For Loop vs While Loop, Which is Faster?

by Panchajanya Sarkar

Introduction Link to heading

Most of the times we are confused between the for loop and the while loop. We don’t know which one to use. We don’t know which one is faster. We don’t know which one is better. In this article, we will try to find out which one is faster and which one is better.

The Code Link to heading

We will be using the following code to test the speed of the for loop and the while loop.

## Python script to compare the difference between for and while LOOPS

# Import modules
import os
import timeit

LOOPS = 100000000 # 10 million

# pure for loop to find the sum of the elements of a range of numbers
def for_loop():
    total = 0
    for i in range(LOOPS):
        total += i
    return total

# while loop to find the sum of the elements of a range of numbers
def while_loop():
    total = 0
    i = 0
    while i < LOOPS:
        total += i
        i += 1
    return total


def main():
    runs = 3
    while runs > 0:
        print("Number of runs left: ", runs)
        print("-------------------- PURE LOOPS ----------------------")
        print("While loop time: ", timeit.timeit(while_loop, number=1), " seconds")
        print("For loop time: ", timeit.timeit(for_loop, number=1), " seconds")
        runs -= 1

if __name__ == "__main__":
    # clear the screen
    os.system("clear")
    main()

Code Explanation Link to heading

Let’s understand the codes one by one. One function at a time!

The for loop Link to heading

LOOPS = 100000000 # 10 million
# pure for loop to find the sum of the elements of a range of numbers
def for_loop():
    total = 0
    for i in range(LOOPS):
        total += i
    return total

This for loop is a pretty basic for loop which basically iterates through the range of 10 million digits, starting from 0. The variable total inside the loop stores the sum of each iteration and finally returns the sum of all the eleements in the given range. Here, i is used to iterate throught the range of digits. The loop runs until the value of i is less than 10 million.

The while loop Link to heading

LOOPS = 100000000 # 10 million
# while loop to find the sum of the elements of a range of numbers
def while_loop():
    total = 0
    i = 0
    while i < LOOPS:
        total += i
        i += 1
    return total

This while loop also calculates the sum of the elements of a range of numbers. The range of numbers is from 0 to 10 million. The while loop is used to iterate over the range of numbers and add them to the variable total. The total variable is then returned. Check here, i is used which is the counter variable. The counter variable is incremented by 1 in each iteration. The loop will run until the counter variable is less than 10 million.

Results of the Code Link to heading

We ran the code 3 times and timed the execution of the code on my Linux Machine. The results are as follows:

Run 1

Number of runs left:  3
-------------------- PURE LOOPS ----------------------
While loop time:  9.289  seconds
For loop time:  3.946  seconds

Run 2

Number of runs left:  2
-------------------- PURE LOOPS ----------------------
While loop time:  8.456  seconds
For loop time:  3.976  seconds

Run 3

Number of runs left:  1
-------------------- PURE LOOPS ----------------------
While loop time:  8.502  seconds
For loop time:  4.028  seconds

Explanation of the Results Link to heading

The results are pretty clear. The for loop is faster than while loop. Can you have the slightest guess why? If you can’t, then let me explain it to you.

Well, let’s explain this in layman’s terms, the for loop in Python is faster than while loop because of the way the interpreter optimizes the operation. It’s designed to iterate over items from collections, iterators which allows the language to process these using efficient C loops under the hood, rather havng to manage loop counts or condiotions at the Python level. This leads to signoficant speed and performance gains.

Conclusion Link to heading

So, here we have seen that the for loop is much faster than the while loop. So, we should use the for loop whenever possible. But, there are some cases where we have to use the while loop. For example, when we don’t know the number of iterations, we have to use the while loop. But, if we know the number of iterations, we should use the for loop.