Python Loops

Introduction

Loops are one of the fundamental concepts in programming that allow you to execute a block of code repeatedly. In Python, loops provide an efficient way to automate repetitive tasks, process collections of data, and implement algorithms that require iteration.

Without loops, you would need to write the same code multiple times, making your programs longer, harder to maintain, and less efficient. Python provides two main types of loops: for loops and while loops, each serving different purposes in your code.

In this tutorial, you'll learn:

• How for and while loops work
• When to use each type of loop
• How to control loop execution with break, continue, and else
• Common loop patterns and best practices

For Loops

A for loop in Python is used to iterate over a sequence (such as a list, tuple, dictionary, string, or range) and execute a block of code for each item in the sequence.

Basic Syntax

python

for item in sequence:
    # Code to execute for each item

Simple For Loop Example

Let's start with a basic example that prints each number in a list:

python

numbers = [1, 2, 3, 4, 5]
for num in numbers:
    print(num)

Output:

1
2
3
4
5

Looping Through a Range

The range() function is commonly used with for loops to execute a block of code a specific number of times:

python

for i in range(5):  # range(5) generates numbers from 0 to 4
    print(f"Iteration {i}")

Output:

Iteration 0
Iteration 1
Iteration 2
Iteration 3
Iteration 4

You can customize the range() function with start, stop, and step parameters:

python

# range(start, stop, step)
for i in range(2, 10, 2):  # Start at 2, stop before 10, step by 2
    print(i)

Output:

2
4
6
8

Looping Through Strings

Strings are sequences of characters, so you can loop through each character:

python

word = "Python"
for char in word:
    print(char)

Output:

P
y
t
h
o
n

Looping Through Dictionaries

When looping through dictionaries, the default behavior is to iterate through the keys:

python

student = {
    "name": "Alice",
    "age": 21,
    "major": "Computer Science"
}

# Iterating through keys
for key in student:
    print(key, ":", student[key])

# Alternatively, using items() method to get both key and value
print("\nUsing items() method:")
for key, value in student.items():
    print(key, ":", value)

Output:

name : Alice
age : 21
major : Computer Science

Using items() method:
name : Alice
age : 21
major : Computer Science

While Loops

A while loop continues to execute a block of code as long as a specified condition is True.

Basic Syntax

python

while condition:
    # Code to execute while condition is True

Simple While Loop Example

python

count = 1
while count <= 5:
    print(f"Count: {count}")
    count += 1  # Increment count to eventually end the loop

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

Infinite Loops and How to Avoid Them

A common mistake is creating an infinite loop by forgetting to update the condition variable:

python

# WARNING: This is an infinite loop (don't run this)
# while True:
#     print("This will run forever!")

To avoid infinite loops, always make sure that:

1. The condition will eventually become False
2. You have a way to break out of the loop

python

# Safe example with a break statement
counter = 0
while True:
    print(f"Counter: {counter}")
    counter += 1
    if counter >= 5:
        print("Breaking the loop")
        break

Output:

Counter: 0
Counter: 1
Counter: 2
Counter: 3
Counter: 4
Breaking the loop

Loop Control Statements

Python provides statements that allow you to control the flow of loops:

Break Statement

The break statement immediately terminates a loop and moves to the code after the loop:

python

for i in range(10):
    if i == 5:
        print("Found 5! Breaking loop...")
        break
    print(i)

Output:

0
1
2
3
4
Found 5! Breaking loop...

