7.7 Do-Until...Loop

Do-Until is another pretest loop. the logic of Do-Until is the opposite of the Do-While loop. The condition is for termination of the loop. In other words, the loop stops when the condition evaluates to True. To better understand the logic of Do-Until, let us consider drinking a glass of iced water on a hot summer day. You might say "I will drink the water, until the glass is empty." You might consider a Do-Until loop, whenthe number of iterations is not known and the logic of the problem suggests that the loop should continue until the condition becomes True. There is usually a loop variable used to form the condition. Like a Do-While loop, the initialization and updating of the loop variable should be done explicitly. Below is the general syntax fo a Do-Until loop:

General Syntax (keywords are in bold):

Do Until condition
Body of the loop
Loop

Key Points to Remember

Do-Until is a pretest loop.
The body of the loop is executed until the condition evaluates to True.
Usually there is a loop variable that is used in the condition.
The loop variable should be explicitly updated within the loop.

Visual Basic VB.NET - EXAMPLE 4

Assignment: Write a Do-Until loop to add up all the numbers from 0 to 100, i.e. 0+1+2+3...+100.

Although this is a good problem to code using a For loop, you can use a Do-Until loop as well. To write a computer program for this problem, you need two variables: one variable to add up the numbers and a loop variable to assume the values 0,1,2...100. The variables should be initialized to 0 before the loop. The condition of the loop should be for stopping the loop. Hence, the condition should be until the loop variable is greater than 100. In the body of the loop, the loop variable should be added to the sum of the numbers, and then incremented by 1 to assume the next value. Below is the pseudocode for solving this problem:

Pseudocode for Example 7-4

Declare two variables: a loop variable and a variable to sum the number.
Initialize both variables to 0.
Start a Do-Until loop, with condition: LoopVariable>100.

Add the loop variable to the sum of the numbers.
Increment the loop variable by 1.
Loop back to Step 3, as long as the condition evaluates to False.

After the loop completes, display the sum of the numbers in the Label.

Create a Label for output changing the characteristics as indicated in the diagram below. Change the Form Text to Do-Until Sum 0 to 100 and double-click on the form to enter the text in the diagram below:

Solution to 7.4

Form at Design:

Form at Runtime:

lblShow Properties

Public Class Form1

Private Sub Form1_Load( ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim Sum As Integer
Dim Num As Integer

Do Until Num > 100
Sum += Num
Num += 1
Loop

lblShow .Text = "Sum Total: " & Sum .ToString

End Sub
End Class

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A Loop That Does Not Execute

A pretest loop will not execute at all if the loop condition does not hold at the very beginning of the loop. In a For loop with a positive step value, the loop will not execute if the initial value is greater than the end value, and vice versa.

Posttest Loops

In a posttest loop, the condition of the loop is checked at the end of each cycle. The body of a posttest loop gets executed at least once, regardless of the condition. Sometimes the logic of the problem's solution fits well into a posttest loop. However, every solution that requires a loop can be coded using one of the pretest loops covered in Sections 7.5 to 7.7.

7.8 Do...Loop-While

The statements surrounded with Do and Loop get executed at least once. The condition is examined at the end of each cycle. The loop continues while the condition evaluates to True.