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Python if...else Statement

Video link: https://youtu.be/497MClrekMY

In this video, we learned about decision making in Python using the if...else statement.

Programs in the Video

Python if statement

In Python, we use the if statement to create programs that can make decisions. The syntax of the if statement is:

if test_condition:
    statement(s)

If test_condition is True, the body of the if statement is executed.

However, if test_condition is False, the body of the if statement is skipped from execution.

Example: Check if student passed the exam

Suppose you are a university student and to pass the examination you need to score 50 or more. Lets look at a program to check if you passed the exam.

score = int(input("Enter a number: "))

if score >= 50:
   print("You have passed your exams.")
   print("Congratulations!")

Output

For a number greater than or equal to 50:

Enter a number: 75
You have passed your exams.
Congratulations!

Otherwise (number smaller than 50):

Enter a number: 35

As we can see, there is no output as the if block is skipped. We can add one more if statement to handle this as:

score = 35

if score >= 50:
   print("You have passed your exam.")
   print("Congratulations!")

if score < 50:
   print("Sorry, you have failed your exam.")

Output

Sorry, you have failed your exam.

Python if...else statement

The if statement can have an optional else clause to run when test_condition is False. The syntax of if..else is:

if test_condition:
    statement(s)
else:
    statement(s)

Now, we can simply write the above program as:

score = 35

if score >= 50:
   print("You have passed your exam.")
   print("Congratulations!")
else:
   print("Sorry, you have failed your exam.")

Python if...elif...else statement

The if statement can also have multiple elif clauses to handle more than two test cases. The syntax of if...elif...else is:

if test_condition1:
    statements_1
elif test_condition2:
    statement_2
...
else:
    statements_3

The marks scored by a student is from 0 to 100. We can add the elif to above program to validate the marks entered as:

score = 105

if score > 100 or score < 0:
   print("Score is invalid.")
elif score >= 50:
   print("You have passed your exam.")
   print("Congratulations!")
else:
   print("Sorry, you have failed your exam.")

Output

Score is invalid.

Programming Task

Can you create a program to check whether a number is positive or negative or 0?

To create this program, create a variable named number and assign a float value to it based on the user input. Then using a if statement, check if the number variable is positive or negative or 0.

  • If number is positive, print "The number is positive"
  • If number is 0, print "The number is 0"
  • If number is negative, print "The number is negative"
# Solution 1

number = float(input("Enter a number: "))

if number > 0:
    print("The number is positive")
elif number == 0:
    print("The number is 0")
else:
    print("The number is negative")
# Solution 2

number = float(input("Enter a number: "))

if number > 0:
    print("The number is positive")
elif number < 0:
    print("The number is negative")
else:
    print("The number is 0")
# Solution 3: Using Nested if else

number = float(input("Enter a number: "))

if number >= 0:
    # At this point, number is either 0 or a positive number
    if number > 0:
        print("The number is positive")
    else:
        print("The number is 0")
else:
    print("The number is negative")