1. Microsoft Visual Basic 2008: Reloaded Third Edition
Chapter Seven
The For…Next Loop
and String Manipulation

2. Objectives After studying this chapter, you should be able to:
Include the For…Next loop in both pseudocode and a flowchart
Write a For…Next statement
Calculate a periodic payment using the Financial.Pmt method
Select the existing text in a text box
Code the TextChanged event procedure
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3. Objectives (continued)
Include a combo box in an interface
Manipulate strings
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4. The For...Next Statement
For...Next statement: processes a set of instructions a known number of times
Is a pretest loop
Counter variable: used to keep track of the number of times the loop has been processed
Startvalue, endvalue, and stepvalue items
Control the number of times the loop is processed
Must evaluate to numeric values
Can be positive or negative
A negative stepvalue causes the loop counter to count down
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5. Figure 7-1: How to use the For…Next statement
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6. The For...Next Statement (continued)
Figure 7-2: Processing tasks for the For…Next statement
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7. Figure 7-3: Processing steps for the code shown in Example 1
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8. The For...Next Statement (continued)
Flowchart symbol for the For...Next loop is a hexagon
Values for the counter variable, startvalue, stepvalue, and endvalue are shown within the hexagon
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9. Figure 7-4: Pseudocode and flowchart for Example 1
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10. The Financial.Pmt Method
Calculates a periodic payment on a loan or investment
Returns the periodic payment as a Double type value
Rate and number of periods arguments must be expressed in the same units (monthly, annual, etc.)
Also calculates the amount that must be saved each period to accumulate a specific sum
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11. Figure 7-5: How to use the Financial.Pmt method
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12. The Monthly Payment Calculator Application
Figure 7-5: Sample run of the Monthly Payment Calculator application
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13. Figure 7-7: Two procedures in the Monthly Payment Calculator application
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14. Figure 7-7: Two procedures in the Monthly Payment Calculator application (continued)
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15. Selecting the Existing Text in a Text Box
Windows standard: highlight the existing text when a text box receives the focus
SelectAll method: selects all text in a text box
Enter event: occurs when the text box receives the focus
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16. Selecting the Existing Text in a Text Box (continued)
Figure 7-8: How to select the existing text in a text box
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17. Figure 7-9: The principalTextBox’s Enter event procedure
Figure 7-10: Result of processing the Enter event procedure
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18. Clearing the PaymentsLabel Control
TextChanged event
Occurs when a change is made in a control’s Text property
Change may be made by user or the program
Figure 7-11: New principal entered in the Principal text box
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19. Figure 7-12: The principalTextBox’s TextChanged event procedure
Figure 7-13: Status of the paymentsLabel after entering a value in the text box
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20. Clearing the PaymentsLabel Control (continued)
Figure 7-14: The termListBox’s SelectedValueChanged event procedure
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21. Using a Combo Box in an Interface
Combo Box control:
Similar to a list box with a list of choices
May contain a text field that allows the user to type an entry that is not on the list
List portion may be hidden
Three styles of combo boxes:
Simple
DropDown (the default)
DropDownList
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22. Using a Combo Box in an Interface (continued)
Figure 7-15: Examples of the combo box styles
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23. Using a Combo Box in an Interface (continued)
Figure 7-15: Code used to fill the combo boxes with values
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24. Using a Combo Box in an Interface (continued)
Items.Add method: used to add items to a combo box
SelectedItem property: contains the value of the selected item in the list
Text property: contains the value that appears in the text portion of the control (item selected or typed in)
Items.count property: used to obtain the number of items in the combo box
Sorted property: used to sort the items in the list
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25. Using a Combo Box in an Interface (continued)
Figure 7-17: Sample run of the Monthly Payment Calculator application using a combo box
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26. Using a Combo Box in an Interface (continued)
Figure 7-15a: Code for the Monthly Payment Calculator using a combo box
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27. Figure 7-15b: Code for the Monthly Payment Calculator using a combo box
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28. String Manipulation
Most applications need to manipulate string data in some fashion
String properties and methods are used to manipulate string data
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29. Determining the Number of Characters Contained in a String
Figure 7-19a: How to use the Length property
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30. Determining the Number of Characters Contained in a String (continued)
Figure 7-19b: How to use the Length property
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31. Removing Characters from a String
TrimStart method: removes one or more characters from the beginning of a string
TrimEnd method: removes one or more characters from the end of a string
Trim method: removes one or more characters from both the beginning and end of a string
Each of these methods returns a string with the appropriate characters removed
trimChars argument:
Comma-separated list of characters to be removed
If omitted, spaces will be removed
Default value is the space character: “ ”
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32. Figure 7-20: How to use the TrimStart, TrimEnd, and Trim methods
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33. The Remove Method Remove method: startIndex argument:
Removes characters from a string
Can remove one or more characters located anywhere in the string
Returns a string with the appropriate characters removed
startIndex argument:
The position of a character in a string
Is zero-relative (starts with 0 as first position)
count argument: number of characters to remove
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34. The Remove Method (continued)
Figure 7-21: How to use the Remove method
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35. Replacing Characters in a String
Replace method: replaces a sequence of characters in a string with another sequence of characters
Arguments:
string: the original String value
oldValue: sequence of characters you want to replace
newValue: replacement characters
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36. Figure 7-22: How to use the Replace method
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37. The Mid Statement Mid statement: Arguments:
Replaces a specified number of characters in a string with characters from another string
Arguments:
targetString: the string in which characters are to be replaced
replacementString: the replacement characters
start: the starting position for the replacement
count: number of characters to replace
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38. The Mid Statement (continued)
Character position of characters in the string starts with 1 (not the same as index)
Count argument is optional
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39. Figure 7-23: How to use the Mid statement
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40. Inserting Characters in a String
PadLeft method: inserts characters at the beginning of a string
PadRight method: inserts characters at the end of a string
Arguments:
length: represents the total length of the desired resulting string
character: the character used to pad the string; default value is the space character
string: the string on which the operation takes place
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41. Figure 7-24: How to use the PadLeft and PadRight methods
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42. The Insert Method
Insert method: used to insert characters anywhere within a string
Arguments:
startIndex: specifies where in the string to insert the value
string: the string being manipulated
value: the character(s) to be inserted
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43. Figure 7-25: How to use the Insert method
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44. Searching a String
StartsWith method: determines whether a specific sequence of characters occurs at the beginning of a string
EndsWith method: determines whether a specific sequence of characters occurs at the end of a string
Arguments:
subString: sequence of characters to be searched for
StartsWith and EndsWith methods both return Boolean values and perform a case-sensitive search
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45. Figure 7-26: How to use the StartsWith and EndsWith methods
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46. Searching a String (continued)
Figure 7-26: How to use the StartsWith and EndsWith methods (continued)
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47. The Contains Method Contains method: Arguments:
Determines if a string contains a specific sequence of characters
Is a method of the String class
Returns a Boolean value
Performs a case-sensitive search
Arguments:
subString: represents the sequence of characters to be searched for
string: the string in which to search
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48. Figure 7-27: How to use the Contains method
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49. The IndexOf Method
IndexOf method: returns an integer representing the location of a substring within a string
Performs a case-sensitive search
Arguments:
subString: sequence of characters to be searched for
string: the string to be searched
startIndex: the starting position for the search (zero-relative)
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50. Figure 7-28: How to use the IndexOf method
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51. Accessing Characters Contained in a String
Substring method: accesses any number of characters contained in a string
Arguments:
startIndex: index of the first character to be accessed (zero-relative)
string: the string containing the characters to be accessed
count: number of characters to be accessed
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52. Figure 7-29: How to use the Substring method
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53. Comparing Strings String.Compare method: compares two strings
Arguments:
string1, string2: the two strings to be compared
ignoreCase: a Boolean value indicating whether to perform a case-sensitive (False) or case-insensitive (True) search
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54. Comparing Strings (continued)
String.Compare returns an integer result:
Returns 0 when string1 equals string2
Returns 1 when string1 > string2
Returns -1 when string1 < string2
String.Compare uses word sort rules:
Numbers are less than lowercase letters
Lowercase letters are less than uppercase letters
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55. Figure 7-30: How to use the String.Compare method
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56. The Like Operator Like operator: Arguments:
Uses pattern-matching characters to determine if one string is equal to another
Returns a Boolean value (True/False)
Arguments:
pattern: contains one or more pattern-matching characters
string: the string to be evaluated
charList: a listing of characters to be matched
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57. Figure 7-31: How to use the Like operator
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58. Figure 7-31: How to use the Like operator (continued)
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59. Programming Tutorial Creating the Hangman Game
Figure 7-33: User interface
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60. Programming Example Principal and Interest Calculator
Figure 7-46: User interface
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61. Summary
For...Next statement: a pretest loop that will process the instructions a fixed number of times
A variable declared in a For clause has block scope
Use a hexagon in a flowchart for a For…Next loop
Financial.Pmt method: calculates a periodic payment on a loan or investment
SelectAll method: highlights text in a text box
TextChanged event: occurs when a control’s text changes
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62. Summary (continued)
Combo box: similar to a list box but may not expose the list items until clicked
Three styles of combo boxes: Simple, DropDown, and DropDownList
Combo box’s Items.Add method: adds an item to the list
Use SelectedItem, SelectedIndex, or Text property of a combo box to set the default item in a combo
Combo box’s Items.Count property: used to obtain the number of items listed in the combo box
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63. Summary (continued)
Combo box’s Sorted property: used to sort items listed in a combo box
Combo box’s SelectedItem property: contains the value of the item selected in the list portion of the combo box
Combo box’s Text property: contains the value that appears in the text portion of the combo box
Combo box’s TextChanged event: occurs when the user either selects an item in the list portion or types a value in the text portion
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64. Figure 7-50: String manipulation techniques
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