Debugging

Invariably, you will encounter errors in your code. Design-time and compile-time errors are relatively easy to deal with because Excel helps us out with error messages and by indicating the offending code. Logical errors are much more difficult to detect and to fix. This is where debugging plays a major role. The Excel IDE provides some very powerful ways to find bugs.

Debugging can be quite involved, and we could include a whole chapter on the subject. There are even special software applications designed to assist in complex debugging tasks. However, for most purposes, a few simple techniques are sufficient. In particular, Excel makes it easy to trace through our programs, executing one line at a time, watching the effect of each line as it is executed.

Let us try a very simple example, which you should follow along on your PC. If possible, you should arrange your screen as in Figure 4-8. This will make it easier to follow the effects of the code, since you won't need to switch back and forth between the Excel window and the Excel VBA window. The code that we will trace is shown in Example 4-1. Note that lines beginning with an apostrophe are comments that are ignored by Excel.

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