Handout-4
Structured Programming - Repetition
The syntax for repetition structure is:
For counter=startvalue to endvalue [Step stepvalue]
instructions
Next counter
An example code is the following:
Dim x as integer
For x = 0 to 10 step 1
Some instructions
next x
Create an application and add a button and a combo box (default name is ComboBox1). Add the
following code:
Me.ComboBox1.Items.Clear()
Dim x As Integer
For x = 1995 To 2004 Step 1
Me.ComboBox1.Items.Add(x)
Next
The statement Me.ComboBox1.Items.Clear() will clear all the contents of the combo box. To
add items to the list, we use the items.add method. Change the step to 2 and then 3 and
evaluate its impact.
Now load a picture box with the name (plan.ico) into the form. Place the picture on the left edge
of the form and note down the value in the Location (X and Y) property (X refers to the
distance from the left edge and Y being the distance from the top edge of the form). Do the same
by moving the image to the right edge of the form. Write the code to move the image left to right.
Dim i As Single
For i = 0 To 500 Step 0.1
Me.PictureBox1.Left = i
Next
How do you speed up and down the movement? Now add another picture called PictureBox2
(cloud.ico). When the car reaches the right edge of the form we want to replace the picture (car)
with the picture of the cloud. In the form_load event make PictureBox2 invisible. To replace
the picture in PictureBox1 the syntax is Me.PictureBox1.Image =
Me.PictureBox2.Image. Similarly write the code to make it move top to bottom and the
code to stretch the image.
Update Shopping cart example:
Dim i As Integer
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For i = clbCart.Items.Count - 1 To 0 Step -1
If clbCart.GetItemChecked(i) = True Then
clbCart.Items.RemoveAt(i)
Else
strOrder = strOrder & vbNewLine & clbCart.Items(i)
End If
Next
Practice Problems: Calculate answers for
1. 10*10 + 11*11 + 12*12+......+ 22*22.
2. (5-1) + (6-2) + (7-3) + (8-4) + (9-5) + (10-5)
Other Repetition Structures
VB.NET supports two other forms of repetition and these are:
Do While condition
instructions
loop
Do Until condition
instructions
Loop
The first one loops as long as a condition is true. The second one loops until a condition becomes
true. Add a combo box and a button and write the following code behind the button.
Dim i As Integer
Me.ComboBox1.Items.Clear()
i = 1
Do While i <= 10
Me.ComboBox1.Items.Add(i)
i = i + 1
Loop
For i= 1 to 10 step 1
Me.ComboBox1.Items.Add(i)
next
How many times did the loop execute? It is important to remember that instructions within the
loop should change the condition. Otherwise either the instructions will never be executed or they
will be an infinite loop. Experiment by changing the increment value to i = 2*i.
Use the above repetition structures to figure out the following:
Write a program that will calculate the number of years in which an initial deposit amount will
reach $1million, with a 7% interest rate. Design an interface with a textbox (name:txtamnt), a
button and a label (name:lblyears). With a click of the button the program should calculate
the number of years in which the amount will accumulate to $1m. Define two counter variables
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called sum and nyears. Set the nyears initially to 0 and the sum to the value in the textbox.
Now write a loop that will repeat until the sum becomes 1000000 and keep track of the number of
years. Complete the program, test and run it. Now what happens when you put 0 in the initial
amount? This is an infinite loop. Avoid this by checking if the value is a positive number and if it
is then exit the program.
Practice Program
Suppose $15,000 dollars is deposited in a bank. The bank provides a return of 5% at the end of
the year. Also, at the end of each year $1000 is withdrawn. In how many years does the money in
the bank get completely depleted?
Dim amount, year as integer
Amount=15000
year=0
Do while amount >=0
Amount=amount*1.05
year=year+1
Amount=amount-1000
Loop
LblDisplay.text=year
Random Number Generation
We use RANDOMIZE() and RND() function. RND() generates a random number that lies within
the following range: 0 <= RND() < 1. To produce random integers in a given range, use this
formula:
Int((upperbound - lowerbound + 1) * Rnd() + lowerbound)
In our example we want to move the two rockets by a random integer number between 1 and 10.
