Control Flow

Learn how to control the flow of your Python programs using conditional statements and decision-making logic

🔀 if, elif, else Statements

Control flow allows your program to make decisions and execute different code based on conditions.

Think of it like this: Just like you make decisions in daily life ("If it's raining, take an umbrella"), Python can make decisions based on conditions.

Basic if Statement

age = 18

if age >= 18:
    print("You are an adult")
    print("You can vote!")

# Expected output:
# You are an adult
# You can vote!

if-else Statement

age = 16

if age >= 18:
    print("You are an adult")
else:
    print("You are a minor")
    print("Wait a few more years!")

# Expected output:
# You are a minor
# Wait a few more years!

if-elif-else Statement

score = 85

if score >= 90:
    print("Grade: A")
    print("Excellent work!")
elif score >= 80:
    print("Grade: B")
    print("Good job!")
elif score >= 70:
    print("Grade: C")
    print("Keep improving!")
else:
    print("Grade: F")
    print("Study harder next time!")

# Expected output:
# Grade: B
# Good job!

elif score >= 80: print("Grade: B") elif score >= 70: print("Grade: C") elif score >= 60: print("Grade: D") else: print("Grade: F")

## 🔢 **Comparison Operators**

```python
a = 10
b = 20

print(a == b)   # False (equal to)
print(a != b)   # True (not equal to)
print(a < b)    # True (less than)
print(a > b)    # False (greater than)
print(a <= b)   # True (less than or equal to)
print(a >= b)   # False (greater than or equal to)

🔗 Logical Operators

age = 25
has_license = True

# AND operator
if age >= 18 and has_license:
    print("Can drive")

# OR operator
if age < 18 or not has_license:
    print("Cannot drive")

# NOT operator
if not has_license:
    print("Need to get a license")

✅ Truthy and Falsy Values

Falsy Values

# These evaluate to False
print(bool(0))          # False
print(bool(""))         # False (empty string)
print(bool([]))         # False (empty list)
print(bool({}))         # False (empty dict)
print(bool(None))       # False

Truthy Values

# These evaluate to True
print(bool(1))          # True
print(bool("hello"))    # True (non-empty string)
print(bool([1, 2, 3]))  # True (non-empty list)
print(bool(-1))         # True (non-zero number)

🎯 Nested Conditions

weather = "sunny"
temperature = 75

if weather == "sunny":
    if temperature > 70:
        print("Perfect day for a picnic!")
    else:
        print("Sunny but a bit cold")
else:
    print("Not a sunny day")

🚀 Practice Examples

Example 1: Age Checker

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

if age < 0:
    print("Invalid age")
elif age < 13:
    print("You are a child")
elif age < 20:
    print("You are a teenager")
elif age < 60:
    print("You are an adult")
else:
    print("You are a senior")

Example 2: Login System

username = "admin"
password = "secret"

user_input = input("Enter username: ")
pass_input = input("Enter password: ")

if user_input == username and pass_input == password:
    print("Login successful!")
else:
    print("Invalid credentials")

Example 3: Number Classifier

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

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

# Check if even or odd
if number % 2 == 0:
    print("Even number")
else:
    print("Odd number")

🎯 Key Takeaways

• Use if for single conditions
• Use elif for multiple conditions
• Use else for default case
• Combine conditions with and, or, not
• Empty values are generally falsy
• Non-empty values are generally truthy
• Proper indentation is crucial

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