Loops in Python – For, While, and Nested Loops

Loops are very important in Python. They help run a block of code many times without writing the same code again and again. They are useful for checking the condition, processing a sequence, and automating repetitive tasks. Python gives different types of loops, and each one has its own usage. Loops can also be placed inside other loops, which helps work with many sequences at the same time and do more complex jobs. It is important to know which loop to use and when to use it. This helps make the code easier to read and run faster. In this article, you will understand what loops are in Python, their types, syntax, and examples for each in detail.

Table of Contents:

• What are loops in Python?
• Features of loops in Python
• Types of loops in Python
  1. For loop in Python
     • Using the else statement with a For Loop in Python
  2. While Loop in Python
     • Using the else statement with a While Loop in Python
  3. Nested Loops in Python
     • Using the else statement with Nested Loops in Python
• Infinite loops in Python
  1. Infinite For Loop in Python
  2. Infinite While Loop in Python
• Performance Comparison: For Loop vs While Loop vs Nested Loop
• Best Practices for Using Loops in Python
• Conclusion

What are loops in Python?

Loops are very important in Python as they allow you to iterate over a set of conditions till the conditions are satisfied. This means repeating tasks without writing the code again and again. There are two types of loops in Python. They are for loops, which are great when you know how many times you are going to run it (or when you are going to loop through items in a list, string, or some other collection). While loops keep running as long as a condition is true. Using loops in Python makes it easier to do repeated tasks and keeps the program shorter and more efficient.

Features of loops in Python

• Automatic Iteration: Loops help in automatically moving through items in the sequence without the need for manual indexing.
• Break and Continue: Loops allow for controlling the flow of execution by providing the break statement, which helps to stop the loop early, and the continue statement helps in skipping the current step and moving to the next.
• Simplifies Code: Loops help in reducing the repetition of the code, which makes the program faster and efficient.
• Flexible Control: You have the power to control the frequency of the execution of a loop based on some conditional statement or a counter.
• Infinite: You can have infinite running loops as well with a while True loop that must be exited manually or through a break statement.
• Works well with Range(): The for loops work well with the Python function range() to repeat the task a fixed number of times.
• Else with Loops: Python allows you to put an optional else block that will execute after a loop has finished normally.

Elevate Your Python Skills – Start Your Journey Now!

Enroll now to gain in-demand Python expertise and stand out in the job market.

Explore Program

Types of loops in Python 

Let’s now explore the different types of loops and how each of them works.

1. For loop in Python

A for loop is used to go through each item in a sequence. It runs a block of code one by one for every item. This makes it good for tasks where the same steps need to be done a fixed number of times.

Flowchart:

Syntax:

for iterating_var in sequence:

    statement(s)

Example:

Output:

Explanation: Here, the for loop is used to go through each course in the list and show a message to learn the course at Intellipaat.

Advantages of Using a for Loop in Python:

• Helps go through items in a list or other sequence easily.
• Makes the code short and easy to understand.
• Handles the counting or moving to the next item automatically.

Disadvantages of Using a for Loop in Python:

• Does not give much control to stop the loop in the middle.
• It cannot be used when the task needs to run forever.
• Not the best choice when the condition is more complex.

Using the else statement with a For Loop in Python

The else statement after a for loop executes only when all the items are completed by the loop without any interruption. If the loop is ended early with a break statement, then the else block does not run.

Syntax:

for item in sequence:
    statement(s)
else:
    statement(s)

Example:

Output:

Explanation: Here, the for loop runs through all courses. After all the courses are finished, the else block runs, which shows the completion message. 

2. While Loop in Python

A while loop in Python keeps running a set of instructions as long as a certain condition is true. It works well when we don’t know how many times we need to repeat the task. The loop stops when the condition becomes false. This is very helpful for tasks like waiting for the input of user or checking the values. 

Flowchart:

Syntax:

while condition:
    statement(s)

Example:

Output:

Explanation: Here, the loop is continued till the study hours reach 3.

Advantages of Using a While Loop in Python:

• It can be used when the number of times is not known.
• It can be used for checking values again and again.
• It can be used when input is needed from the user.
  

