Conditional Statements

Dr Ian Cornelius

Hello

Learning Objectives
1. Understand what a conditional statement is
2. Demonstrate the ability to use conditional statements

Previously…

You were introduced to comparison operators previously
These operators are used with conditional statements to ensure certain conditions have been met
Recap on the comparison operators:

Symbol	Explanation
`==`	The Same
`!=`	Not the Same
`>`	Greater Than
`>=`	Greater Than or Equal To
`<`	Less Than
`<=`	Less Than or Equal To !

Introduction to Conditional Statements

A basic decision statement which is done using a selection structure
The decision will be described to the interpreter by a conditional statement
- whereby a result can only be True or False
Python allows the following:
- if statements
- if ... else ... statement
- if ... elif ... else statement
- Nested if ... else ... statements

If Statements (1)

Often referred to as a decision-making statement
Used to control the flow of execution for statements and to test an expression
- tests logically whether a condition is True or False

if variable == value:
    ...

If Statements (2)

x = 1

if x == 1:
    print(True)

True

If … Else Statements (1)

Known as an alternative execution, whereby there are two possibilities
- the condition statement determines which of the two statements gets executed
The else is used as the ultimate result for a test expression
- this result is only met if all other statements are False

if variable == value:
    ...
else:
    ...

If … Else Statements (2)

def is_equal(x):
    if x == 1:
        return True
    else:
        return False

is_equal(1)
is_equal("Hello")

is_equal(1) –> True

is_equal(“Hello”) –> False

Else-If Statements (1)

elif is a keyword in Python to replace the else if conditions from other languages
The condition allows for two or more possibilities, known as a chained conditional

if variable > value:
    ...
elif variable < value:
    ...
else:
    ...

Else-If Statements (2)

The first check is to determine whether x is equal to 1
- x == 1 the check passes
- (True, "x is 1") is returned

def is_equal(x, y):
    if x == 1:
        return True, "x is 1"
    elif y == 2:
        return True, "y is 2"
    else:
        return False, "x is not 1 and y is not 2"

is_equal(x=1, y=3)

is_equal(x=1, y=3) –> (True, ‘x is 1’)

Else-If Statements (3)

The first check is to determine whether x is equal to 1
- x == 2, the check fails, move onto the next check
The second check is actioned, and checks whether y is equal to 2
- y == 2, the check passes
- (True, "y is 2") is returned

def is_equal(x, y):
    if x == 1:
        return True, "x is 1"
    elif y == 2:
        return True, "y is 2"
    else:
        return False, "x is not 1 and y is not 2"

is_equal(x=2, y=2)

is_equal(x=2, y=2) –> (True, ‘y is 2’)

Else-If Statements (3)

The first check is to determine whether x is equal to 1
- x == 2, the check fails, move onto the next check
The second check is actioned, and checks whether y is equal to 2
- y == 3, the check fails, move onto the next check
As both checks have failed, the final case is reached
- (False, "x is not 1 and y is not 2") is returned

def is_equal(x, y):
    if x == 1:
        return True, "x is 1"
    elif y == 2:
        return True, "y is 2"
    else:
        return False, "x is not 1 and y is not 2"

is_equal(x=2, y=3)

is_equal(x=2, y=3) –> (False, ‘x is not 1 and y is not 2’)

Nested If … Else Statements (1)

if ... else statements can be written inside each other
- this is known as nesting

if variable == value:
    if variable == value:
        ...
    elif variable == value:
        ...
    else:
        ...
elif variable != value:
    ...
else:
    ...

Nested If … Else Statements (2)

The first check is to determine whether x is equal to 1
- x == 1, the check passes, move onto the next check
The nested if statement is now actioned and a second check is performed on whether y is equal to 2
- y == 3, the check fails, move onto the next check
The third check is actioned and checks whether y is equal to 4
- y == 3, the check fails, move onto the next check
The final case is reached
- (False, "x is 1 and y is not 2 or 4") is returned

def is_equal(x, y):
    if x == 1:
        if y == 2:
            return True, "x is 1 and y is 2"
        elif y == 4:
            return True, "x is 1 and y is 4"
        else:
            return False, "x is 1 but y is not 2 or 4"
    else:
        return "False", "x is not 1"

is_equal(x=1, y=3)

is_equal(x=1, y=3) –> (False, ‘x is 1 but y is not 2 or 4’)

Nested If … Else Statements (2)

The first check is to determine whether x is equal to 1
- x == 2, the check fails, move onto the next check
The nested if statement does not execute and the final case is reached
- ("False", "x is not 1") is returned

def is_equal(x, y):
    if x == 1:
        if y == 2:
            return True, "x is 1 and y is 2"
        elif y == 4:
            return True, "x is 1 and y is 4"
        else:
            return False, "x is 1 but y is not 2 or 4"
    else:
        return "False", "x is not 1"

is_equal(x=2, y=3)

is_equal(x=2, y=3) –> (‘False’, ‘x is not 1’)

Nested If … Else Statements (3)

The first check is to determine whether x is equal to 1
- x == 1, the check passes, move onto the next check
The nested if statement is now actioned and a second check is performed on whether y is equal to 2
- y == 2, the check passes
- (True, "x is 1 and y is 2") is returned

def is_equal(x, y):
    if x == 1:
        if y == 2:
            return True, "x is 1 and y is 2"
        elif y == 4:
            return True, "x is 1 and y is 4"
        else:
            return False, "x is 1 but y is not 2 or 4"
    else:
        return "False", "x is not 1"

is_equal(x=1, y=2)

is_equal(x=1, y=2) –> (True, ‘x is 1 and y is 2’)

Nested If … Else Statements (4)

The first check is to determine whether x is equal to 1
- x == 1, the check passes, move onto the next check
The nested if statement is now actioned and a second check is performed on whether y is equal to 2
- y == 4, the check fails, move onto the next check
The third check is actioned and checks whether y is equal to 4
- y == 4, the check passes
- (True, "x is 1 and y is 4") is returned

def is_equal(x, y):
    if x == 1:
        if y == 2:
            return True, "x is 1 and y is 2"
        elif y == 4:
            return True, "x is 1 and y is 4"
        else:
            return False, "x is 1 but y is not 2 or 4"
    else:
        return "False", "x is not 1"

is_equal(x=1, y=4)

is_equal(x=1, y=4) –> (True, ‘x is 1 and y is 4’)

Nested If … Else Statements (5)

The first check is to determine whether x is equal to 1
- x == 1, the check passes, move onto the next check
The nested if statement is now actioned and a second check is performed on whether y is equal to 2
- y == 5, the check fails, move onto the next check
The third check is actioned and a check is performed on whether y is equal to 4
- y == 5, the check fails, move onto the next check
The final case is reached
- (False, "x is 1 but y is not 2 or 4") is returned

def is_equal(x, y):
    if x == 1:
        if y == 2:
            return True, "x is 1 and y is 2"
        elif y == 4:
            return True, "x is 1 and y is 4"
        else:
            return False, "x is 1 but y is not 2 or 4"
    else:
        return "False", "x is not 1"

is_equal(x=1, y=5)

is_equal(x=1, y=5) –> (False, ‘x is 1 but y is not 2 or 4’)

Goodbye

Questions?
- Post them in the Community Page on Aula
Contact Details:
- Dr Ian Cornelius, ab6459@coventry.ac.uk