Decision Making With If-Else

Decision-making is the heart of any program. The if, elif and else keywords let your script choose between different paths depending on conditions — checking ages, comparing scores, validating data or branching workflows. Master these and your programs stop being linear and start being smart.

• Conditions are any expression that evaluates to True or False.
• The colon : marks the end of a header line; the indented block below is the body.
• elif stands for else if and can be chained as many times as needed.
• Indentation is part of the syntax — use 4 spaces consistently.

if condition:
    # runs when condition is True
elif another_condition:
    # runs when the first is False but this one is True
else:
    # runs when none of the above is True

• Boolean conditions: Any expression Python can interpret as truthy or falsy works. 0, 0.0, None, empty strings/lists/dicts are falsy; everything else is truthy.
• if statement: Use if when you have a single check and no alternative branch. The body runs only when the condition is True.
• if-else statement: else handles the opposite case. Exactly one of the two branches will always run.
• if-elif-else ladder: When you have more than two possibilities (grade A/B/C/D/F, traffic-light states, menu choices), chain elif blocks. The first true condition wins; the rest are skipped.
• Nested if: Place an if inside another if to build multi-stage checks (e.g. login succeeded → user is admin → show admin panel). Keep nesting shallow for readability.
• Ternary operator: Python supports a one-line conditional: value = a if condition else b. Great for assignments but avoid nesting ternaries.

n = int(input("Number: "))
if n % 2 == 0:
    print(n, "is even")
else:
    print(n, "is odd")

a, b, c = 10, 25, 18
if a >= b and a >= c:
    print("Largest:", a)
elif b >= a and b >= c:
    print("Largest:", b)
else:
    print("Largest:", c)

marks = 76
if marks >= 90:
    grade = "A"
elif marks >= 75:
    grade = "B"
elif marks >= 60:
    grade = "C"
elif marks >= 40:
    grade = "D"
else:
    grade = "F"
print("Grade:", grade)

y = 2024
if y % 4 == 0:
    if y % 100 == 0:
        if y % 400 == 0:
            print("Leap year")
        else:
            print("Not a leap year")
    else:
        print("Leap year")
else:
    print("Not a leap year")

age = 16
price = 100 if age >= 18 else 50
print("Ticket price:", price)

pwd = "Hello@123"
if len(pwd) < 8:
    print("Too short")
elif pwd.isalpha() or pwd.isdigit():
    print("Weak password")
else:
    print("Strong password")

• Indent every block exactly 4 spaces — mixing tabs and spaces raises IndentationError.
• Use == for equality comparison, not = (which is assignment).
• Combine conditions with and, or, not — Python short-circuits them.
• Always include an else branch when results must cover all cases.
• Prefer elif over deeply nested ifs — it reads top-to-bottom like English.
• Use parentheses to clarify complex boolean expressions even when not strictly required.

• Using = instead of ==: if x = 5 is a syntax error in conditions.
• Forgetting the colon: if x > 0 without : raises SyntaxError.
• Wrong indentation: Python uses indentation to mark blocks; misaligned lines change the meaning.
• Comparing strings as numbers: "10" > "9" is False (lexicographic compare).
• Mutable default-style logic: writing if list: works, but if list == []: is clearer for empties.
• Deep nesting: beyond 2–3 levels, refactor into functions or use elif ladders.

• Problem 1: Read a number and print whether it is positive, negative or zero.
• Problem 2: Accept three sides and decide if they form an equilateral, isosceles or scalene triangle.
• Problem 3: Take a year and determine if it is a leap year using a single boolean expression.
• Problem 4: Build a simple calculator: ask two numbers and an operator (+, -, *, /) and print the result.
• Problem 5: Read marks for five subjects, compute the percentage, and print the final grade.
• Problem 6: Ask the user for a character and determine if it is a vowel, consonant, digit, or special symbol.

• Q1. Explain the difference between if, if-else and if-elif-else.
• Q2. Does Python support switch-case? How do you achieve similar behaviour?
• Q3. What is short-circuit evaluation in Python's and / or operators?
• Q4. Write a ternary expression to assign "adult" or "minor" based on age.
• Q5. How does Python decide truthiness for objects like empty lists or zero?
• Q6. What happens if two conditions in an if-elif chain are both true at the same time?

• Q1: Does Python have a switch statement?
  Classic Python did not have one. Python 3.10 introduced match-case, but for most decision logic an if-elif-else chain is idiomatic.
• Q2: Can I omit the else block?
  Yes — else and elif are both optional. Use them only when you need alternative paths.
• Q3: What is the ternary operator in Python?
  It is a one-line conditional: result = a if condition else b. Useful for compact assignments.
• Q4: How many elif blocks can I have?
  There is no hard limit, but if you have more than 4–5 elifs consider a dictionary lookup or polymorphism.
• Q5: Why is indentation so strict in Python?
  Python uses indentation as the block syntax — there are no braces. Consistent indentation keeps code readable and parsing unambiguous.
• Q6: Can I nest if statements?
  Yes, but limit nesting to 2–3 levels. Beyond that, refactor with elif or helper functions for clarity.