Variables

Programming is a process that requires us to organize our actions into three stages. These are:
  1. Entering information into the program.
  2. Processing the entered data.
  3. Outputing the results to a screen or a printer.
In this process we often find that we need to temporarily store data because we are not ready to process it just yet. For example when we calculate the payment due on a loan, we need to input the following information before the processing can occur.

Inputs Necessary for Loan Calculation
  • The amount of the loan.
  • The current interest rate.
  • The duration of the loan.
Until all three datum have been input, we cannot perform the calculation to determine the payment which will be due each period. So we cannot trigger the loan calculation after any of these three individual inputs occur, but we must wait until all three inputs have occurred before we can calculate.

The question is what do we do with each datum entered so that we will have it when we are ready to perform the calculation. The answer is simple. We put it in a safe place where we can go back and get it whenever we need it. If programming terms that 'safe place' is called a variable.

The beginning programmer in Visual Basic will usually say: "Well it is in a safe place, it is in the textbox where the user entered it," or "It is in the radiobutton which the user selected." They are correct, but only partly. In a simple programming we can indeed leave our data on the form and expect it to be there when we need it. However, as a person learns more about programming you learn that there are advantages to making a copy of the data which has been entered and storing it in a place which will not permit the user to accidentally change or remove it. The only real disadvantage is a bit of extra coding to create the variable and store the information in it.

Some of these adavantages are:
  • Data stored in a textbox is a string. If it is to be used as a number it must first be converted from string data type to one of the number data types. If we leave the data in the textbox then this conversion must be done everytime the data is needed. So the question is: should you do this conversion every time you need the datum or do it once and store the number as a number? Experience has shown that fewer conversions are more efficient and safer because there are fewer opportunities for errors to occur.
  • By storing the data we want to keep track of, we are certain to have it when we need it. A form's main purpose is to allow the user to input data, and to display output, not for holding or storing data. Information in textboxes and radiobuttons could be changed by the user. If we do not store the data we need to keep track of, it may not be there when we need it because it may have been accidentally or deliberately altered by the user. By placing the data in a variable we can control it better preventing the accidental or deliberate changing of our data.
  • Separating the entry of the data from the storage of the data allows us to check each time data is entered to see if it is the right kind of data. If it passes the error checks then we can store it in a variable. This way we are only asking our program to do one thing at a time. This is an approach which has been proven to be a more reliable way to do things for over fifty years.
  • Processing is sometimes done in stages. If we continually convert our data from number to string and back again we can lose precision. We should convert between data types as seldom as possible. Ideally that means twice: once just after the data is entered and second when we display the result for the user. The best way to do this is to use variables for data storage.
Examples of When a Variable can be Used
  • Q The data which has been input is stored in a textbox but it is not text data.
  • A Create a variable then convert the date to the proper data type and store it in that variable.
  • Q You have two numbers stored in two variables. How do you swap the numbers?
  • A Declare a third variable. Move second number into the third variable. Move first number into second variable. Now move second number, currently located in the third variable, into first variable.
  • Q You need to keep track of whether the data in your program has changed so that you will know whether the file needs to be saved.
  • A Declare a boolean variable and name it appropriately e.g. fileNeedsSaving When this variable is set to True the flie needs to be saved and when set to False it does not need to be saved. Now use this variable to note when your file needs saving. For example when we add data the file needs saving, When the file has just been saved it does not need saving.


 

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