2026-01-268 min read

looping statements (Python Programming)

Learn looping statements (Python Programming) step by step with clear examples and exercises.

Title: Python Looping Statements - Mastering Iteration and Repetition

Why This Matters

Looping statements are crucial in Python programming as they enable you to iterate over a sequence of data or perform repetitive tasks efficiently. Understanding how to use loops can help you solve complex problems, write optimized code, and prepare for real-world coding challenges and interviews.

Prerequisites

Before diving into looping statements, it's important that you have a good grasp of the following concepts:

Variables and data types in Python
Basic arithmetic and string operations
Conditional statements (if-else)
Understanding functions and their syntax
Comprehending how to define and use variables within functions

Core Concept

Python offers two main types of loops: for loops and while loops.

For Loops

The for loop is used to iterate over a sequence (such as lists, tuples, strings, or range objects) and execute the code block for each item in the sequence. Here's an example:

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
print(fruit)

In this example, we have a list of fruits, and we use a for loop to iterate over the items in the list. For each item (fruit), we execute the code block inside the loop (in this case, printing the fruit).

While Loops

The while loop allows you to repeat a block of code as long as a certain condition is true. Here's an example:

i = 0
while i < 5:
print(i)
i += 1

In this example, we initialize a variable i to 0 and use a while loop to execute the code block as long as i is less than 5. Inside the loop, we print the current value of i and increment it by 1.

Nested Loops

You can also have loops within other loops, which are called nested loops. Here's an example:

for i in range(3):
for j in range(2):
print(f"i={i}, j={j}")

In this example, we have two for loops nested inside each other. The outer loop iterates over the numbers 0, 1, and 2 (using the range() function), and for each iteration, the inner loop iterates over the numbers 0 and 1. Inside the inner loop, we print the current values of both i and j.

Break and Continue Statements

You can use the break statement to exit a loop prematurely, and the continue statement to skip an iteration and move on to the next one. Here's an example:

for i in range(5):
if i == 2:
break
print(i)

In this example, we use a for loop to iterate over the numbers from 0 to 4. Inside the loop, we check if the current iteration is equal to 2 (using the if statement). If it is, we exit the loop using the break statement and stop printing any further numbers.

for i in range(5):
if i % 2 == 0:
continue
print(i)

In this example, we use a for loop to iterate over the numbers from 0 to 4. Inside the loop, we check if the current iteration is even (using the modulo operator %). If it is, we skip that iteration and move on to the next one using the continue statement. Otherwise, we print the current iteration number.

Worked Example

Let's consider a real-world example where you need to calculate the sum of the first 100 even numbers. Here's how you can do it using a loop:

sum = 0
for i in range(1, 101):
if i % 2 == 0:
sum += i
print("The sum of the first 100 even numbers is:", sum)

In this example, we initialize a variable sum to 0 and use a for loop to iterate over the numbers from 1 to 100. Inside the loop, we check if the current iteration number is even (using the modulo operator %). If it is, we add the number to our running total (stored in the sum variable) and continue with the next iteration. After the loop finishes, we print the final sum of the first 100 even numbers.

Common Mistakes

Forgetting to initialize a counter or using an incorrect starting value
Using for instead of while (or vice versa) when the other loop type would be more appropriate
Not properly handling edge cases, such as empty lists or sequences with only one item
Incorrectly incrementing or decrementing a counter variable
Misusing the break and continue statements to exit or skip iterations unintentionally
Neglecting to handle exceptions that might occur during loop execution
Not properly indenting code inside loops, which can lead to syntax errors
Failing to consider the order of operations when using multiple loops (e.g., nested loops)
Using a for loop with an infinite sequence or an unbounded condition, leading to an infinite loop
Not properly defining the stop condition for a while loop, causing it to run indefinitely

Subheadings under Common Mistakes:

Infinite Loops and Unbounded Conditions
Improperly Defined Stop Conditions for While Loops
Not Properly Handling Empty Sequences or Lists with Only One Item

Practice Questions

Write a Python program that prints the numbers 1 through 10 using a for loop.
Write a Python program that calculates the sum of the first 50 odd numbers.
Write a Python program that finds all prime numbers less than 100 using a while loop.
Write a Python program that reverses the order of the items in a list using a for loop.
Write a Python program that calculates the factorial of a given number (using recursion is not allowed) using a while loop.
Write a Python program that finds all Fibonacci numbers up to 100 using a for loop.
Write a Python program that counts the number of vowels in a string using a for loop and a dictionary.
Write a Python program that checks if a given year is a leap year using a while loop.
Write a Python program that finds the largest prime factor of a given number using a while loop.
Write a Python program that generates all permutations of a given string using a for loop and recursion.

FAQ

What happens when you use a for loop with an empty list or sequence?

When you use a for loop with an empty list or sequence, the loop does not execute any iterations since there are no items to iterate over.

Can I use a `for` loop to iterate over a dictionary in Python?

Yes, you can iterate over a dictionary using a for loop in Python. Here's an example:

my_dict = {"apple": 1, "banana": 2, "cherry": 3}
for key, value in my_dict.items():
print(f"Key: {key}, Value: {value}")

In this example, we have a dictionary with three items, and we use a for loop to iterate over the keys and values of the dictionary simultaneously. For each iteration, we print both the key and value.

What is the difference between a `break` and a `continue` statement in Python?

The break statement exits the current loop prematurely, while the continue statement skips the current iteration and moves on to the next one. Here's an example:

for i in range(5):
if i == 2:
break
print(i)

for i in range(5):
if i % 2 == 0:
continue
print(i)

How can I generate all permutations of a given string in Python?

You can generate all permutations of a given string using a for loop and recursion. Here's an example:

def permute(string, index, length):
if index == length:
print(string)
else:
for i in range(index, length):
string[index], string[i] = string[i], string[index]
permute(string, index + 1, length)
string[index], string[i] = string[i], string[index]

permute("abc", 0, len("abc"))

In this example, we define a recursive function permute() that takes a string, an index, and the length of the string as arguments. The function generates all permutations by swapping each character with every other character from the current index to the end of the string, then recursively calls itself with the updated index. After generating a permutation, it swaps back the characters to their original positions before moving on to the next iteration.

How can I find the largest prime factor of a given number in Python?

You can find the largest prime factor of a given number using a while loop and trial division. Here's an example:

def largest_prime_factor(number):
i = 2
while True:
if number % i == 0:
number /= i
print("Largest prime factor:", i)
break
else:
i += 1

largest_prime_factor(98)

In this example, we define a function largest_prime_factor() that takes a number as an argument. The function initializes the divisor to 2 and checks if the number is divisible by it. If it is, the function updates the number by dividing it by the divisor and continues with the next iteration. If the number is not divisible by the current divisor, it increments the divisor and continues with the next iteration. The loop continues until the number becomes 1 or an odd prime factor is found, at which point the function prints the largest prime factor and exits.