Conditionals in Python

Conditionals allow us to make decisions in code.
We can run different blocks of code depending on whether a condition is True or False.

Why use conditionals?

• Conditionals allow the program to respond differently based on input or situation.
• They are essential for decision-making, validation, and branching logic.
• Widely used in real-world applications where outcomes depend on conditions.

Examples of real-time usage:

• Checking if a user is logged in or not before granting access
• Determining grades based on marks
• Processing different payment methods in an e-commerce app
• Running a program only if certain conditions are met
• Validating form inputs or user commands

We will cover:

1. if statement
2. if-else statement
3. if-elif-else ladder
4. Nested if
5. Shorthand if expressions (Ternary operator)
6. Combining conditions with and / or

Example: if statement

# ---- Example: if statement ----
# We use "if" to check a condition.
# If the condition is True, the indented block of code runs.

x = 10

if x > 0:   # condition: is x greater than 0?
    print("x is positive")   # this runs because 10 > 0

x is positive

Example: if-else

# ---- Example: if-else ----
# Use "if-else" when we want two possible outcomes:
# - if condition is True → run the 'if' block
# - if condition is False → run the 'else' block

num = -5

if num >= 0:   # check if number is non-negative
    print("Number is non-negative")
else:
    print("Number is negative")

Number is negative

Example: if-elif-else

# ---- Example: if-elif-else ----
# Use "if-elif-else" when we want to check multiple conditions in sequence.
# Only the FIRST matching condition will run.

score = 85

if score >= 90:
    print("Grade: A")   # runs if score is 90 or above
elif score >= 75:
    print("Grade: B")   # runs if score is between 75 and 89
elif score >= 50:
    print("Grade: C")   # runs if score is between 50 and 74
else:
    print("Grade: Fail")  # runs if score is below 50

Grade: B

Example: nested if

# ---- Example: nested if ----
# A "nested if" means one if-statement inside another.
# Useful when we want to check multiple levels of conditions.

number = 15

if number > 0:                  # check if positive
    print("Number is positive")
    if number % 2 == 0:         # inside positive check, we check even/odd
        print("It is even")
    else:
        print("It is odd")
else:
    print("Number is negative or zero")

Number is positive
It is odd

Example: shorthand if (ternary operator)

# ---- Example: shorthand if (ternary operator) ----
# Use this when we want to assign a value based on a condition in ONE line.

age = 20

status = "Adult" if age >= 18 else "Minor"
# if age >= 18 → "Adult", otherwise "Minor"

print("Status:", status)

Status: Adult

Example: combining conditions

# ---- Example: combining conditions ----
# Use logical operators:
# - "and" → both conditions must be True
# - "or"  → at least one condition must be True

marks = 70

if marks >= 50 and marks <= 100:   # both conditions must hold
    print("Pass")
else:
    print("Fail")

temperature = 35
if temperature < 0 or temperature > 30:   # either condition can hold
    print("Extreme weather")

Pass
Extreme weather

Summary

We learned about conditional statements:

• if → check a single condition
• if-else → two possible outcomes
• if-elif-else → multiple conditions, only first match runs
• Nested if → one condition inside another
• Shorthand if (ternary operator) → compact one-line condition
• and / or → combine multiple conditions

Next notebook: 05_loops.ipynb