Data validation helps in ensuring that entries made to cells are valid. For example, in the Linear Equation Solver tool, this feature is used to ensure that the number of equations being entered is an integer between 1 and 20. Lets see how this is done.
Select the cell on which this feature is to be used and choose Data -> Validation. We want the number entered to be between 1 and 20, and we also do not want fractional numbers. So, our settings for data validation are as shown in the screen shot below.
If an invalid entry is made, we can generate a message to inform the user of the correct requirements. To do this, click the Error Alert tab and key in the appropriate message.
Now, when the user keys in an invalid value, an error message is generated.