If statements

Data types starter - identify the data types

String

Character

Float

Boolean

Float

Character

value = "wizard123"

value = "0"

value = 94.3

value = True

value = 99.

value = "@"

Reveal

Learning Intentions

Understand basic logic in a program
Know how to use if statements in Python
Know when to use if statements in Python

Success Criteria

I can explain what a condition is
I can write a condition in Python
I can explain what an if statement does in Python
I can write an if statement to make a choice in Python

If statements

This lesson will be all about conditions. In programming, a condition is something which we must meet for something to happen.
Conditions use our Boolean data type!

Say if it’s true or false

Is it true or false?

1. 50 is greater than 43

2. 70 is less than 70

3. 50 is equal to 50

4. "Word 1" is equal to "Word 2"

False

True

False

True

False

True

False

True

Operators

Up until now we’ve been working with some mathematical operators shown below.
What do they all do?

Comparators

Say if it’s true or false

Is it true or false?

1. 25 > 13

2. 25 < 13

3. 19 == 90

4. 19 != 90

5. 90 >= 90

False

True

False

True

False

True

False

True

False

True

If statements

The if statement is one of the most useful pieces of code you will learn in any language. Almost every program ever written that does something useful will have an if statement.
An if statement is all about making a choice.

Structure of an if statement

If something is true then

do something

Otherwise

do something else

Structure of an if statement

if bob > 15:
   #do something
else:
   #do something else

If statements in python

user_age = int(input())

if user_age > 60:
  print("You are old enough to claim a Senior’s RailCard")
else:
  print("You are too young to claim a Senior’s RailCard")

Note the indentation!

If statements in python

username = input()

if username == "johnsmith":
  print("Welcome John Smith")
else:
  print("Username not found")

Key things about if statements

In Python, if statements will not work without indentation!
Do not forget the colon ( : ) at the end of the if statement line and after the else
When checking if something is equal with something else we use the double equals sign ( == ). The single equals single is used when we are putting a value into a variable.

Nested if statements

#Users who over 60 and younger than 80 years
user_age = int(input())

if user_age >= 60:
  if user_age <= 80:
    print("You are old enough to attend the ball")
  else:
    print("You are too old to attend the ball")
else:
  print("You are too young to attend the ball")

What is missing from the code below?

print(Please insert a number)
i = int(input())

if i > 10
print(Please insert a number)
x = int(input())

if x > 10
print(Thank you)

What is missing from the code below?

print("Please insert a number")
i = int(input())

if i > 10:
  print("Please insert a number")
  x = int(input())

if x > 10:
  print("Thank you")

Create a program that asks the user for their age. It should store their age in a variable and then check the user’s age to see if they are 17 or over. If they are it should print “You are old enough to drive” otherwise it would say “You are too young to drive”.

Task

Now work on the tasks on if statements (selection) in the booklet. Open the Python booklet to page 18 and read through it.

After that, try the tasks on page 20 and 21 and complete the tasks there.

Task

COMPLEX IF STATEMENTS

Nested if statements get complicated quite quickly.
There are different ways of writing them. Consider the following:

username = input()
password = input()
if username == "John":
  if password == "password123":
    print("Welcome, John")
  else:
    print("Incorrect details provided")
else:
  print("Incorrect details provided")

Complex if statements

Nested if statements get complicated quite quickly.
There are different ways of writing them. Consider the following:

username = input()
password = input()
if username == "John" and password == "password123":
  print("Welcome, John")
else:
  print("Incorrect details provided")

Notice the use of the word keyword and

Complex if statements

Notice the use of the Python keyword and
This makes this a complex condition

username = input()
password = input()
if username == "John" and password == "password123":
  print("Welcome, John")
else:
  print("Incorrect details provided")

A	B	A and B	A or B
T	T	T	T
T	F	T	F
F	T	T	F
F	F	F	F

Complex if statements

There are three keywords that turn a condition in to a complex condition:
- and
- or
- not

A	B	A and B	A or B
True	True	True	True
True	False	False	True
False	True	False	True
False	False	False	False

Complex if statements - Truth Tables

This table is called a truth table. It defines what happens with a complex condition:
- And: true when both parts of a condition are true
- Or: true when one or more parts of a condition are true
- Not: flips a true to a false and a false to a true

