Decision Making in Python | if, if-else, if-elif-else and Nested if

In real life, we make decisions every day based on different situations. For example, if it is raining, we carry an umbrella. If we score more than 33 marks, we pass the examination; otherwise, we fail. Similarly, a computer program also needs to make decisions based on certain conditions. Python provides decision-making statements that allow a program to execute different blocks of code depending on whether a condition is True or False.

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Learning Outcomes

After studying this chapter, you will be able to:

• Understand the need for decision making in Python.
• Explain the flow of control in a program.
• Use the if statement.
• Use the if-else statement.
• Understand conditions using relational and logical operators.
• Develop simple Python programs using decision-making statements.

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What is Decision Making?

In Python, decision-making statements evaluate a condition. If the condition is True, one block of statements is executed. If it is False, another block (if available) is executed.

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Why is Decision Making Important?

• Helps programs take intelligent decisions.
• Allows different actions for different situations.
• Makes programs interactive.
• Avoids unnecessary execution of statements.
• Forms the basis of real-world applications such as login systems, ATM software, online shopping websites, and examination systems.

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Real-Life Examples

Condition	Decision
If it rains	Carry an umbrella.
If marks are greater than or equal to 33	Student passes.
If age is 18 or above	Eligible to vote.
If password is correct	Allow login.
If account balance is sufficient	Allow withdrawal.

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Flow of Control

The order in which statements are executed in a program is called the flow of control.

Normally, Python executes statements one after another from top to bottom. Decision-making statements change this normal flow by executing code only when a specified condition is satisfied.

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Conditions in Python

A condition is an expression that evaluates to either True or False.

Examples

10 > 5
25 == 20
15 != 8
100 >= 50

Condition	Result
10 > 5	True
8 < 2	False
20 == 20	True
15 != 15	False

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Relational Operators Used in Conditions

Operator	Meaning	Example
==	Equal to	a == b
!=	Not equal to	a != b
>	Greater than	a > b
<	Less than	a < b
>=	Greater than or equal to	a >= b
<=	Less than or equal to	a <= b

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Logical Operators Used in Conditions

Operator	Purpose
and	Returns True if both conditions are True.
or	Returns True if at least one condition is True.
not	Reverses the result of a condition.

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The if Statement

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Syntax of if Statement

if condition:
    statement(s)

Important: The block of statements must be properly indented.

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Flowchart of if Statement

        Condition
            |
      +-----+------+
      |            |
    True        False
      |
 Execute Block
      |
    Next Statement

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Example 1: Positive Number

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

if num > 0:
    print("Positive Number")

Sample Output

Enter a number: 15
Positive Number

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Example 2: Eligibility to Vote

age = int(input("Enter your age: "))

if age >= 18:
    print("Eligible to Vote")

Sample Output

Enter your age: 21
Eligible to Vote

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Important Note

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The if-else Statement

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Syntax of if-else Statement

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

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Flowchart of if-else Statement

         Condition
             |
      +------+------+
      |             |
    True         False
      |             |
 Execute IF    Execute ELSE
      |             |
      +------+------+
             |
      Next Statement

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Example 3: Pass or Fail

marks = int(input("Enter marks: "))

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

Sample Output

Enter marks: 58
Pass

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Example 4: Even or Odd Number

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

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

Sample Output

Enter a number: 17
Odd Number

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Real-Life Application

An ATM machine checks whether the account balance is sufficient before allowing a cash withdrawal. If the balance is available, the transaction is processed; otherwise, an "Insufficient Balance" message is displayed. This is a practical example of the if-else statement.

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Common Mistakes

Mistake	Correct Practice
Forgetting the colon (:) after if or else.	Always end the condition with a colon.
Incorrect indentation.	Indent all statements inside the block consistently.
Using = instead of ==.	Use == for comparison.
Writing Else instead of else.	Python keywords are case-sensitive.

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Continue with Part 2 → if-elif-else, Nested if, comparison table, solved programs, interview questions, quick revision, CBSE exam tips, and summary.