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      While Loops

      Python Tutorial

      This Python tutorial has been written for the beginners to help them understand the basic to advanced concepts of Python Programming Language. After completing this tutorial, you will find yourself at a great level of expertise in Python, from where you can take yourself to the next levels to become a world class Software Engineer.

      While Loops

      Python while Loops

      Normally, flow of execution of steps in a computer program goe from start to end. However, instead of the next step, if the flow is redirected towards any earlier step, it constitutes a loop.
      A while loop statement in Python programming language repeatedly executes a target statement as long as a given boolean expression is true.

      Syntax

      The syntax of a while loop in Python programming language is
      while expression:
         statement(s)
      The while keyword is followed by a boolean expression, and then by colon symbol, to start an indented block of statements. Here, statement(s) may be a single statement or a block of statements with uniform indent. The condition may be any expression, and true is any non-zero value. The loop iterates while the boolean expression is true.
      As soon as the expression becomes false, the program control passes to the line immediately following the loop.
      If it fails to turn false, the loop continues to run, and doesn't stop unless forcefully stopped. Such a loop is called infinite loop, which is undesired in a computer program.
      The following flow diagram illustrates the while loop
      

      Example 1

      In Python, all the statements indented by the same number of character spaces after a programming construct are considered to be part of a single block of code. Python uses indentation as its method of grouping statements.
      count=0
      while count<5:
         count+=1
         print ("Iteration no. {}".format(count))
      
      print ("End of while loop")
      We initialize count variable to 0, and the loop runs till "count<5". In each iteration, count is incremented and checked. If it's not 5 next repetion takes place. Inside the looping block, instantenous value of count is printed. When the while condition becomes false, the loop terminates, and next statement is executed, here it is End of while loop message.

      Output

      On executing, this code will produce the following output 
      Iteration no. 1
      Iteration no. 2
      Iteration no. 3
      Iteration no. 4
      Iteration no. 5
      End of while loop

      Example 2

      Here is another example of using the while loop. For each iteration, the program asks for user input and keeps repeating till the user inputs a non-numeric string. The isnumeric() function that returns true if input is an integer, false otherwise.
      var='0'
      while var.isnumeric()==True:
         var=input('enter a number..')
         if var.isnumeric()==True:
            print ("Your input", var)
      print ("End of while loop")

      Output

      On executing, this code will produce the following output
      enter a number..10
      Your input 10
      enter a number..100
      Your input 100
      enter a number..543
      Your input 543
      enter a number..qwer
      End of while loop

      The Infinite Loop

      A loop becomes infinite loop if a condition never becomes FALSE. You must be cautious when using while loops because of the possibility that this condition never resolves to a FALSE value. This results in a loop that never ends. Such a loop is called an infinite loop.
      An infinite loop might be useful in client/server programming where the server needs to run continuously so that client programs can communicate with it as and when required.

      Example 3

      Let's take an example to understand how the infinite loop works in Python
      var = 1
      while var == 1 : # This constructs an infinite loop
         num = int(input("Enter a number :"))
         print ("You entered: ", num)
      print ("Good bye!")

      Output

      On executing, this code will produce the following output
      The above example goes in an infinite loop and you need to use CTRL+C to exit the program.
      Enter a number :20
      You entered: 20
      Enter a number :29
      You entered: 29
      Enter a number :3
      You entered: 3
      Enter a number :11
      You entered: 11
      Enter a number :22
      You entered: 22
      Enter a number :Traceback (most recent call last):
         File "examples\test.py", line 5, in
            num = int(input("Enter a number :"))
      KeyboardInterrupt

      The while-else Loop

      Python supports having an else statement associated with a while loop statement.
      If the else statement is used with a while loop, the else statement is executed when the condition becomes false before the control shifts to the main line of execution.
      The following flow diagram shows how to use else with while statement
      

      Example

      The following example illustrates the combination of an else statement with a while statement. Till the count is less than 5, the iteration count is printed. As it becomes 5, the print statement in else block is executed, before the control is passed to the next statement in the main program.
      count=0
      while count<5:
         count+=1
         print ("Iteration no. {}".format(count))
      else:
         print ("While loop over. Now in else block")
      print ("End of while loop")

      Output

      On executing, this code will produce the following output
      Iteration no. 1
      Iteration no. 2
      Iteration no. 3
      Iteration no. 4
      Iteration no. 5
      While loop over. Now in else block
      End of while loop

      Practice with Online Editor

      Note: This Python online Editor is a Python interpreter written in Rust, RustPython may not fully support all Python standard libraries and third-party libraries yet.
      Remember to save code(Ctrl + S Or Command + S) before run it.