Difference between For Loop and While Loop in Python

Loops are an important part of programming that helps in executing a block of code repeatedly. In Python, the two most common loops are the For loop and the While loop. Both these loops help in iteration but work differently and have different use cases. Understanding these differences can do magic to your programming approach and efficiency. In this article, we will compare For Loop and While Loop differences in Python. Before we get a deep dive into the difference, let us first understand the basic concept of loops.

Also read: Python Programming

What are Loops?

Loops are the most basic things in programming. A loop is a programming technique that repeats a block of code or set of instructions over and over again without explicitly writing it out. This piece of code is executed based on a certain condition. Loops constitute the control structures of a program.

Loops are one of the core concepts in programming and are used often in making programs. Every programming language has loops as a fundamental concept; be it C++, Java, Python, and many others. There are different types of loops in every other programming language like For loop, While loop, do-while, etc. In Python, there are only two types of loops: The for loop and the While loop; let’s discuss them one by one in detail.

A For loop is used to iterate over a sequence of items in Python, such as a tuple, list, string, or range. For each element in the sequence, the loop runs a specific block of code.

Syntax:

How Does a For Loop Work?

Step 1: Initialisation

• The for loop starts with setting the loop variable to the first element of the iterated object (list, range) and then begins traversing it.
• For example, if a loop is iterating over a sequence, i.e., range(5), it assigns the first value of the range to that variable.

Step 2: Condition Check

• In a for loop the condition is implicit. Unlike in a while loop where you explicitly define a condition that stops it, the for loop goes on to execute until all elements in the sequence are covered.
• Python internally checks if there are more elements in the sequence.

Step 3: Loop body execution

• The loop body is run; it contains the code to be executed on each iteration. This may include carrying out operations like print, adjusting variables, and compute.

Step 4: After executing the loop body, Python moves to the next element in the sequence. The next value in the iterable automatically updates the loop variable.

Step 5: Repeat steps 2 to 4, for each element in the sequence.

Step 6: Once the sequence has been completely iterated over (no more elements are left), the loop exits, and the control moves to the first statement after the for loop.

Also read : Tuples in Python

Characteristics of For Loop

• It loops through a sequence (such as a range, list, or string).
• The length of the sequence shows how many iterations there will be.
• Improved and more concise syntax while utilising sequences.
• Frequently used when the total number of iterations is predetermined.

Example:

Output:

While Loop in Python

The while loop continues to execute a block of code as long as a given condition is true. The condition is checked before every iteration, and the loop stops when the condition becomes false.

Syntax:

How Does a While Loop Work?

Step 1: Initialisation

• Before entering the while loop, the loop controlling condition must be initialised. This typically means setting up a variable that is going to control the condition (e.g., a counter).

Step 2: Condition Check

• It tests the condition at the beginning of each iteration. If True, it executes the body; if False, it ends.

Step 3: Execution of Loop Body

• If the condition is true, the code inside the loop body runs. This means performing calculations, printing values, or changing variable values.

Step 4: Updation of Condition

• At this step, those variables in the loop that control the condition (like a counter) are updated. If this is not done and the condition is never updated elsewhere, then if need be the loop would go on endlessly (an infinite loop).

Step 5: Repeat steps 2 to 4, after the loop body executes, check the condition. If the condition is true, repeat the loop.

Step 6: The while loop ends and control shifts to the first statement after the loop when the condition is no longer true.

Characteristics of While Loop

• It is a loop with conditions.
• Useful when it’s unclear how many iterations there will be.
• Every time an iteration begins, the condition is verified.
• It may result in never-ending cycles if the condition is never broken.

Example:

Output:

Difference Between For and While Loops in Python

Below are the key differences between the For loop and the While loop in Python:

Infinite Loop in Python

An infinite loop is characterised by the absence of a working exit strategy. As a result, the loop keeps repeating until the operating system detects it and ends the program with an error or until another event happens (such as setting an automatic program termination after a predetermined amount of time).

Infinite For Loop

In a For loop, an infinite loop is impossible but may occur if it is used with generators or iterators. See the below example.

Example:

In this example, count(0) creates an infinite series of numbers beginning at 0, which keeps the loop running forever.

Infinite While Loop

The infinite loop problem is more common in a While loop. See the below example.

Example:

In this example, the condition True is always true, therefore this loop will continue forever. Either manually interrupt the loop or use a break statement to end it.

When to Use For Loops

A For loop must be used at the correct time according to the program requirements. Here’s when to use the For loops in Python:

• Iterating through a sequence: Lists, tuples, or strings are the perfect objects to iterate over using the for loop. For example, Processing characters in a string or iterating through a list of items.
• Working with ranges: When processing a fixed range of integers, for example, or when you need to iterate a certain amount of times, use for loops.
• More straightforward and readable: The For loops are typically simpler to read and comprehend, particularly when dealing with sequences that have a definite number of iterations.

When to Use While Loops

A While loop must be used at the correct time according to the program requirements. Here’s when to use a while loop in Python:

• Condition-based loops: The while loop works better when the loop’s termination is dependent on a dynamic condition. For example, keep requesting input from the user until the right response is given.
• Unknown number of iterations: A while loop is a better option if you’re not sure how many times the loop will execute.
• Managing complex conditions: The while loop offers flexibility if you need to run the loop depending on a condition involving several variables or outside variables.

Best Practices for Both For and While Loops

      1.Avoid Unnecessary Computation

If loops execute pointless calculations throughout each iteration, they may be inefficient. For example, calculating the same value repeatedly within the loop can cause it to execute more slowly.

      2.Using Break when required

Try using the break statement to end a loop early if you know that its task is finished before all of its iterations are completed. By reducing pointless iterations, performance can be enhanced in this way.

       3. Use enumerate()

To make the code more readable and Pythonic, use Python’s enumerate() method rather than a counter when you need both the index and the value in a for loop.

Conclusion

Python has two fundamental loop constructs: while and for loops. Knowing the difference between the two will help you develop more effective code. The while loop performs better in scenarios where the number of iterations is dependent on a dynamic condition, whereas the for loop is better suited for looping over sequences where the number of iterations is known.