Chapter 3 – Examples

1. Chapter 3 – Examples The examples from chapter 3, combining the data types, variables, expressions, assignments, functions and methods with Windows controls (list boxes, text boxes) These examples illustrate the concepts discussed earlier

2. Example 3.1.2

3. Example 3.1.2 This is a ListBox called lstResults

4. Example 3.1.2 Variable declarations Variable a is uninitialized so starts with a value of 0. Variable b is initialized to 3.

5. Example 3.1.2 Calling methods of the ListBox object’s Items property The Clear method empties the Listbox. The Add method adds a row to the ListBox.

6. Anatomy of a Method Call When calling the Add method, you pass it the data as an argument. The ListBox lstResults.Items.Add(a) The Items property contains the collection of data that are displayed in the ListBox The Items property’s Add method is a subroutine that places an item into the collection

7. Example 3.1.2 Assigning a value into the a variable

8. Example 3.1.2 Adding a third item to the list

9. Method Call passing a complex expression as an argument lstResults.Items.Add(a * (2 + b)) • An argument can be a complex expression. The expression will be fully evaluated before the resulting data is sent. In this case, the following steps take place in this order: • 2 + b innermost parentheses  5 • Multiply a times the result if (1)  25 • Pass the result of (2) to the Add method  Order of operations from innermost to outermost based on parentheses

10. Example 3.1.4

11. Example 3.1.4 This is a ListBox called lstResults

12. Example 3.1.4 Variable declarations

13. Example 3.1.4 Variable assignments

14. Example 3.1.4 Empty the list box items

15. Example 3.1.4 Add three values to the list box

16. Order of operations lstResults.Items.Add(Math.Sqrt(5 * b + 1)) 15 16 4 Add to list

17. Order of operations lstResults.Items.Add(Int(a ^ b + 0.8)) 8 8.8 8 Add to list The Int function truncates the number…it will not round up, but rather just chops off the fractional part.

18. Order of operations lstResults.Items.Add(Math.Round(a / b, 3)) 0.6666666666… 0.667 Add to list • The Math.Roundmethod rounds the number either up or down, depending on which rounded value is nearer. It can take two arguments: • The number to be rounded • The total number of decimal places for the rounded number • If the second argument is not provided, 0 is assumed. There will be no decimal places, so the result will be a whole number. Multiple arguments to methods are separated by commas.

19. Example 3.1.5

20. Example 3.1.5 This example converts 41 inches into 3 feet, 5 inches

21. Example 3.1.5 Variable declarations…three in one statement

22. Example 3.1.5 Backslash is for integer division. Truncates the fractional part. So, feet = 3

23. Example 3.1.5 Mod gives the remainder of an integer division. So, inches = 5. 41 divided by 12 is 3, with a remainder of 5

24. Example 3.2.1 This is a string literal, which will be displayed literally. This is a string variable, so its contents will be displayed

25. Example 3.2.2

26. Example 3.2.2 This example adds the values in two textboxes and places them in a third

27. Example 3.2.2 The CDbl function takes a string value and attempts to convert it into a Double value. Note: the string must consist of digits (and perhaps one dot for a decimal point). Otherwise an exception (run-time error) would occur.

28. Example 3.2.2 The CStr function takes a numeric value and converts it to a string. This is necessary because a TextBox’s Text property requires a string value.

29. 3 Program Modes • Design mode • Run mode • Debug or Break mode

30. Stepping through program • Execute one line of code at a time (Stepping into) • Execute one procedure (Stepping over) • Execute remaining lines of code (Stepping out) • Execution will stop a pre-specified line of code (Break point) • Hoover to see value of particular variable or object

31. Stepping through program • Each line is highlighted before execution • Hoover to see values • Num1 & both Textboxes • num2 is 0 – Why?

32. Break points • Set a break point & run • Program stops • Hoover to see values

33. Example 3.2.3

34. Example 3.2.3 Declaring three String variables

35. Example 3.2.3 Assigning String literals into String variables.

36. Example 3.2.3 Concatenating the contents of two String variables and assigning them into another String variable Note: remember that the right side of an assignment statement is an expression. In this example, the expressions are String expressions.

37. Example 3.2.3 The final result placed in the Text property of txtOutput is the result of another concatenation

38. Example 3.2.5 This example illustrates the use of some String methods and properties.

39. Example 3.2.5 • The Substring method takes two arguments: • The beginning position of the substring (first position of the string is 0) • The length of the substring (number of characters to return)

40. Example 3.2.5 This IndexOf method returns the first position of a substring within a string.

41. Example 3.2.5 This ToUpper method converts the characters of a string to upper case. There is also a ToLower method In this case, the string expression is a concatenation. Note that (str1 & str2) is a concatenation of two strings. Because this is in parentheses, the concatenation occurs BEFORE the conversion to upper case. What would happen if you did not have the parentheses around str1 & str2?

42. Example 3.2.5 The Trim method removes beginning and end spaces from a string. Here, the trim takes place for str1, and the result is concatenated with str2

43. Example 3.2.5 The Length property of a string gives the total number of characters in the string. As you can see, str2 contains “a wink”, which has a total of six characters In this case, the Substring method is only taking ONE argument (str2.Length – 4)  2. If a second argument is not provided Substring returns the remainder of the string, starting at the specified beginning position So, at the end of the assignment statement, str3 contains the string “wink”.

44. Example 3.2.5 Note: These methods (SubString, IndexOf, Trim, ToUpper) return a string expression that can be used for display or assignment. But they do NOT change the original contents of the variables str1 and str2. Only an assignment changes them. So, at the end of this program, the contents of str1 and str2 are still “Quick as ” and “a wink”.

45. Example 3.2.7 It is good programming practice to include comments in your code in order to explain what is being done.

46. Example 3.2.8 In previous examples, the variables were declared INSIDE the procedure. Here, it is declared OUTSIDE any subroutines. • Two things to note about class-level variables: • Their data remain in existence throughout the entire time that the Form is running • Their data is accessible by ALL subroutines and functions of the form

47. Example 3.2.8 Compare previous to what would happen if numTimes were declared INSIDE the procedure. In this case, numTimes would only exist as long as the subroutine was running, and would disappear when it ended. So, EVERY TIME you run the routine, it will reinitialize to zero. Wouldn’t be able to keep a count of how many times the button was pushed!

48. Example 3.3.1 Working with dates In this case, we have declared a variable as a Date.

49. Example 3.3.1 Note that we have set a mask in the mtbDayOfBirthMaskedTextBox control. The MaskedTextBox’s Mask property helps guide the user and prevent invalid input. Here we guarantee that only numbers are entered separated by slashes

50. Masked Text Box Control Click on the Tasks button to reveal the Set Mask property. Click Set Mask to invoke the Input Mask dialog box.