Python Loops and Conditionals

Control flow is essential in programming. Python provides powerful tools for making decisions and repeating actions. This tutorial covers conditional statements (if/elif/else) and loops (for/while), which let your programs make choices and handle repetitive tasks efficiently.

Conditional Statements

Conditional statements allow your program to make decisions based on conditions. The most common is the if statement.

Basic if Statement

age = 18

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

print("This always runs")  # Not indented, runs regardless

if-else Statement

temperature = 25

if temperature > 30:
    print("It's hot outside")
else:
    print("It's not too hot")

# With user input
name = input("Enter your name: ")
if name:
    print(f"Hello, {name}!")
else:
    print("You didn't enter a name")

if-elif-else Statement

Use elif (else if) for 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 is: {grade}")

Nested Conditionals

You can nest if statements inside each other.

age = 25
has_license = True

if age >= 18:
    if has_license:
        print("You can drive")
    else:
        print("You need a license to drive")
else:
    print("You're too young to drive")

Logical Operators

Combine conditions with and, or, and not.

age = 20
is_student = True

# and: both conditions must be true
if age >= 18 and is_student:
    print("You're an adult student")

# or: at least one condition must be true
if age < 18 or age > 65:
    print("You might qualify for a discount")

# not: reverses the condition
if not is_student:
    print("You're not a student")

Ternary Operator

A compact way to write simple if-else statements.

age = 20
status = "adult" if age >= 18 else "minor"
print(status)  # "adult"

# Multiple conditions
temperature = 25
weather = "hot" if temperature > 30 else "cold" if temperature < 10 else "moderate"
print(weather)  # "moderate"

Loops

Loops allow you to repeat code multiple times. Python has two main types: for and while loops.

For Loops

For loops iterate over sequences like lists, strings, or ranges.

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

# Loop through a string
for letter in "Python":
    print(letter)

# Loop through a range
for number in range(5):  # 0 to 4
    print(number)

for number in range(1, 6):  # 1 to 5
    print(number)

for number in range(0, 10, 2):  # Even numbers: 0, 2, 4, 6, 8
    print(number)

While Loops

While loops continue as long as a condition is true.

count = 1

while count <= 5:
    print(f"Count: {count}")
    count += 1

# Infinite loop with break
while True:
    user_input = input("Enter 'quit' to exit: ")
    if user_input == 'quit':
        break
    print(f"You entered: {user_input}")

Loop Control Statements

break: Exit the loop immediately
continue: Skip the rest of the current iteration and move to the next

# break example
for number in range(10):
    if number == 5:
        break
    print(number)  # Prints 0, 1, 2, 3, 4

# continue example
for number in range(5):
    if number == 2:
        continue
    print(number)  # Prints 0, 1, 3, 4 (skips 2)

Nested Loops

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()  # Empty line between rows

List Comprehensions

A compact way to create lists using loops.

# Traditional way
squares = []
for x in range(5):
    squares.append(x**2)

# List comprehension
squares = [x**2 for x in range(5)]
print(squares)  # [0, 1, 4, 9, 16]

# With condition
even_squares = [x**2 for x in range(10) if x % 2 == 0]
print(even_squares)  # [0, 4, 16, 36, 64]

# Nested list comprehensions
matrix = [[i*j for j in range(3)] for i in range(3)]
print(matrix)  # [[0, 0, 0], [0, 1, 2], [0, 2, 4]]

Practical Examples

Finding Maximum Value

numbers = [3, 7, 2, 9, 5, 1]
maximum = numbers[0]

for number in numbers:
    if number > maximum:
        maximum = number

print(f"The maximum value is: {maximum}")

Simple Calculator

while True:
    print("1. Add")
    print("2. Subtract")
    print("3. Multiply")
    print("4. Divide")
    print("5. Quit")
    
    choice = input("Choose an operation (1-5): ")
    
    if choice == '5':
        break
    
    if choice in ['1', '2', '3', '4']:
        num1 = float(input("Enter first number: "))
        num2 = float(input("Enter second number: "))
        
        if choice == '1':
            result = num1 + num2
            print(f"{num1} + {num2} = {result}")
        elif choice == '2':
            result = num1 - num2
            print(f"{num1} - {num2} = {result}")
        elif choice == '3':
            result = num1 * num2
            print(f"{num1} * {num2} = {result}")
        elif choice == '4':
            if num2 != 0:
                result = num1 / num2
                print(f"{num1} / {num2} = {result}")
            else:
                print("Cannot divide by zero")
    else:
        print("Invalid choice. Please try again.")
    
    print()

Password Validation

def validate_password(password):
    has_upper = False
    has_lower = False
    has_digit = False
    
    for char in password:
        if char.isupper():
            has_upper = True
        elif char.islower():
            has_lower = True
        elif char.isdigit():
            has_digit = True
    
    return len(password) >= 8 and has_upper and has_lower and has_digit

password = input("Enter a password: ")
if validate_password(password):
    print("Password is valid")
else:
    print("Password must be at least 8 characters with upper, lower, and digits")

Common Patterns

Iterating with Index

fruits = ["apple", "banana", "orange"]

# Using enumerate
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

# Manual index tracking
for i in range(len(fruits)):
    print(f"{i}: {fruits[i]}")

Looping Over Dictionaries

person = {"name": "Alice", "age": 30, "city": "New York"}

for key in person:
    print(f"{key}: {person[key]}")

for key, value in person.items():
    print(f"{key}: {value}")

Finding Items in Lists

numbers = [1, 3, 5, 7, 9]
target = 5
found = False

for number in numbers:
    if number == target:
        found = True
        break

if found:
    print(f"{target} was found in the list")
else:
    print(f"{target} was not found in the list")

Best Practices

Use for loops when you know the number of iterations
Use while loops for indefinite repetition
Avoid infinite loops by ensuring exit conditions
Use meaningful variable names in loops
Consider list comprehensions for simple transformations
Use enumerate() when you need both index and value
Break complex nested loops into functions

Performance Considerations

For loops are generally faster than while loops for iterating over known sequences
List comprehensions are more efficient than traditional loops for creating lists
Use break and continue sparingly to keep code readable
Avoid modifying lists while iterating over them

Summary

Control flow statements are fundamental to programming:

Statement	Purpose	Example
`if`	Execute code if condition is true	`if x > 0:`
`elif`	Check alternative condition	`elif x < 0:`
`else`	Execute if no conditions match	`else:`
`for`	Iterate over sequence	`for item in items:`
`while`	Repeat while condition is true	`while x < 10:`
`break`	Exit loop immediately	`break`
`continue`	Skip to next iteration	`continue`

Mastering these concepts will allow you to write more complex and useful Python programs.

External Resources:

Related Tutorials:

Learn about Python variables and data types here to understand the values you can use in conditions.
Check out Python functions here to see how to organize your conditional and looping code.

Last updated on December 30, 2025

Python Variables and Data Types