Understanding the For ... Next Structure

Macros in Excel are written in a language called Visual Basic for Applications (VBA). Like any other programming language, VBA includes certain programming structures which are used to control how the program executes. One of these structures is the For ... Next structure. The most common use of this structure has the following syntax:

For X = 1 To 99
    program statements
Next X

You are not limited to using the X variable; you can use any numeric variable you desire. You are also not limited to the numbers 1 and 99 in the first line; you can use any numbers you desire, or you can use numeric variables. When a macro is executing, and this structure is encountered, Excel repeats every program statement between the For and Next keywords a certain number of times. In the syntax example, the statements would be executed 99 times (1 through 99). The first time through the structure, X would be equal to 1, the second time through it would be equal to 2, then 3, 4, 5, and so on, until it equaled 99 on the last iteration.

Normally, as Excel is working through the For ... Next structure, it increments the counter by one on each iteration. You can also add a Step modifier to the For ... Next structure, thereby changing the value by which the counter is incremented. For instance, consider the following example:

For X = 1 To 99 Step 5
    program statements
Next X

The first time through this example, X will be equal to 1, and the second time through, X is equal to 6 because it has been incremented by 5. Similarly, the third time through X is equal to 11. You can also use negative numbers for Step values, which allows you to count downwards. For instance, take a look at the following:

For X = 24 To 0 Step -3
    program statements
Next X

In this example, the first time through the structure X is equal to 24, the second time it is equal to 21, and the third time it is equal to 18.