Lesson 02-05: Boolean Expressions¶

Learning Target: I can predict the outcome of boolean expressions.

Right now the programs that we’ve been writing have beem very straightforward - they run instruction after instruction, in the order that it was written. Computers act smart when they can make decisions, and in order to make decisions, we have to learn about boolean expressions.

Simple Boolean Operators¶

A boolean expression is simply any expression that evaluates to be True or False. While we learned about the PEMDAS operators before, none of those apply to boolean expressions. There are three basic operators covered in this lesson:

and

or

not

and and or are both operators that require two boolean terms - like True and False. The not operator, however, is only applied to a single term.

We can evaluate the expression of boolean expressions with truth tables.

Truth Tables¶

Truth Tables are tables that list all possible outcomes for a given operator. The simplest example of a truth table is the table for the not operator.

`not`¶

The not operator takes a boolean expression and turns it into its opposite. So if I have True as my statement, then using not would turn it into not True - which is False.

Here’s how the truth table for not looks:

If A is: Then not A is:

True False

False True

If `A` is:	Then `not A` is:
`True`	`False`
`False`	`True`

The way this table is read is as follows:

if A is True, then not A is False

if A is False, then not A is True

`and`¶

With and, we require two boolean terms - we’ll call them A and B. This time, when we construct our truth table, we will have to list all possible combinations of A and B where either one of them can be True or False.

If A is: If B is: Then A and B is:

True True True

True False False

False True False

False False False

If `A` is:	If `B` is:	Then `A and B` is:
`True`	`True`	`True`
`True`	`False`	`False`
`False`	`True`	`False`
`False`	`False`	`False`

I think it’s important to understand the logic behind why this is.

Think of A and B being two independent statements that can be either True or False. A and B is a single statement that speaks to the truthiness or falseness of both statements together. As an example, let’s make the following statements:

A means that the Earth is roundish

B means that the sky is made of lemons

Individually, A is obviously True and B is obviously False. However, when we think about it in the context of our real-life examples, if you were to say them together as one statement: “The Earth is roundish AND the sky is made of lemons”, then the statement as a whole is false. This is why True and False ==> False, and False and True ==> False.

It should be pretty obvious that when you say two truthful statements together, then the entire thing is truthful. Or if you say two false statements together, then the entire thing is false.

As a general rule for and statements: “A and B” is True as long as both of them are True.

`or`¶

With or, we can logic our way through the truth table. I like making statements for my A and B, it makes it easier to understand. Let’s say that:

A means “it is raining outside”

B means “it is cloudy”

Individually, A and B could be True or False depending on the weather in your local area. However, if we consider the statement A or B, meaning “it is raining outside or it is cloudy”, then either one of those statements being True makes the whole statement True.

We can still maintain that both being True is truthful and both being False is false. Here’s how our truth table looks:

If A is: If B is: Then A or B is:

True True True

True False True

False True True

False False False

If `A` is:	If `B` is:	Then `A or B` is:
`True`	`True`	`True`
`True`	`False`	`True`
`False`	`True`	`True`
`False`	`False`	`False`

As a general rule for or statements: “A or B” is True whenever either one of them is True.

Order of Operations & Sample Exercise¶

The order of operations for boolean operators and, or, not are as follows:

Any parentheses () first

Then not

Then and

The or last

Make sure you memorize this information! You will need it on the exercises below.

Example: Evaluate the following boolean expression: True and False and not False or True

Step 1: Evaluate all not operators:

True and False and not False or True

True and False and True or True

Step 2: Evaluate all and operators:

True and False and True or True

False and True or True

False and True or True

False or True

Step 3: Evaluate all or operators:

False or True

True

Thus our final answer is True!

Checks for Understanding¶

With booleans, each question really only has one answer - True or False. Therefore, try to make sure you are certain of the answer before you attempt it, otherwise it will be ruined! Try not to guess - guessing will not help you get more comfortable with booleans. Use paper and pencil if you need it - it’s good to show your steps, just like in math class.

Q#1¶

        ch0205-1: Drag the blocks in the order in which they would be evaluated.Parentheses
NOT
AND
OR

Q#2¶

(A) True
(B) False
Feel free to check the charts above for reference. Make sure you go step by step!

Q#3¶

(A) True
Feel free to check the charts above for reference. Make sure you go step by step!
(B) False

Q#4¶

(A) True
Feel free to check the charts above for reference. Make sure you go step by step!
(B) False

Q#5¶

(A) True
(B) False
Feel free to check the charts above for reference. Make sure you go step by step!

Q#6¶

(A) True
Feel free to check the charts above for reference. Make sure you go step by step!
(B) False

Next Section - Lesson 02-06: Comparison Operators