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LESSON 2 - How to write code




The Code Editor

As we saw in the previous lesson, getting to the Code Editor is as simple as hitting the proper button. You may have discovered that you can also call-up the Editor by double-clicking on an object. It is also possible to select "View code" with the right mouse button.





You will note that the Editor has all the functions of a text editor and then some. The most commonly used functions will be Cut ... Copy ... Paste which you can call from the Menu, from the Toolbar or from the right mouse button. You also have access to the usual Find and Replace functions ....

Getting help

There is a lot of documentation available on VB. There is so much, in fact, that it's easy to get lost in it. However, the on-line Help available from the Menu should be used regularly. Very often just doing a search on a word in particular will be sufficient to get you out of a jam. If you want to go into more detail check out the Contents part of MSDN (Microsoft Developers' Network) and surf through it.

Writing code

VB is not very particular about presentation - spaces, indents, lower case or upper case, it doesn't make too much difference to the compiler. But it may make a whole lot of difference to the programmer who has to maintain your code in 2 years, after you've moved up to being CEO of the company.



Apply "Best Programming Practices"
When you work with RAD (Rapid Application Development) tools like VB in a graphical interface environment, you become more than just a programmer, a writer of code. You are a developer. We will cover that in the next lesson.

But at the moment, you are still a Programmer. And unless you are developing an application for your own personal use, that nobody else will see, you have to think of the environment, of the team you are working with.

"No man (or woman) is an island!"
Especially when it comes to programming. The code you write may have to be checked by an Analyst. It will have to go through testing. It may have to be modified by other team members and it almost certainly will go through modifications, maybe several times, in the months and years ahead when you probably won't be around to defend yourself. "The evil that men do lives after them...". You do not write code for the VB compiler. You write it for other developers and programmers. What you want others to say behind your back is: "That Jane was blindingly efficient, brilliant, a genius with comments ...". You do not want to be remembered as "...the Picasso of code, master of the abstract".





If you are just starting out with the language, why not pick up a few good habits right now and it may make your life a lot easier down the road.
  1. Use comments when appropriate but not so many as to overwhelm the code; the apostrophe ' is the comment identifier; it can be at the beginning of a line or after the code.

    ' This is a comment
    ' on 2 lines.
    DIM intNumber AS Integer       'This is a comment


  2. Use indents - code must be indented under control structures such as If ... Then or Sub - it makes it so much easier to follow the logic.

    FOR i = 1 TO 5
          value(i) = 0      ' Indent used in control structures
    NEXT i


  3. Use standard capitalization - keywords like If, Dim, Option, Private start with a capital letter with the rest in lower case; variable names, control names, etc. are usually mixed case: ClientName, StudentId, etc.

  4. Write extra-long statements on 2 lines using the continuation character _ (space underscore); in VB each line is assumed to be an individual statement unless there is a continuation at the end of the first line.


    Data1.RecordSource =  _
        "Select * From Titles"       ' One statement on 2 lines is OK





Naming conventions

These are the rules to follow when naming elements in VB - variables, constants, controls, procedures, and so on:
  • A name must begin with a letter.

  • May be as much as 255 characters long (but don't forget that somedy has to type the stuff!).

  • Must not contain a space or an embedded period or type-declaration characters used to specify a data type ; these are ! # % $ & @

  • Must not be a reserved word (that is part of the code, like Option, for example).

  • The dash, although legal, should be avoided because it may be confused with the minus sign. Instead of Family-name use Family_name or FamilyName.

Data types

Data type Storage size Range
Byte 1 byte 0 to 255
Boolean 2 bytes True or False
Integer 2 bytes -32,768 to 32,767
Long (long integer) 4 bytes -2,147,483,648 to 2,147,483,647
Single (single-precision floating-point) 4 bytes -3.402823E38 to -1.401298E-45 for negative values; 1.401298E-45 to 3.402823E38 for positive values
Double (double-precision floating-point) 8 bytes -1.79769313486232E308 to -4.94065645841247E-324 for negative values; 4.94065645841247E-324 to 1.79769313486232E308 for positive values
Currency (scaled integer) 8 bytes -922,337,203,685,477.5808 to 922,337,203,685,477.5807
Decimal 14 bytes +/-79,228,162,514,264,337,593,543,950,335 with no decimal point; +/-7.9228162514264337593543950335 with 28 places to the right of the decimal; smallest non-zero number is +/-0.0000000000000000000000000001
Date 8 bytes January 1, 100 to December 31, 9999
Object 4 bytes Any Object reference
String (variable-length) 10 bytes + string length 0 to approximately 2 billion
String (fixed-length) Length of string 1 to approximately 65,400
Variant (with numbers) 16 bytes Any numeric value up to the range of a Double
Variant (with characters) 22 bytes + string length Same range as for variable-length String
User-defined (using Type) Number required by elements The range of each element is the same as the range of its data type.


