Introduction to Programming. Writing Programs Syntax, Logic and Run-time Errors

Transcription

1 Introduction to Programming Writing Programs Syntax, Logic and Run-time Errors

2 Error Types in Visual Basic There are three main types of errors that can occur while programming in Visual Basic. Knowing the different types can help a programmer make less mistakes therefore having more robust code.

3 Syntax Errors (Compiler Errors) Syntax errors are those that appear while you write code. Visual Basic checks your code as you type it in the Code Editor window and alerts you if you make a mistake, such as misspelling a word or using a language element improperly. Syntax errors are the most common type of errors. You can fix them easily in the coding environment as soon as they occur.

4 The Option Explicit statement is one means of avoiding syntax errors. It forces you to declare, in advance, all the variables to be used in the application. Therefore, when those variables are used in the code, any typographic errors are caught immediately and can be fixed.

5 To correct this error 1. Examine the documentation for each of the keywords used in the source code line. Compare the example code in the documentation against the source line generating this error. 2. If you cannot identify the cause of the error, gather information about the circumstances and notify Microsoft Product Support Services. Try this lesson. Go here.

6 Logic Errors Logic errors are errors that prevent your program from doing what you intended it to do. Your code may compile and run without error, but the result of an operation may produce a result that you did not expect.

7 For example, you might have a variable named FirstName that is initially set to a blank string. Later in your program, you might concatenate FirstName with another variable named LastName to display a full name. If you forgot to assign a value to FirstName, only the last name would be displayed, not the full name as you intended.

8 Logic errors are the hardest to find and fix, but Visual Basic has debugging tools that make this job easier, also. Try this lesson. Go here.

9 Run-time Errors Run-time errors are those that appear only after you compile and run your code. These involve code that may appear to be correct in that it has no syntax errors, but that will not execute. For example, you might correctly write a line of code to open a file. But if the file is corrupted, the application cannot carry out the Open function, and it stops running.

10 You can fix most run-time errors by rewriting the faulty code, and then recompiling and rerunning it. When a run-time error does occur, you can use the debugging tools in Visual Basic to determine the cause. Try this lesson. Go here.

11 RadioButton Enables the user to select a single option from a group of choices when paired with other RadioButton controls. The RadioButton control can display text, an Image, or both. When the user selects one option button (also known as a radio button) within a group, the others clear automatically.

12 All RadioButton controls in a given container, such as a Form, constitute a group. To create multiple groups on one form, place each group in its own container, such as a GroupBox or a Panel. RadioButton and CheckBox controls have a similar function: they offer choices a user can select or clear.

13 The difference is that multiple CheckBox controls can be selected at the same time, but option buttons are mutually exclusive. Use the Checked property to get or set the state of a RadioButton. The option button's appearance can be altered to appear as a toggle-style button or as a standard option button by setting the Appearance property.

14 ComboBox A combo box control combines the features of a text box and a list box. This control allows the user to select an item either by typing text into the combo box, or by selecting it from the list. Combo boxes present a list of choices to the user.

15 If the number of items exceeds what can be displayed in the combo box, scroll bars will automatically appear on the control. The user can then scroll up and down or left to right through the list.

16 Combo Box Styles There are three combo box styles. Each style can be set at design time and uses values, or equivalent Visual Basic constants, to set the style of the combo box.

17 Drop-down Combo Box With the default setting (Style = 0 Dropdown Combo), a combo box is a drop-down combo box. The user can either enter text directly (as in a text box) or click the detached arrow at the right of the combo box to open a list of choices. Selecting one of the choices inserts it into the text portion at the top of the combo box. The user also can open the list by pressing ALT+ DOWN ARROW when the control has the focus.

18 Simple Combo Box Setting the Style property of a combo box to 1 Simple Combo specifies a simple combo box in which the list is displayed at all times. To display all entries in the list, you must draw the list box large enough to display the entries. A vertical scroll bar is automatically inserted when there are more entries than can be displayed. The user can still enter text directly or select from the list. As with a drop-down combo box, a simple combo box also allows users to enter choices not on the list.

19 Drop-down List Box A drop-down list box (Style = 2 Dropdown List) is like a regular list box it displays a list of items from which a user must choose. Unlike list boxes, however, the list is not displayed until you click the arrow to the right of the box. The key difference between this and a drop-down combo box is that the user can't type into the box, he can only select an item from the list. Use this type of list box when space is at a premium.

20 CheckBox A CheckBox control displays an X when selected; the X disappears when the CheckBox is cleared. Use this control to give the user a True/False or Yes/No option. You can use CheckBox controls in groups to display multiple choices from which the user can select one or more. You can also set the value of a CheckBox programmatically with the Value property.

21 In today s activity, make a simple quiz program. The topic of your questions is completely up to you. Your quiz must contain at least 5 questions. The answers may be multiple choice, True/False, Yes/No or a combination of the above. When the user presses the RESULTS button, you will verify the user entered the correct answers and give them the result.

22 As a bonus, if the user gets all the questions correctly, have something special happen. Copy your document to the DropOff folder on the X:drive. This is activity 4_3_Quiz.