A	B	A and B	A or B
True	True	True	True
True	False	False	True
False	True	False	True
False	False	False	False

Boolean Heroes

RED OR GREEN

TRUE

sUIT OF METAL AND Gold

TRUE

ORANGE OR YELLOW

FALSE

TRUE

RED AND NOT GREEN

TRUE

FALSE

BLUE AND NOT GREEN

FALSE

TRUE

Asgardian AND NOT GREEN

TRUE

FALSE

Wears a cape AND green

FALSE

TRUE

Wears a cape OR green

TRUE

FALSE

Write a statement that uses and in every day life.
- For example, you are expected to have a Chromebook and be in school uniform in school
Write a statement that uses or in every day life.
- For example, you are expected to be learning history or geography in social subjects
Work through the tasks on page 24 and 25 in the Python booklet.

Task

Write a statement that uses and in every day life.
- For example, you are expected to have a pen and be in school uniform in school
Write a statement that uses or in every day life.
- For example, you are expected to be learning history or geography in social subjects
Work through the tasks on page 24 and 25 in the Python booklet.

FALSE

Multiple selection

Sometimes an if statement becomes overly complicated:

age = int(input())

if age < 21:
  print("You are under 21")
else:
  if age < 31:
    print("You are under 31")
  else:
    if age < 41:
      print("You are under 41")
    else:
      print("You are much older")

Multiple selection - Else If

Introducing the else if statement.
We can use an else if statement (in Python this is written as elif) to combine two or more if statements into one and prevent us needing to have nested if statements.
Else if is more efficient than having many if statements:
- it takes fewer lines of code
- it is easier to read
- it runs faster

Multiple selection - Else if

We can use an else if statement (in Python elif) to write that same program from before:

age = int(input())

if age < 21:
  print("You are under 21")
elif age < 31:
  print("You are under 31")
elif age < 41:
  print("You are under 41")
else:
  print("You are much older")

Combining complex conditions and else if

print("What age are you?")
age = int(input())

if age <= 11 and age >= 5:
  print("You should be in primary school")
elif age <= 17 and age >= 12:
  print("You should be in primary school")
elif age < 5:
  print("You should be in nursery!")
else:
  print("You are much older")

Write a program, using complex conditions and else if statements, to:
- Take in a user's percentage
- If the percentage is above 70%
  - Tell them they got an A
- If the percentage is above 60%
  - Tell them they got a B
- If the percentage is above 50%
  - Tell them they got a C
- Otherwise tell them they did not pass

Task

print("What percentage did you get?")
percentage = int(input())

if percentage > 70:
  print("You got an A")
elif percentage > 60:
  print("You got a B")
elif percentage > 50:
  print("You got a C")
else:
  print("You didn't pass.")

Write a program, using complex conditions and else if statements, to:
- Take in a user's percentage
- If the percentage is above 70%
  - Tell them they got an A
- If the percentage is above 60%
  - Tell them they got a B
- If the percentage is above 50%
  - Tell them they got a C
- Otherwise tell them they did not pass

Task - Solution

print("What percentage did you get?")
percentage = int(input())

if percentage > 70:
  print("You got an A")
elif percentage > 60:
  print("You got a B")
elif percentage > 50:
  print("You got a C")
else:
  print("You didn't pass.")

Work through the tasks in the Python booklet on pages 26 and 27.

Once you have done that, you can create your own Python quiz using Python.

Work through the tasks in the Python booklet on pages 26 and 27.

Once you have done that, you can create your own Python quiz using Python.

Task

print("What percentage did you get?")
percentage = int(input())

if percentage > 70:
  print("You got an A")
elif percentage > 60:
  print("You got a B")
elif percentage > 50:
  print("You got a C")
else:
  print("You didn't pass.")

Plenary

Identify the errors in the code below:

print("Please input your username.")
username = input()

if username == "John Smith":
  print("Welcome John Smith")
else
  print("Your password is incorrect")
print("Please input your password.")
password = input()
if password == "Halo":
  print("Welcome")
else:
  print("Your password is incorrect")

Presentation Overview

If statements

11:49 | 29-04-2024

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