Python - While Loops

Python while Loop

Awhile loop in Python programming language repeatedly executes a target statement as long as the specifiedboolean expression is true. This loop starts withwhile keyword followed by a boolean expression and colon symbol (:). Then, an indented block of statements starts.

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. 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.

Syntax of while Loop

The syntax of a while loop in Python programming language is −

while expression:   statement(s)

InPython, 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.

Flowchart of While loop

The following flow diagram illustrates the while loop −

Example 1

The following example illustrates the working of while loop. Here, the iteration run till value of count will become 5.

count=0while count<5:   count+=1   print ("Iteration no. {}".format(count))print ("End of while loop")

On executing, this code will produce the following output −

Iteration no. 1Iteration no. 2Iteration no. 3Iteration no. 4Iteration no. 5End of while loop

Example 2

Here is another example of using thewhile loop. For each iteration, the program asks foruser input and keeps repeating till the user inputs a non-numeric string. The isnumeric() function returns true if input is an integer, false otherwise.

var = '0'while var.isnumeric() == True:   var = "test"   if var.isnumeric() == True:      print ("Your input", var)print ("End of while loop")

On running the code, it will produce the following output −

enter a number..10Your input 10enter a number..100Your input 100enter a number..543Your input 543enter a number..qwerEnd of while loop

Python Infinite while 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

Let's take an example to understand how the infinite loop works in Python −

var = 1while var == 1 : # This constructs an infinite loop   num = int(input("Enter a number :"))   print ("You entered: ", num)print ("Good bye!")

On executing, this code will produce the following output −

Enter a number :20You entered: 20Enter a number :29You entered: 29Enter a number :3You entered: 3Enter a number :11You entered: 11Enter a number :22You entered: 22Enter a number :Traceback (most recent call last):   File "examples\test.py", line 5, in      num = int(input("Enter a number :"))KeyboardInterrupt

The above example goes in an infinite loop and you need to use CTRL+C to exit the program.

Python while-else Loop

Python supports having anelse statement associated with awhile loop. If theelse statement is used with awhile loop, theelse statement is executed when the condition becomes false before the control shifts to the main line of execution.

Flowchart of While loop with else Statement

The following flow diagram shows how to useelse statement withwhile loop −

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=0while count<5:   count+=1   print ("Iteration no. {}".format(count))else:   print ("While loop over. Now in else block")print ("End of while loop")

On running the above code, it will print the following output −

Iteration no. 1Iteration no. 2Iteration no. 3Iteration no. 4Iteration no. 5While loop over. Now in else blockEnd of while loop

Single Statement Suites

Similar to theif statement syntax, if yourwhile clause consists only of a single statement, it may be placed on the same line as the while header.

Example

The following example shows how to use one-line while clause.

flag = 0while (flag): print ("Given flag is really true!")print ("Good bye!")

When you run this code, it will display the following output −

Good bye!

Change the flag value to "1" and try the above program. If you do so, it goes into infinite loop and you need to press CTRL+C keys to exit.