Iterations

Iterations in Visual Basic 2010 are performed via For..Next and For Each loops. Let’s analyze them more in details.

For..Next

A For..Next loop enables repeating the same action (or group of actions) for a finite number of times. The following code shows an example in which the same action (writing to the Console window) is performed 10 times:

In such loops you need to define a variable of a numeric type (i in the preceding example) that acts as a counter.

The above code produces the following result:

Notice that you can also initialize the counter with zero or with any other numeric value.

You can also decide how the counter must be incremented. For example, you could decide to increment the counter of two units instead of one (as in the previous example). This can be accomplished via the Step keyword:

This code produces the following output:

Step can also work with negative numbers and allows performing a going-back loop:

You can also decide to break a For loop when a particular condition is satisfied, and you do not need to still perform the iteration. This can be accomplished with the Exit For statement, as shown in the following example:

In the preceding example, when the counter reaches the value of 4, the For loop is interrupted and control is returned to the code that immediately follows the Next keyword. There is also another way for controlling a For loop; there could be situations in which you need to pass the control directly to the next iteration of the loop when a particular condition is satisfied (which is the opposite of Exit For). This can be accomplished with the Continue For statement, as shown in the following code snippet:

In the preceding example we are doing some edits on the counter. Notice that each time you invoke a Continue For, the counter itself is incremented one unit.

For Each

A For Each loop allows performing an action or a group of actions on each item from an array or a collection. Although collections are discussed in Chapter 16, it is a good idea to provide a code example with them, because this is the typical usage of a For Each loop. For example, consider the following code:

In the preceding code snippet, there is a collection containing references to all the running processes on the machine. Each process is represented by an instance of the System.Diagnostics.Process class; therefore List(Of Process) is a collection of processes. Supposing we want to retrieve some information for each process, such as the name and the identification number, we can iterate the collection using a For Each statement. You need to specify a variable that is the same type of the item you are investigating. In the preceding code you are just performing reading operations, but you can also edit items’ properties. For example, you might have a collection of Person objects, and you could retrieve and edit information for each Person in the collection, as in the following code:

This code will add the Dr. prefix to the LastName property of each Person instance.

You can still use Exit For when you need to break out from a For Each statement. Basically a For Each loop has better performances with collections than with arrays, but you have to know that you can use it in both scenarios.