The code is:
Dim num1, num2 As Integer
num1 = Int(10 * Rnd() + 1)
num2 = Int(10 * Rnd() + 1)
End If
Now we will write a program to race two rocket ships. Place two picture box controls near the
bottom of the form and add the picture of the rocket ship to the two images. Name the two images
picrock1 and picrock2. Place the two rockets side-by-side. Note that at the top of the form
the top property is 0. Add three buttons for start, reset, and exit.
Note the Y value of the Location property. Suppose it is 280. In the Reset button write the
following code.
Me.picrock1.Top = 280
Me.picrock2.Top = 280
The above code puts the two pictures at the bottom of the form. Now add a timer control to the
form. Set the interval property to 200. Add the following code:
Start Button:
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Me.Timer1.Enabled = True
Reset Button:
Me.Timer1.Enabled = False
Now write the following code in the timer’s Tick event.
Me.picrock1.Top = Me.picrock1.Top - 50
Me.picrock2.Top = Me.picrock2.Top - 50
If Me.picrock1.Top <= 0 Or Me.picrock2.Top <= 0 Then
Me.Timer1.Enabled = False
End If
The logic for the above is that the two pictures keep moving up until one of them reaches the top.
Then the timer is turned off. To make the program interesting we need to add an element of
randomness to it.
Complete this code by checking for the winner. You simply need to compare the top properties of
the two pictures. Code the reset button and ensure that start and reset are enabled correctly.
Now provide an option for the user to bet on one of the rockets to win. You can do this through
radio buttons. Hint: a person wins the bet if the rocket they choose wins – if there is a draw or the
rocket loses then they lose.
Practice Programs
1. Simulate rolling of a dice. Use a label to display the number (1 to 6) that comes up.
2. Create a program for users to take a math quiz. Two random numbers (integers between 1
and 200) should be generated. Then the user would enter the sum of the two numbers
generated. If the answer they provide is correct, increment the number of correct answers
by 1. Otherwise, increment the number of incorrect answers by 1. Note that the user
should be able to repeatedly take the quiz.
3. Create an automatic bouncing ball program (a ball automatically bounces horizontally,
vertically or any directions).
Scroll Bars
We will now explore another object from the toolbox - scroll bar (actually there are two of
these, a horizontal and a vertical bar). The scroll bar has an integer value attached to it. The other
properties of interest are: minimum, maximum, smallchange, largechange. The
value assigned to a scroll bar can be accessed through the property Value. The event that is
commonly used here is the Scroll (which is activated when the position of the scroll bar
changes). Create a scroll bar with min, max, smallchange and largechange set at 0,100,1,5. Create
a label and have the value of the scroll bar displayed in the label.
Now attempt the following: Recall that all color schemes are based on three base colors: Red,
Green and Blue. By varying the intensity of these three colors, one can produce all other colors.
In VB, each of these colors can take an integer value between 0 and 255
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Create a program to visually see this effect - by changing the background color of a label and
setting the intensity of three colors through scroll bars. Note that in the above the same code
needs to be written for all the three scroll bars.
Private Sub HScrollBar1_Scroll(ByVal sender As System.Object, ByVal e
As System.Windows.Forms.ScrollEventArgs) Handles HScrollBar1.Scroll,
HScrollBar2.Scroll, HScrollBar3.Scroll
Me.Label1.BackColor = Color.FromArgb(Me.HScrollBar1.Value,
Me.HScrollBar2.Value, Me.HScrollBar3.Value)
End Sub
Practice Problems:
1. Create a program to add 4 integer numbers (range of each is from 1 to 500). For each
number to be entered use a scrollbar and a corresponding textbox.
2. Create a program to test a user’s color blindness. Pick about half a dozen colors and see if
the user can correctly guess those colors.
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