Calculator Program

Here you will learn about:

We will make a calculator that can do the basic functions of addition, subtraction, multiplication and division. Instead of trying to make the calculator that looks like a pocket calculator, which would be difficult, we will set it up as below.

Make a new Windows Application project with the name 'calculator'. Save the project immediately with File>Save All (not Ctrl-S, for that only saves the form file). Give the name Calculator, the location H:\VB if you are at EEB3 (otherwise use the default location), give the solution name Calculator and leave the box checked for the automatic creation of a folder with that name to hold your project. ALWAYS set the location to H:\VB if you are saving a project at school.

Add the eight controls to the form and set the name and text properties as shown above. When making controls of the same type, you may wish to save time by copying a control and then pasting additional copies onto the form. 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! For the label, set the backcolor to white and the autosize property to false otherwise the control may keep changing width to accommodate answers of different lengths. Set the font size to 16 for all the controls - the easy way to do this is to first select all the controls simultaneously inside a selection rectangle made with the pointer tool. Set the form's text property to 'Calculator'.

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 radio buttons.

Double-click the button and type the following code between the btnEquals_Click wrapper lines:

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 text 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 copying the numbers into variables that have been declared to contain numbers (not text)

Visual Basic usually forces you to declare (create) variables before you use them (you can switch off this obligation by including the line Option Explicit off as we did in the Greeting project but that makes your program more error-prone and is not recommended). Let's do this properly then and declare to variables x and y which will be set up so that they can only contain numbers (that way VB will add them as 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 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 (it's called 'double because it holds numbers more precisely than the 'single' type).

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:

If the variables are only going to be used in one subroutine, as is the case in our program, then the Dim statement should be placed inside the subroutine, just under the first wrapper line.

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

To make sure you're not lost yet, here's a snapshot of the code window, including the class and subroutine wrapper lines. To make that annoying long line legible I have put an underscore character '_' in the middle of the line - that makes the line 'wrap' into tow 'half-lines' but please understand that as far as VB is concerned this is still a single line of code, not two. You can use the underscore trick yourself when you want a very long line of code to wrap for easy reading but make sure that you put the underscore immediately after a space like I did otherwise it will not work properly.

By the way, what happens if you try to add 'book' + 'case' now? Is the result what you would expect?

If you are busy clicking the different radio buttons then you haven't understood yet that our program is paying no attention to them since we haven't referred to them in our code yet. The code above always does addition no matter which radio button is selected.

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

Including code that tests 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 pressing the Tab key character at the beginning of the line - this makes the program more legible since we can more easily the If/End If pairing.

Run the program, and you should find that the addition happens only when the radio button for addition is selected - this is a slight improvement on what we had before.

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):

Lost? Here's another snapshot of the code window as it should look now:

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: