String variables are used to process user input in Visual Basic. As you have seen in previous labs and examples, you can use a VB "textbox" for users to enter data to your programs. The following window shows a project with a form containing a label asking the user to enter some words, a text-box (named "txtInput") for entering the words, a picture-box for displaying the results of running the program and a command-button to initiate the processing of the users entered words.
We can capture the words that the user types into the text-box by using the text-box.TEXT property, as shown in the following VB code which is associated with the "Go" command-button:
In this code, two variables are declared: x is a string and y is an integer. The variable x is used to store the user entered text from the "txtInput" text-box through the command
x = txtInput.Text
Next, the length of the string x is computed using the function Len( ).
y = Len(x)
Now, if we want to print out the last character of the string stored in x, we can use the command
picOutput.Print Mid(x, y, 1)
The "Mid(i, j, k)" function picks out the k characters of the string variable i starting at character j.
Now lets add in the rest of the characters of the text that we entered. We can do this using the code in the following window:
The output looks like:
Well, that works well as long as you know exactly the length of the text that the user is going to enter. However, what if you don't? A loop would help!
One of the most significant advantages of using computers is to do a great many repetitive operations in a fraction of the time it would take a human to do the same thing. It is “loop” structures that enable us to perform those repetitive operations in the programming language. In many cases, it is necessary to exit from a loop when or if a certain condition is satisfied. This exiting process is analogous to the decision-making and branching with “If…Then” statements. The concept of controlling loops and exiting from loops is critical for most programming languages. This concept is often called “Flow of Control” in the programming language. After this assignment, you will have a basic idea about the two main looping techniques that are most likely to be encountered in computer programs for engineering application.
There are several ways to create the “loop” structure in Visual Basic. Let’s try out some fundamentals of looping as follows:
1. “For…Next” Loops
For n = first_n To final_n
...
Next
The “Do” loop is an
alternative way to repeat a block of code.
You can achieve the same results using the “For…Next” loop, but
using “Do” loop sometimes makes your code more elegant and intuitive due
to its inclusion of an exiting function.
Do while (n <= final_n)
...
Loop
The previous program can be rewritten and generalized using a "For" loop:
Now the input text can be any length up to the length of a string variable.
1. Implement the reverse text program shown above with the loop structure for output. Enter some example text in the program to ensure that it works. (Palindromes are not very good for demonstrating this code, but they are fun. For example, try: "Stanley Yelnats")
2. Using the Taylor’s (or Maclaurin’s) expansion theorem, the function
sin(x) can be rewritten as:
Write a VB program to determine sin(x) using the first n terms of the series expression and then compare this value with the more accurate value returned from the VB "sin" function. The values of x and n will be input values. WHen evaluating the series expansion, make use of the factorial function discussed in class. The Graphic User Interface should look like the following:
| Object | Property | Setting |
| Form1 | Caption Name |
Sine of x frmSin |
| Label1 | Caption Name Font |
Series approximation for sin(x) lblLabel1 MS Sans Serif, 12-point |
| Label2 | Caption Name Font |
x = lblLabel2 MS Sans Serif, 10-point |
| Label3 | Caption Name Font |
n = lblLabel3 MS Sans Serif, 10-point |
| Label4 | Caption Name Font |
Sin(x) = lblLabel4 MS Sans Serif, 10-point |
| Label5 | Caption Name Font |
Series approximation lblLabel5 MS Sans Serif, 10-point |
| Label6 | Caption Name Font |
Correct value lblLabel6 MS Sans Serif, 10-point |
| Text1 | Caption Name Font |
none txtText1 MS Sans Serif, 10-point |
| Text2 | Caption Name Font |
none txtText2 MS Sans Serif, 10-point |
| Text3 | Caption Name Font |
none txtText3 MS Sans Serif, 10-point |
| Text4 | Caption Name Font |
none txtText4 MS Sans Serif, 10-point |
| Command1 | Caption Name Font |
Go cmdGo MS Sans Serif, 10-point |
| Command2 | Caption Name Font |
Clear cmdClear MS Sans Serif, 10-point |
| Command3 | Caption Name Font |
Quit cmdQuit MS Sans Serif, 10-point |
Write the code for the Sub Procedure "Sub cmdGo_Click( )". This sub procedure should get the required values of x and n from the appropriate text boxes and place the values into appropriately named variables.
You will need a loop to add each term in the series expansion to an accumulation variable "sum". The "factorial" function should be called from this loop to calculate the denominator in each term.
After completing the loop your program should print the values of the approximate value and the correct value in the appropriate text boxes.
Include code for the "Clear" button that clears all the text in the text boxes.
Test your code for x = 1 and n = 4, and calculate sin(x).
Compute and plot (using a spreadsheet) the results for x = 1 and n = 1, 2, 3, 4, 5, 6, 7, 8
Compute and plot (using a spreadsheet) the error between the approximate value and the exact value for each of the values of n. Compute the error for each n as
To Be Turned In
1. String code:
A copy of the source code and a screenshot of the user interface of the Assignment. The screenshot should show the result of testing your program
2. Sine expansion
A copy of the source code and a screenshot of the user interface of the Assignment (Taylor Expansion Series). The screenshot should show the result of testing your program for x = 1 and n = 4.
Printouts of the plots of the results and the errors.