Disadvantages of Using a While Loop in Python:

• It may keep running if the stop condition is not given.
• It can be hard to check and fix if something goes wrong.
• It must be updated inside the loop, or it won’t stop.

Using the else statement with a While Loop in Python

The else statement after a while loop runs only when the loop finishes by itself. If the loop is stopped early using a break statement, the else statement does not run.

Syntax:

while condition:
    statement(s)
else:
    statement(s)

Example:

Output:

Explanation: The loop runs for 5 days. When all days finish, the else block runs to celebrate the success.

3. Nested Loop in Python

A nested loop is a loop placed inside another loop that lets you work with data made up of lists inside lists. The inner loop runs through its items completely for each pass of the outer loop. It is very useful for handling table-like data or grouping related data items together.

Syntax:

for i in outer_sequence:
    for j in inner_sequence:
        statement(s)

Example:

Output:

Explanation: Here, the loop runs one time and stops using break to prevent it from running for infinite number of times.

Advantages of Using Nested Loops in Python:

• It can be used to make pairs or groups of data.
• It can be used when one loop depends on another.
• It can be used to repeat actions for every combination of two sets of data.
  

Disadvantages of Using Nested Loops in Python:

• The code will get larger and less readable.
• Hard to find mistakes if too many loops are used.
• The execution becomes slower when there are many items, as each inner loop should run for every item in the outer loop.

Using the else statement with a Nested Loop in Python

In nested loops, an else block after the inner loop executes if the inner loop does not stop and completes all of its iterations. An else block associated with the outer loop only gets executed if the outer loop finishes all iterations and stops before it is completed.

Syntax:

for outer_item in outer_sequence:
    for inner_item in inner_sequence:
        statement(s)
    else:
        statement(s)
else:
    statement(s)

Example:

Output:

Explanation: Here, the inner loop completes all tasks each day and runs the inner else. After both days are completed, the outer else runs that shows the final message.

Infinite loops in Python

An infinite loop keeps running continuously because its condition never becomes false. This can happen by accident or on purpose when a program needs to run continuously until it is stopped manually.

There are two types of infinite loops in Python

1. Infinite for loop
2. Infinite while loop

1. Infinite For Loop in Python

An infinite for loop in Python occurs when the loop is designed to repeat continuously using a special iterator like itertools.cycle(). It continues until a break statement stops it.

Syntax:

import itertools
for item in itertools.cycle(sequence):
    statement(s)

Example:

Output:

Explanation: Here, the loop repeats the list [1, 2, 3] continuously without stopping. The break stops it after 6 times to prevent it from running endlessly.

2. Infinite While Loop in Python

An infinite while loop occurs when the condition of the loop is always true, which causes the loop to be executed continuously. The loop is stopped only when the break statement is used or the program is manually stopped.

Syntax:

while True:
    statement(s)

Example:

Output:

Explanation: Here, the while loop runs without stopping because the condition is always True. The break statement ends the loop after 5 rounds.

Performance Comparison: For Loop vs While Loop vs Nested Loop

Best Practices for Using Loops in Python

• Use descriptive variable names: Use names that describe their purpose, like count, item, or score, so that it is clear when reading the code.
• Keep your loop code simple: Include only the necessary steps in the loop to make it easy to read.
• Use break and continue: Use break to exit the loop early, and continue to skip the looping steps at the right time.
• Make sure the loop will end: Make sure to set your loop condition to ensure it will end, or we can end up with an endless running loop.
• Using else with loops: The else statement in a loop runs only if the loop is not ended by a break. This must be planned out in advance.

Learn Python for Free – Build Real Skills Today!

Sign up for our free course and start mastering Python

Explore Program

Conclusion

Python loops are quite useful for doing similar tasks over and over, controlling the flow of the program, and reducing the amount of code you have to write every time. Using different types of loops, while and nested loops, allows handling different tasks effectively. Understanding how to use the else statement with loops is useful in handling exceptions. It allows code to execute only when the loop completes without any interruption. By practicing the tips and techniques, you will be able to write clear, efficient, and organized programs in Python.

To take your skills to the next level, enroll in our Python Training Course and gain hands-on experience. Also, prepare for job interviews with our Python Interview Questions, designed by industry experts.

Loops in Python – FAQs