Software Development

  Assignment 3 Guide

Debugging using Breakpoints


There is a tool in VB called a Breakpoint which can help you find out where your code is incorrect. It will help you locate logical errors. Logical errors occur when your program runs ok but you are getting unexpected results, such as incorrect answers.

For example, suppose you have a line of code...

 AreaRectangle = Length + Width

VB will run this code perfectly happily but you will still get an incorrect answer because your formula is wrong. VB does not know you want to calculate an area because you have specified a formula for adding a length and width together. The formula should be ..

AreaRectangle = Length * Width

Using breakpoints will help you track down incorrect code like the example above.

Setting a Breakpoint

When VB runs a program, it executes the code at lightening speed. You use a breakpoint to tell VB to stop your programme at a particular place, so you can step through the code line by line.

It's easy to add a breakpoint. You just add one by clicking in the margins. A brown circled then appears, indicating where the code will break. The following two images show how to add one:

Adding a breakpoint

The code below does not have a breakpoint

Now I will add a breakpoint

When you click in the margins, to the left of a line of code, a brown circle appears:

Notice that the line where you want VB to break is highlighted brown. You can put on as many breakpoints as you like but until you are used to them. For now it's best just to stick with one breakpoint.

It is just as easy to remove breakpoints, just click on the brown spot and it will go away.

~~~~ When you have added your breakpoint...

...all you have to do then is run your program as normal~~~~

When you run a program with a breakpoint, VB will halt at any line of code with a breakpoint and show you the coding window. The place where you put the breakpoint will now have a yellow arrow on top of the brown circle. The brown highlighted line will now be yellow:

Running the Program

VB stops at a breakpoint

The yellow highlight indicates where in your code VB is. You can now instruct VB to step line by line through your code by pressing F8 on your keyboard. When you press F8, it will execute the currently highlighted line and then move onto the next line which will be then be highlighted.

Stepping Through your Code

Use F8 to step to the next line

The yellow arrow, and the yellow highlight, jump down one line.

If you press the F8 key again, it will jump to the next line. In fact, every time you press F8, VB executes the highlighted line and then jumps to the next line and stops again, waiting for you to press F8 again.

~~~~You should note VB does not execute the highlighted line until...

...after you have pressed F8 and it has moved on to the next line.~~~~

Now here is the important part. The step-by-step execution of code can help you locate the errors in your code. If you hover your mouse over a variable, the value this variable currently holds will be displayed: See below. You can see a little tooltip showing the value of the variable.

You can use the tooltip information to find out where a value is incorrect. This will help you locate any incorrect lines

When you have had enough of debugging you can clicking the stop button as normal to end your program.





  Unit Information



Scheme of Work

Notes &Tutorials



Books & Things