Input in a While Loop:

The script below repeats the procedure of prompting a user, the reading in a positive number, and echo-printing it, as long as the user properly enters positive numbers whenever prompted. As soon as the user types in a negative number, the program will print the OK and end.

Whenever the program is executed, the input/output mayt look like this:

>> whileposnum

Enter a positive number: 6

You entered a 6.

Enter a positive number: -2

OK!

Whenever the user enters a negative number at the first time, no values will be echo-printed:

>> whileposnum

Enter a positive number: -33

OK!

This describes a very important feature of the while loops: it is possible that the action will not be executed at all, if the value of the condition is false at the first time it is computed.

As we have seen before, the MATLAB will give an error message when a character is entered instead of a number.

>> whileposnum

Enter a positive number: a

??? Error using ==> input

Undefined function or variable 'a'.

Enter a positive number: -4

OK!

Though, if the character is really the name of variable, it use the value of that variable as the input. For illustration:

>> a = 5;

>> whileposnum

Enter a positive number: a

You entered a 5.

Enter a positive number: -4

OK!