A calculator comprises a processor and a keyboard coupled to the processor. The keyboard comprises a calculation key that causes a calculation to be performed on input data. The calculator also comprises a display coupled to the processor. When the calculation key is pressed, the processor determines whether the input data has already been entered. If the input data has not already been input, the processor prompts a user of the calculator to enter the input data via the keyboard by displaying an alphanumeric description of the input data required for the calculation.