Obtain the input - Algorithms:

From where does the input come? The two possible choices would be from an external file on a disk, or from the user, who enters the number by typing it from keyboard. For every system, one of these will be the default input device (that means, if not specified or else, this is where the input comes from). If the user is assumed to enter the radius, the user has to be told to type it. Telling the user what to enter is termed as prompting. Therefore, the input step actually becomes two steps: prompt the user to enter the radius, and then read it into the program.