Python if-else Statement (CBSE Class 11 Computer Science)

In many situations, a program must choose between two alternative actions. For example, if a student's marks are 33 or above, the result is Pass; otherwise, the result is Fail. Python provides the if-else statement to handle such situations.

Learning Objectives

• Understand the if-else statement.
• Learn its syntax and working.
• Understand flowcharts and algorithms.
• Trace program execution.
• Write decision-making programs using if-else.

Quick Syntax Reference

if condition:
    statement(s)
else:
    statement(s)

What is the if-else Statement?

The if-else statement checks a condition. If the condition is True, the statements inside the if block are executed. Otherwise, the statements inside the else block are executed.

Flowchart

          Start
            │
            ▼
     Check Condition
            │
      ┌─────┴─────┐
      │           │
    True        False
      │           │
      ▼           ▼
 Execute IF    Execute ELSE
    Block          Block
      │           │
      └─────┬─────┘
            ▼
           End

Algorithm

1. Start.
2. Read the condition.
3. If the condition is True, execute the if block.
4. Otherwise, execute the else block.
5. Stop.

Pseudocode

IF condition THEN

    Execute IF block

ELSE

    Execute ELSE block

END IF

Example 1: Pass or Fail

marks = 45

if marks >= 33:
    print("Pass")
else:
    print("Fail")

Output

Pass

Dry Run Table

Trace Table

Python IDLE Editor Snapshot

>>> marks = 45

>>> if marks >= 33:
...     print("Pass")
... else:
...     print("Fail")

Pass

Example 2: Even or Odd

num = 17

if num % 2 == 0:
    print("Even Number")
else:
    print("Odd Number")

Output

Odd Number

Flow of Execution

Condition True
      │
      ▼
 Execute IF Block
      │
      ▼
 Continue Program

Condition False
      │
      ▼
Execute ELSE Block
      │
      ▼
 Continue Program

Think Like a Programmer

Predict the output.

age = 15

if age >= 18:
    print("Adult")
else:
    print("Minor")

print("Completed")

Minor
Completed

Output Prediction Challenge

x = 12

if x % 3 == 0:
    print("Divisible")
else:
    print("Not Divisible")

Divisible

Debugging Exercise

Find and correct the errors.

marks = 50

if marks >= 33
    print("Pass")
else
    print("Fail")

Errors:

• Missing colon (:) after if and else.
• Incorrect indentation.

Competency-Based Question

A school charges a library fine only if a book is returned after the due date. Write a Python program that displays "Fine Applicable" if the number of delayed days is greater than 0; otherwise, display "No Fine".

Real-Life Examples

• If attendance ≥ 75%, allow the student to appear for the examination; otherwise, do not allow.
• If the password is correct, log in; otherwise, display an error message.
• If the balance is sufficient, complete the withdrawal; otherwise, show "Insufficient Balance".

Common Beginner Mistakes

• Forgetting the colon after if or else.
• Incorrect indentation.
• Using = instead of == in conditions.
• Writing multiple else blocks for one if.

Visual Memory Box

Memory Trick

Exam Tips

• Always use a colon after if and else.
• Maintain proper indentation.
• Practice both True and False cases while tracing programs.
• Remember that only one block executes.

CBSE Important Questions

2 Marks

1. What is the purpose of the if-else statement?
2. Differentiate between if and if-else.

3 Marks

1. Write a program to check whether a number is even or odd using if-else.
2. Draw the flowchart of an if-else statement.

5 Marks

1. Write a Python program to check whether a student has passed or failed and explain the execution using a dry run and trace table.

Practice Yourself

1. Check whether a person is eligible for voting.
2. Check whether a number is positive or negative.
3. Check whether a student has passed or failed.
4. Write a program to compare two numbers and display the greater one.
5. Trace the execution of the above programs.

Frequently Asked Questions (FAQs)

1. Can both the if and else blocks execute?

No. Only one block executes based on the condition.

2. Is the else block mandatory?

No. It is optional and used only when an alternative action is required.

3. Does the else statement have a condition?

No. The else block executes automatically when the if condition is false.

4. Why is indentation important?

Indentation defines which statements belong to the if or else block.

5. Can an if-else statement be nested?

Yes. An if-else statement can be placed inside another conditional statement.

Summary

• The if-else statement is used to choose between two alternative actions.
• If the condition is true, the if block executes; otherwise, the else block executes.
• Only one block executes during program execution.
• Proper indentation and colon usage are mandatory.
• The if-else statement is widely used in decision-making programs.