7.12 Nested Loops

A loop can be coded within another loop thus forming nested loops. There can be many levels of nesting. Needless to say, there can be a selection structure coded inside a loop, a loop coded in a selection structure, and so on. Such nesting is possible as long as the inner structure is completely enclosed in the outer structure. Nested loops do have application in programming. For example, nested loops are needed to display a table of values, to process the elements of a two-dimensional array, or to sort array elements using the Bubble Sort algorithm. Let us look at an example:

Off-By-One Error

This error means that the loop is iterated one extra, or one fewer times than expected. To avoid an Off-By-One error, be careful when forming the loop condition. For example, the following loop condition will process all the numbers from 1 to 100, excluding 100. Assume that N as Int = 1.

Do While N<100
...
N=N+1
Loop

Visual Basic VB.NET - Example 10

Assignment: Design a VB.NET project to display a multiplication table for numbers 1 to 10 on the screen. To do this, you need to display 10 rows, and in each row you have to display 10 values, which is the product of the row # by the column #. The outer loop will go through the rows, and the inner loop will go through the columns. Notice that for each cycle of the outer loop, i.e. for each row, the inner loop has to through all the cycles, i.e., all the columns. Let us design a simple GUI for this project with one Button and one Label, and write the code in the click event procedure of the Button.

Solution to 7.10

Modify the properties of the Label, Button and Form as indicated in the diagrams below. Double-Click on the Display button to open the code box, and enter the code supplied below.

Form at Design:

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Form at Runtime:

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lblShow Properties

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Public Class Form1

Private Sub btnDisplay_Click( ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnDisplay.Click
Dim Row As Integer
Dim Col As Integer
Dim Length As Integer
Dim Product As Integer
Dim strProduct As String

lblShow .Text =""

For Row = 1 to 10
For Col=1 to 10
Product=Row*Col
strProduct = Product .ToString
For Length=4 to strProduct .Length step -1
strProduct = " " & strProduct
Next
lblShow .Text &= strProduct
Next
lblShow .Text &= vbCrLf
Next

End Sub
End Class

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