Decision Making

Programs often need to choose what to do based on a condition. The if statement runs a block only when its condition is True.

if / elif / else Syntax

if condition1:
    # runs if condition1 is True
elif condition2:
    # runs if condition1 False and condition2 True
else:
    # runs if all above are False

• Each condition ends with a colon :.
• The block under it is indented (4 spaces).
• elif = "else if"; you can have many. else is optional.

Program 1: Pass or Fail

marks = int(input("Enter marks: "))
if marks >= 33:
    print("Result: PASS")
else:
    print("Result: FAIL")

If marks is 33 or more, the first block runs; otherwise the else block runs.

Program 2: Largest of Three Numbers

a = int(input("a: "))
b = int(input("b: "))
c = int(input("c: "))

if a >= b and a >= c:
    print("Largest is", a)
elif b >= c:
    print("Largest is", b)
else:
    print("Largest is", c)

• First check if a beats both others.
• If not, elif b >= c decides between b and c.
• Otherwise c is largest.

Program 3: Grade Calculator

pct = float(input("Enter percentage: "))

if pct >= 90:
    grade = "A+"
elif pct >= 75:
    grade = "A"
elif pct >= 60:
    grade = "B"
elif pct >= 33:
    grade = "C"
else:
    grade = "Fail"

print("Grade:", grade)

Python checks each elif top to bottom and stops at the first true one — so order matters (highest first).

Program 4: Positive, Negative or Zero

n = int(input("Enter a number: "))
if n > 0:
    print("Positive")
elif n < 0:
    print("Negative")
else:
    print("Zero")

Nested if

An if can sit inside another if for two-level decisions.

age = int(input("Age: "))
if age >= 18:
    has_id = input("Have ID? (yes/no): ")
    if has_id == "yes":
        print("Allowed to vote")
    else:
        print("Bring your ID")
else:
    print("Too young to vote")

Common Mistakes

• Using = instead of == in the condition.
• Forgetting the colon :.
• Wrong indentation inside the block.
• Putting elif conditions in the wrong order.

Practice Tasks

1. Check whether a number is even or odd.
2. Take a year and check if it is a leap year.
3. Ask a person's age and print child / teen / adult / senior.
4. Take three sides and check if they can form a triangle.

Summary

• if runs a block when its condition is True.
• elif adds more choices; else is the fallback.
• Conditions need a colon and an indented block.
• elif order matters — check from highest/most-specific first.