixir cell below, turn the power on in our light control program to see the light turn on! Replace false with true.

power = false
content = if power, do: "/images/on.png", else: "/images/off.png"!(__DIR__ <> content), :png)

For more on booleans consider this video by Crash Course Computer Science."")

Boolean Operators

Using the boolean operators and, or and not we can compare and manipulate booleans.

and: Check If Two Booleans Are true.

Returns true if left and right-hand side are true. Returns false if either the left side and/or right side of the operator are false.

  flowchart LR
  A[true] --> C
  B[true] --> C
  C[and] --> D[true]
true and true
true and false
false and true
false and false

or: Check If One Or More Booleans Are True.

Returns true if either the left side or the right side of the operator are true.

  flowchart LR
  A[false] --> C
  B[true] --> C
  C[or] --> D[true]
true or true
true or false
false or true
false or false

not: Flip A Booleans Value

Flips false -> true and true -> false.

not true
not false

Strict And Non-Strict Operators

and, or, and not are strict boolean operators, meaning they should only be used with booleans, otherwise they will raise an error.

1 and 1

Sometimes we want to make logical comparison using truthy and falsy values. nil and false are falsy values, and everything else is a truthy value.

We can use non-strict boolean operators && (and), || (or), and ! (not) to work with truthy and falsy values.

&& Check If Two Values Are Truthy.

Returns the right-hand side value if the left-hand side value is truthy.

1 &amp;&amp; 2

Returns the left-hand side value if it is falsy.

nil &amp;&amp; 2
false &amp;&amp; 2

|| Checks If One Or More Values Are Truthy.

Returns the first truthy value on either the left-hand side or the right-hand side.

2 || nil
1 || 2
nil || 2

!: Flips The Falsy Or Truthy Value To true Or false.

Flips truthy -> false and falsy -> true.


When Should You Use Strict Or Non-strict Operators?

You might wonder why you shouldn’t simply always use non-strict operators &&, ||, and ! since they also work with booleans.

true &amp;&amp; true
false || true

The primary reason is communication. We want our code to communicate to other developers, so it’s useful to use and, or, and not to communicate to other developers reading our code that we are working with boolean values.

Generally, prefer and, or, and not with booleans, and use &&, ||, and ! with non-boolean values.

Control Flow

By comparing booleans, you can create branching paths in your code depending on certain conditions.

For example, let’s say we’re building a weather app. This app might send morning notifications to users to let them know that it’s going to be a sunny day.

You might check that the user is subscribed, that it is morning, and that it is sunny. If all of these things are true, then we want to send a notification.

flowchart LR
  A[is subscribed] --> and
  B[is morning] --> and
  C[is sunny] --> and
  and --> true
  true --> D[send notification]
  and --> false
  false --> E[don't send notification]

We can cleverly use boolean operators to write this logic. Try changing any of the true values to false and you’ll see we no longer simulate sending a notification.

is_subscribed = true
is_morning = true
is_sunny = true

is_subscribed and is_morning and is_sunny &amp;&amp; "send notification!"


Rather than rely on boolean operators entirely, it’s more common to use specific control flow structures. For example, we can replicate the code above using if statements.

if is_subscribed and is_morning and is_sunny do
  "send notification"


if statements allow use to trigger some code when some condition is truthy. We often use if statements in combination with else statements to create two separate branching paths in our code.

For example, let’s say we’re creating a coin flip game. The user provides a guess of :heads or :tails, and we have coin flip of :heads or :tails.

flowchart LR
if/else --> c[coin == guess] --true--> w[You win!]
c --false--> l[You lose!]

Here’s how we could write this in code. Try changing guess to :heads to win the coin flip game.

coin = :heads
guess = :tails

if coin == guess do
  "You win!"
  "You lose!"

Further Reading

Consider the following resource(s) to deepen your understanding of the topic.

Commit Your Progress

DockYard Academy now recommends you use the latest Release rather than forking or cloning our repository.

Run git status to ensure there are no undesirable changes. Then run the following in your command line from the curriculum folder to commit your progress.

$ git add .
$ git commit -m "finish Booleans reading"
$ git push

We’re proud to offer our open-source curriculum free of charge for anyone to learn from at their own pace.

We also offer a paid course where you can learn from an instructor alongside a cohort of your peers. We will accept applications for the June-August 2023 cohort soon.


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