Conditional Statements

Conditional statements let your program make decisions — execute one block of code or another based on whether a condition is True or False. This is where Python starts thinking!

1. The if Statement

Executes a code block only if the condition is True. If the condition is False, the block is skipped entirely.

if condition:
    code block    # runs only when condition is True

marks = 80

if marks >= 50:
    print("You passed!")

print("Program continues...")   # always runs

From your file (control-flow.py), notice the difference between these two code sets:

# Code set 1 — if without else:
if False:
    print("Statement 1")   # skipped

print("Statement 2")       # always runs — outside if block

# Code set 2 — if with else:
if False:
    print("Statement 1")   # skipped
else:
    print("Statement 2")   # runs because if was False

2. The if-else Statement

Runs one block when the condition is True, and a different block when it's False. One of them always runs.

if condition:
    code block    # runs if True
else:
    code block    # runs if False

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

if age >= 18:
    print("You are an adult.")
else:
    print("You are a minor.")

3. The if-elif-else Statement

Checks multiple conditions in sequence. The first condition that is True wins — its block runs, and the rest are skipped. The else at the end is the fallback when no condition is True.

if condition_1:
    code block
elif condition_2:
    code block
else:
    code block

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

if marks >= 90:
    print("Grade: A")
elif marks >= 75:
    print("Grade: B")
elif marks >= 60:
    print("Grade: C")
elif marks >= 50:
    print("Grade: D")
else:
    print("Grade: F — Failed")

From your file example:

if True:
    print("Statement 1")   # runs — first True condition
elif True:
    print("Statement 2")   # skipped — first block already ran
else:
    print("Statement 3")   # skipped

4. Nested Conditions

A condition inside another condition. Use this when you need to check a second condition only after the first one passes.

if condition_1:
    if condition_2:       # only checked if condition_1 is True
        code block
    else:
        code block
else:
    code block

# From control-flow.py teacher example:
if True:
    if True:
        print("Statement 1")   # runs
    else:
        print("Statement 2")
else:
    print("Statement 3")

Real-world example:

age = int(input("Enter age: "))
has_ticket = input("Have a ticket? (yes/no): ")

if age >= 18:
    if has_ticket == "yes":
        print("Welcome! Enjoy the event.")
    else:
        print("Adult, but no ticket.")
else:
    print("You must be 18+ to enter.")

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

if marks >= 50:
    print("Pass")
else:
    print("Fail")

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

if num > 0:
    print("Positive")
elif num < 0:
    print("Negative")
else:
    print("Zero")

marks = int(input("Enter marks (0-100): "))

if marks >= 90:
    print("Grade: A")
elif marks >= 75:
    print("Grade: B")
elif marks >= 60:
    print("Grade: C")
elif marks >= 50:
    print("Grade: D")
else:
    print("Grade: F")

age = int(input("Enter your age: "))
has_voter_id = input("Do you have a voter ID? (yes/no): ")

if age >= 18:
    if has_voter_id == "yes":
        print("You can vote!")
    else:
        print("You need a voter ID to vote.")
else:
    print("You must be 18+ to vote.")

x = 15

if x > 20:
    print("A")
elif x > 10:
    print("B")
elif x > 5:
    print("C")
else:
    print("D")

# Output: B
# Why: x=15 → 15>20? No. → 15>10? YES → print "B" → stop.
# Even though 15>5 is also True, elif stops at first match.