Part 3 Boolean Expressions

What Is It?

A boolean expression is an expression that is either true or false.

Examples

== 5         # True

== 6         # False

> 6          # False

< 6          # True

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Relational Operators

What Is It?

One of the operators that compares its operands.

Examples

Operator	Description	Example
==	is equal to	x == y
!=	is not equal to	x != y
>	is greater than	x > y
<	is less than	x < y
>=	greater than or equal to	x >= y
<=	less than or equal to	x <= y

==

0 == 0         # True

'a' == 'b'     # False

True == True   # True

True == False  # False

!=

0 != 0         # False

'a' != 'b'     # True

True != True   # False

True !=  False # True

>

1 > 0          # True

0 > 1          # False

'a' > 'b'      # False

'b' > 'a'      # True

True > False   # True

False > True   # False

<

1 < 0          # False

0 < 1          # True

'a' < 'b'      # True

'b' < 'a'      # False

True < False   # False

False < True   # True

>=

0 >= 0         # True

0 >= 1         # False

'a' >= 'a'     # True

'a' >= 'b'     # False

<=

0 <= 0         # True

0 <= 1         # True

'a' <= 'b'     # True

'b' <= 'a'     # False

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Logical Operators

What Is It?

One of the operators that combines boolean expressions.

There are three logical operators:

and

Evaluates to True if both conditions are true

x = 5
x > 0 and x < 10
# True

x < 0 and x > 3
# False

or

Evaluates to True if either or both conditions are true

n = 4
n % 2 == 0 or n % 3 == 0
# True

n % 5 == 0 or n % 3 == 0
# False

not

Negates a boolean expression.

not True
# False

not False
# True

not (1 > 0) 
# False

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Compound Statements

Before we begin…

You have already seen compound statements.
Functions are syntactically compound statements.
Don’t get overwhelmed by the terminology.

What Is It?

A compound statement consists of one or more ‘clauses.’
A clause consists of a header and a ‘suite.’
The clause headers of a particular compound statement are all at the same indentation level.
Each clause header begins with a uniquely identifying keyword and ends with a colon.
A suite is a group of statements controlled by a clause.

Compound Statements

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Conditional Execution

Part 1

The if Statement

x = 1
if x > 0:
    print("x is positive")

# x is positive

Breaking Down The if Statement

The boolean expression after the if is called the condition.
If the condition is true, the indented statement runs.
If the condition is false, nothing happens.
If statements have the same structure as function definitions. (a header followed by an indented body). These are called compound statements.

The else Clause

x = 7
if x % 2 == 0:
    print("x is even")
else:
    print("x is odd")

# x is odd

Breaking Down the else Clause

The else clause simply provides an alternative path of execution when the if condition is false.

The elif Clause

x = 10
y = 5
if x < y:
    print("x is less than y")
elif x > y:
    print("x is greater than y")
else:
    print("x and y are equal")

# x is greater than y

Breaking Down The elif Clause:

elif is an abbreviation for “else if”.
If there is an else clause, it has to be at the end, but there doesn’t have to be one.
Each condition is checked in order. The first one to evaluate to true is run and no other is executed or evaluated.

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Conditional Execution

Part 2

Nested Conditions

One condition can be nested within another.

Example

x = 10
y = 5

if x == y:
     print("x and y are equal")
else:
    if x < y:
        print("x is less than y")
    else:
        print("x is greater than y")

# x is greater than y

Just Because You Can, Doesn’t Mean You Should.

Logical operators often provide a way to simplify nested conditional statements.

Nested Conditional Statement

x = 5
if 0 < x:
    if x < 10:
        print("x is a positive single-digit number.")

# x is a positive single-digit number.

Simplified Using Logical Operator

if 0 < x and x < 10:
    print("x is a positive single-digit number.")

# x is a positive single-digit number.

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The End

Parrot