Python while Loop

Introduction

Before we begin talking about Python while loop, let's briefly understand – What is Loop?

Loops are an essential element in programming languages. When you need to repeatedly run a particular block of code until a predetermined condition is met, loops come in handy.

A for loop is a control structure that allows you to iterate over a sequence of elements such as a list, tuple, string, or range of numbers. It executes a block of code repeatedly for each element in the sequence.

The for and while loops are the two fundamental loop building blocks in Python.

The fundamentals of Python's while loops are covered in this tutorial. You will learn how to use the else clause, break, and continue statements as well. We will also address a few FAQs on Python while loop.

Python while Loop

As long as the given condition evaluates to true, the while loop runs its statements an indefinite number of times.

The while loop in Python looks like this:

while EXPRESSION:
    STATEMENT(S)

The while statement begins with the while keyword and is followed by the conditional expression.

Before the statements are executed, the EXPRESSION is evaluated. The STATEMENT(S) is executed if the condition is true. Otherwise, if the condition evaluates to false, the loop is ended and the next statement takes over program control.

The first line of the STATEMENT(S) block is indented, while the last line is the first unintended line. Most individuals either prefer to indent 4-spaces or 2-spaces. The Python Code Style Guide suggests using 4-spaces per indentation level and avoiding combining the usage of tabs and spaces for indentation.

Consider the following code, which increments and prints the current value of the variable i if it is less than five:

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

As long as i is less than or equal to five, the loop iterates. The output will be as follows:

Output

number: 1
number: 2
number: 3
number: 4
number: 5

Standard comparison operations are supported by Python.

• a == b - True if variables a and b are equal.
• a != b - True if variables a and b are not equal.
• a > b - True if variable a is greater than variable b.
• a >= b - True if variable a is equal to or greater than variable b.
• a < b - True if variable a is less than variable b.
• a <= b - True if variable a is equal to or less than variable b.

Use the logical not operator to negate the conditional expression:

i=0
while not i >= 5:
    i += 1
    print('number:', i)

break and continue Statements

You can regulate the while loop's execution using the break and continue statements.

The break statement ends the current loop and transfers control to the statement that comes after the loop that was terminated. Using break to end the loop when a predetermined condition is fulfilled is the most common case.

In the example that follows, the loop is terminated when the item being iterated equals 2.

i=0
while i < 5:
    i += 1
    if i == 2:
        break
    print('number:', i)

Output

Number: 1

A loop's current iteration is terminated by the continue statement, which also transfers program control to the next iteration of the loop.

In the example below, the continue statement will force execution to return to the start of the loop and proceed with the next iteration once the currently iterated item equals 2.

i=0
while i < 5:
    i += 1
    if i == 2:
        continue
    print('number:', i)

Output

number: 1
number: 3
number: 4
number: 5

else Clause

Python features an optional else clause for the while loop, unlike other languages:

while EXPRESSION:
    STATEMENT(S)
else:
    STATEMENT(S)

When the EXPRESSION evaluates to false, the statements inside the else clause are executed. The loop will not be executed if an exception is encountered or if a break statement terminates it.

Here's an example:

i=0
while i < 5:
    i += 1
    print('number:', i)
else:
    print('Loop completed.')

Output

number: 1
number: 2
number: 3
number: 4
number: 5
Loop completed.

Let us check out what happens when you break out of the loop:

i=0
while i < 5:
    i += 1
    if i == 2:
        break
    print('number:', i)
else:
    print('Loop completed.')

Because the expression did not evaluate to false, the statement inside the else clause is not executed:

Output

Number: 1

The else clause in a while loop is rarely used. One such situation is when you expect to break from a loop and the loop continues to run until the condition evaluates to false, you can execute some statement or function.

Infinite while Loop

An endless loop is a loop that keeps repeating indefinitely and does not end until the program does. If the condition constantly evaluates to true, you get an infinite loop.

Usually, programs are made to wait for an external event to happen by using infinite loops. The most common technique to build an infinite loop in Python is with while True: You can also use any other expression that always returns true in place of True.

Here is an example of an endless while loop that will keep asking you to type “Yes”:

while True:
    i = input('Please enter \'Yes\': ')
    if i.strip() == 'Yes':
        break

The while loop will continue to run until you enter “Yes”:

Output

Please enter 'Yes': 3
Please enter 'Yes': l
Please enter 'Yes': lin
Please enter 'Yes': No
Please enter 'Yes': Yes

Pressing CTRL+C is another way to break an infinite loop.

Use the break statement to end the loop at some point when creating infinite loops.

FAQs on Python while loop

Conclusion

As long as the specified condition is true, the while loop repeatedly executes its statements.

If you have any queries, feel free to post a comment below, and we'll be happy to answer them.