1.
Debugging
a programme
Learning
Outcomes
On completion of this chapter you will know:
·
The nature of breakpoints
·
How to set and remove breakpoints in a
programme
·
How to step through a programme line by
line
· How to examine values of variables while stepping through a programme
Introduction
When normally running a programme the processing speed
is very fast and we cannot see the individual lines of programme being
executed. This is as it should be since if we could see each line being
executed the programme would be running so slow that it would be completely
useless. Occasionally, however, it is good to be able to slow down the
processing so we can examine individual lines being executed. Some of the
reasons for this are:
The programme may not be functioning as it should be
due to faulty code. In this case it is good to be able to slow down the
processing so we can examine each command being executed and check the values
of the variables as the data in them changes.
For learners, being able to see the execution sequence
in such instances as which lines are skipped and which lines are executed in a If..Else
or Select..Case construct or which group
of lines are repeated in a loop construct would be helpful in the
understanding of those constructs.
In this chapter we shall concentrate how to examine
the line by line execution of the programme code as well as checking the values
of the variables at each stage of the execution.
Breakpoints
When we manually want to run a Visual Basic programme
we simply click inside the code and then press F5. The programme will then run,
executing one line after another at the rate of perhaps 500,000 lines per
second until the programme finishes. The only interruptions to this would be if
we used either an InputBox or Msgbox command
in the code. In this case the execution would stop until we clicked the OK
button on the dialogue boxes. Even though the programme execution would be
halted by the commands mentioned we would not still be able to examine the
code. In order to be able to do this we need to set a breakpoint in the
programme before we run it.
For our example we shall use the first programme that
we wrote in Listing 2‑1. in order to examine the code. We first list the programme
itself as shown in Figure 9‑1. We want to start
examining the code at the line for calculating the gross. To do this we move to
the grey area to the left of the edit area and click opposite the line for
calculating the gross. This will result
in a dark red circle to appear in the grey area and the line of code itself
will be highlighted in the same colour as shown below
in Figure 9‑2. Our breakpoint is
now positioned at this line. This means that the programme will run normally
until it reaches the breakpoint. It will then stop and at this point we can
examine the values of the variables that have been given values up until then.
Figure 9‑1
Figure 9‑2
In order to try this out we simply press F5 as before.
We shall be asked for the value of the hours in the first line of code and of
the rate in the second line of code. After this the breakpoint will be reached.
The execution of the programme will pause and the line for calculating the
gross will be highlighted in yellow as shown in Figure 9‑3.
Figure 9‑3
The highlighted line has not yet been executed and
thus we can only examine the values of the variables sngHours and curRate.
To examine the value of a variable you simply position the cursor over its
name. In Figure 9‑4 the cursor has
been positioned over sngHours and a small textbox appears
below it, showing us its value. If we were to position the cursor over curGross,
we would see that it has a value of zero because the highlighted line has not
yet been executed and therefore the value of the gross has not been calculated.
Figure 9‑4
To execute the current line we press F8. This causes the
execution of the line to occur and the yellow highlight will move on to the
next line, indicating that it is now the line that is ready for execution. This
is shown in Figure 9‑5 below. Since the
line for calculating the gross has been executed, when we put our cursor over
curGross we can read its updated value.
Figure 9‑5
Pressing F8 again will cause the line for calculating
the tax to be executed and will cause the highlight to move to the line for
calculating the nett. This is shown in Figure 9‑6 where we can read
the value of the variable curTax.
Figure 9‑6
Similarly pressing F8 once more will execute the line
for calculating the nett and allow us to check the
value for curNett as shown below.
Figure 9‑7
Practice
Copy Listing 7‑1 into your code
module and this time debug it instead of running it. To do this
take the following steps:
1. Insert a breakpoint
at line 9 - the first executable line.
2. Press F5 to start
the programme running
3. To make the
programme move from line to line press F8
Notice the control jumping to the functions and then
returning back to the line that initially called the functions. Also ensure that you check the values
calculated within the function bodies and also check that those values are passed
back to the appropriate variables in the main programme..
Exercise
Debug the programme you created for Exercise 5‑1 in the same way as
you debugged Listing 7‑1