Illustration of if - else statement:

The one application of an if-else statement is to check for errors in the inputs to a script. For illustration, a former script prompted the user for a radius, and then used that to compute the area of a circle. Though, it did not check to make sure that the radius be valid (example, a positive number). Here is a modified script which checks the radius:

Illustrations of running this script whenever the user enters invalid and then valid radii are shown here:

>> checkradius

Please enter the radius: -4

Sorry; -4.00 is not a valid radius

>> checkradius

Please enter the radius: 5.5

For a circle with radius of 5.50, the area becomes 95.03

The if-else statement in this illustration selects between two actions: printing an error message, or really using the radius to compute the area, and then printing out the result. Note that the action of the if clause is a single statement, while the action of the else clause is a group of three statements.