Note that once we have finished this Greeting program we will add the next program, a calculator program, to the SAME FORM.

Calculator Program

Here you will learn about:

We will make a calculator that can do the basic functions of addition, subtraction, multiplication and division. It won't look much like the calculator you've got in your bag - it will look like this:

Begin a new project and save it right away with the name 'Calculator'. Add these eight controls to the form and set the name and caption/text properties as shown above. When making controls of the same type, it may occur to you that you can do this by copying a control and then pasting additional copies onto the form - this is OK as long as you choose NOT to create a 'control array' when asked - we will learn about control arrays later. Note that the control at the right is a label not a textbox (you knew this from the name already?) - we don't want to anyone typing in the answer!

It's a good idea to build programs a step at a time, making sure each step is working before proceeding to the next step. Let's make a first step by asking the program to simply add the contents of the two textboxes without paying any attention to the option buttons.

Double-click the command button and type the following code:

It's very important to realize that in Visual Basic the '=' operator does not usually work like an equals sign, instead it is an 'assignment operator' that works out what is on the right hand side and then stores the answer in the variable or property named on the left hand side. So a line such as

would evaluate the right hand side to equal 9 then store that number into the variable called x. In English, you could read the above line as 

'Set x to equal the result of 3 + 6'

In our program, the line

should add together the contents of the txtX textbox and the txtY textbox and store the answer in the caption of the label called lblAnswer. You should find that the program gives you a result, but not the one you expect. You may get answers like 1+1=11 or 2+3=23! Clearly, computers can be pretty stupid sometimes! Or maybe it is NOT being so stupid - is it stupid to say "book" + "case" = "bookcase"? (Try using your calculator to add these words together). So this is what the computer is doing - joining the numbers together as if they were text characters rather than numbers.  Our problem is that sometimes the + operator in Visual Basic can be used to add numbers but sometimes it works like the '&' character and simply joins blocks of text together. We must force the computer to treat the contents of the textboxes as numbers, not as text - this can be done by moving the numbers into variables that have been declared to contain numbers (not text)

Some programming languages force the programmer to declare ALL the variables, but Visual Basic does not. We need to do it here just so that we can specify that the variables will hold numbers, not text or dates or anything else. There are several types of number formats in VB including integer, single and double. We don't want to create variables that can only hold integers otherwise we won't be able to add together non-integer numbers such as 1.3 and 4.84. So we'll take choose the double format (more precise than single).

We can declare a variable x to be of type 'double' with this line ('Dim' is short for 'dimension'):

To declare both x and y we could declare on separate lines or we can declare them together like this:

Note that variable declarations are normally placed outside any subroutine, unlike all other code you have met. So copy and paste the above like into your program, putting it above all the existing code.

Before we add the two numbers, then, we need to copy what is in the text boxes into our two new variables. Replace your cmdEquals_Click procedure with this one and try running the program - it should add correctly now.

Method 2

Now let's try using the Val() function to solve the problem. This forces the computer to convert the 'numbers in the form of text' into real numbers, so we no longer need to use the variable declaration method. Please erase then, the variable declaration line that you added earlier Then modify the code so that it looks like this (I will use bold type to emphasize changes):

and try running the program again. Now you should find that 1+1=2, as expected! What happens if you try to add 'book' + 'case' now? Is the result what you would expect?

As a next step, let's modify the code so that the program only does the addition if the optadd option button is selected. We will use an IF statement to check whether the option button is selected. Note that option buttons have a value property which is true only when the option button is selected.

Including code that test whether a certain condition is true and runs code depending on the result is called conditional branching and it is an extremely important part of all programming languages.

Modify the code like this:

Note how the end of the IF structure has been marked by the line End If, and note how the contents of the IF structure have been indented by putting a Tab character at the beginning of the line - this makes the program more legible.

Run the program, and you should find that the addition happens only when the option button for addition is selected.

Do you agree that the sentence "If it is true that it is raining then take your umbrella" could be shortened to "If it is raining then take your umbrella"? In a similar way, the second line above can be shortened to 

In Visual Basic, a line that says

can always be shortened by omitting the '=True'. We are always trying to write code that is compact and efficient, though should not compress code to the extent that it becomes hard to understand.

Now that we have figured out how to make addition happen only when we want it, it should be easy to modify the code to handle subtraction, division and multiplication and division. Modify the code like this (note that you can save a lot of typing by using copy and paste):

Test the program for subtraction, and then add the necessary code for division and multiplication. The calculator program is now fully functional, though the code is not quite as efficient as it could be. 

Rather than spending time trying to make the code more efficient, concentrate now on adding features to your program, for EXTRA CREDIT. Note that if you share your ideas with your friends you may be harming your own grade, for if lots of people are able to add these features then I will have to assume that it was very easy! Suggestions: