Error-Checking user input in the While Loop:

In many applications, whenever the user is prompted to enter anything, there is a valid range of values. When the user enters a wrong value, instead of having the program carry on with a wrong value, or just printing an error message, the program must repeat the prompt. The program must keep prompting the user, reading the value, and checking it, till the user enters a value which is in the right range. This is a very general application of a conditional loop, looping until the user properly enters a value in the program. This is known as error-checking.

For illustration, the script below prompts the user to enter a positive number, and loops to print an error message and repeat the prompt till the user finally enter a positive number.