Continue Statement

The continue statement skips the current iteration and moves to the next iteration:

python

for i in range(10):
    if i % 2 == 0:  # Skip even numbers
        continue
    print(f"Odd number: {i}")

Output:

Odd number: 1
Odd number: 3
Odd number: 5
Odd number: 7
Odd number: 9

Else Clause in Loops

Python allows an else clause after loops that executes when the loop completes normally (without a break):

python

# Loop with break
print("Loop with break:")
for i in range(5):
    print(i)
    if i == 3:
        break
else:
    print("Loop completed normally")

# Loop without break
print("\nLoop without break:")
for i in range(5):
    print(i)
else:
    print("Loop completed normally")

Output:

Loop with break:
0
1
2
3

Loop without break:
0
1
2
3
4
Loop completed normally

Nested Loops

You can place one loop inside another to create more complex iteration patterns:

python

for i in range(1, 4):
    for j in range(1, 4):
        print(f"({i}, {j})", end=" ")
    print()  # New line after each inner loop completes

Output:

(1, 1) (1, 2) (1, 3) 
(2, 1) (2, 2) (2, 3) 
(3, 1) (3, 2) (3, 3) 

Practical Examples

Example 1: Calculating Factorial

python

def factorial(n):
    result = 1
    for i in range(1, n + 1):
        result *= i
    return result

number = 5
print(f"The factorial of {number} is {factorial(number)}")

Output:

The factorial of 5 is 120

Example 2: Finding Prime Numbers

python

def is_prime(num):
    if num <= 1:
        return False
    if num <= 3:
        return True
    
    # Check divisibility up to square root of num
    i = 2
    while i * i <= num:
        if num % i == 0:
            return False
        i += 1
    return True

# Print prime numbers between 1 and 20
print("Prime numbers between 1 and 20:")
for number in range(1, 21):
    if is_prime(number):
        print(number, end=" ")

Output:

Prime numbers between 1 and 20:
2 3 5 7 11 13 17 19 

Example 3: Processing Data with Loops

Let's analyze a list of student scores to calculate statistics:

python

scores = [85, 92, 78, 65, 97, 88, 72, 81, 89, 94]

# Calculate sum and average
total = 0
count = 0

for score in scores:
    total += score
    count += 1

average = total / count

# Find minimum and maximum scores
min_score = scores[0]
max_score = scores[0]

for score in scores:
    if score < min_score:
        min_score = score
    if score > max_score:
        max_score = score

print(f"Scores: {scores}")
print(f"Total students: {count}")
print(f"Total score: {total}")
print(f"Average score: {average:.2f}")
print(f"Minimum score: {min_score}")
print(f"Maximum score: {max_score}")

Output:

Scores: [85, 92, 78, 65, 97, 88, 72, 81, 89, 94]
Total students: 10
Total score: 841
Average score: 84.10
Minimum score: 65
Maximum score: 97

Loop Performance Considerations

When working with loops, consider these performance tips:

1. Avoid unnecessary work inside loops: Move operations that don't change with each iteration outside the loop.

2. Use list comprehensions for simple transformations:

python

# Instead of:
squares = []
for i in range(10):
    squares.append(i ** 2)

# Use list comprehension:
squares = [i ** 2 for i in range(10)]
print(squares)

Output:

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

3. Consider using enumerate() when you need both index and value:

python

fruits = ["apple", "banana", "cherry", "date"]

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

Output:

Index 0: apple
Index 1: banana
Index 2: cherry
Index 3: date

Summary

In this tutorial, you've learned about:

• For loops: Used to iterate through sequences with a known number of items
• While loops: Used when you need to repeat code based on a condition
• Loop control statements: break, continue, and else clause
• Nested loops: How to create more complex iteration patterns
• Practical examples showing how loops can solve real-world problems

Loops are essential to Python programming and mastering them will significantly improve your ability to write effective and efficient code.

Exercises

To practice your loop skills, try these exercises:

1. Write a program that prints the first 10 Fibonacci numbers using a loop.
2. Create a program that finds all numbers divisible by 7 between 1 and 100.
3. Write a loop to reverse a string without using the built-in reversed() function.
4. Create a pattern of stars that forms a right-angled triangle using nested loops.
5. Write a program that simulates a guessing game where the user has to guess a random number between 1 and 100.

Additional Resources

• Python Official Documentation on Control Flow
• Python for Loops (W3Schools)
• Python While Loops (W3Schools)