If Statements

An if statement lets your program make decisions. It runs a block of code only when a condition is true.

age = 18

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

If-Else

Add an else block to handle what happens when the condition is false:

temperature = 35

if temperature > 30:
    print("It's hot outside!")
else:
    print("The weather is nice.")

If-Elif-Else

Use elif (short for "else if") to check multiple conditions:

score = 85

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(f"Your grade: {grade}")    # Your grade: B

Python checks each condition from top to bottom and runs the first one that's true. If none match, the else block runs.

Comparison and Logical Operators

Use these operators in your conditions:

# Comparison operators
x == y     # Equal to
x != y     # Not equal to
x > y      # Greater than
x < y      # Less than
x >= y     # Greater than or equal
x <= y     # Less than or equal

# Logical operators (combine conditions)
x > 0 and x < 100     # Both must be true
x < 0 or x > 100      # At least one must be true
not (x > 0)            # Inverts the result

Example with logical operators:

age = 25
has_license = True

if age >= 18 and has_license:
    print("You can drive!")
else:
    print("You cannot drive.")

For Loops

A for loop repeats code for each item in a sequence:

# Loop through a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(f"I like {fruit}")

# Output:
# I like apple
# I like banana
# I like cherry

The range() Function

Use range() to loop a specific number of times:

# Count from 0 to 4
for i in range(5):
    print(i)        # 0, 1, 2, 3, 4

# Count from 1 to 5
for i in range(1, 6):
    print(i)        # 1, 2, 3, 4, 5

# Count by 2s
for i in range(0, 10, 2):
    print(i)        # 0, 2, 4, 6, 8

Looping Through Strings

for char in "Python":
    print(char)     # P, y, t, h, o, n

While Loops

A while loop repeats code as long as a condition remains true:

count = 0
while count < 5:
    print(f"Count: {count}")
    count += 1          # Don't forget to update!

# Output: Count: 0, Count: 1, ... Count: 4

Practical While Loop Example

# Simple password checker
password = ""
while password != "secret123":
    password = input("Enter password: ")
    if password != "secret123":
        print("Wrong password, try again.")

print("Access granted!")

Break and Continue

break exits the loop immediately. continue skips to the next iteration:

# break example: stop at the first negative number
numbers = [10, 25, -3, 8, 15]
for num in numbers:
    if num < 0:
        print("Negative number found! Stopping.")
        break
    print(num)
# Output: 10, 25, Negative number found! Stopping.

# continue example: skip even numbers
for i in range(10):
    if i % 2 == 0:
        continue
    print(i)
# Output: 1, 3, 5, 7, 9

Nested Loops

You can put loops inside loops:

# Multiplication table
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} x {j} = {i * j}")
    print("---")

# Output:
# 1 x 1 = 1
# 1 x 2 = 2
# 1 x 3 = 3
# ---
# 2 x 1 = 2
# ...

Summary

• if/elif/else let your program make decisions based on conditions
• for loops iterate over sequences (lists, strings, ranges)
• while loops repeat while a condition is true
• break exits a loop; continue skips to the next iteration
• Indentation (4 spaces) defines code blocks in Python
• Logical operators (and, or, not) combine conditions