In all probability, in 90% of your applications you will use at most six types: String, Integer, Long, Single, Boolean and Date. The Variant type is often used automatically when type is not important. A Variant-type field can contain text or numbers, depending on the data that is actually entered. It is flexible but it is not very efficient in terms of storage.

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Declaring variables

Declaring a variable means giving it a name, a data type and sometimes an initial value. The declaration can be explicit or implicit.

An explicit declaration: variable is declared in the Declarations Section or at the beginning of a Procedure. An explicit declaration looks like:
Dim MyNumber As Integer

Now the variable MyNumber exists and a 2-byte space has been reserved for it.

An implicit declaration: the variable is declared "on the fly", its data type is deduced from other variables. For example:
Dim Total1 As Integer      'Explicit declaration
Dim Total2 As Integer      'Explicit declaration
Total3 = Total1 + Total2      'Implicit declaration

Total3 is not formally declared but is implied, it is "arrived at" from the other declarations.

It is never a good idea to have implicit declarations. It goes against the rules for clarity, readability and ease of use of the code.
To make sure that this rule is followed, start the Declarations with the Option Explicit clause. This tells the compiler to consider implicit declarations as errors and forces the programmer to declare everything explicitly.

Other examples of declarations:
Dim MyName As String
Dim StudentDOB As Date
Dim Amount5, Amount6, Amount7

In the last example the type assigned to each variable will be: Variant. It is the default type when none is specified.

There can be multiple explicit declarations in a statement:
Dim EmpName As String, SalaryMonth As Currency, SalaryYear As Currency

In this final example, what are the types assigned to the three variables:
Dim Amount1, Amount2, Amount3 As Single

All Single-precision floating point, you say. Wrong! Only Amount3 is Single. Amount1 and Amount2 are considered Variant because VB specifies that each variable in a statement must be explicitly declared. Thus Amount1 and Amount2 take the default data type. This is different from what most other languages do.

Constants

A constant is a value that does not change during the execution of a procedure. The constant is defined with:
Const ValuePi = 3.1416


The Scope of variables

The term Scope refers to whether the variable is available outside the procedure in which it appears. The scope is procedure-level or module-level.

A variable declared with Dim at the beginning of a procedure is only available in that procedure. When the procedure ends, the variable disappears. Consider the following example:
          Option Explicit
                    Dim Total2 As Integer

          Private Sub Command1_Click ()
                    Dim Total1 As Integer
                    Static Total3 As Integer
                    Total1 = Total1 + 1
                    Total2 = Total2 + 1
                    Total3 = Total3 + 1
          End Sub

          Private Sub Command2_Click ()
                    Dim Total1 As Integer
                    Total1 = Total1 + 1
                    Total2 = Total2 + 1
                    Total3 = Total3 + 1
          End Sub

Every time Button1 is clicked, Total1 is declared as a new variable during the execution of that clicked event. It is a procedure-level variable. It will always stay at 1. The same for the Button2 event: Total1 is a new variable in that procedure. When the procedure ends, Total1 disappears.
Total2 is declared in the Declarations section. It is a module-level variable, meaning it is available to every control in this Form. When Button1 is clicked, it increments by 1 and it retains that value. When Button2 is clicked, Total2 is incremented from its previous value, even if it came from the Button1 event.
Total3 shows another way of retaining the value of a local variable. By declaring it with Static instead of Dim, the variable acts like a module-level variable, although it is declared in a procedure.

Another scope indicator that you will see when you study examples of code is Private and Public. This determines whether a procedure is available only in this Form (module) or if it is available to any module in the application. For now, we will work only with Private procedures.

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Operators

Mathematical and Text operators

Operator Definition  Example  Result 
Exponent (power of)  4 ^ 2  16 
Multiply  5 * 4  20 
Divide  20 / 4 
Add  3 + 4 
Subtract  7 - 3 
Mod  Remainder of division  20 Mod 6 
Integer division  20 \ 6 
String concatenation  "Joan" & " " & "Smith"  "Joan Smith" 


Note that the order of operators is determined by the usual rules in programming. When a statement includes multiple operations the order of operations is:
Parentheses ( ), ^, *, /, \, Mod, +, -

Logical operators

Operator  Definition  Example  Result 
Equal to  9 = 11  False 
Greater than  11 > 9  True 
Less than  11 < 9  False 
>=  Greater or equal  15 >= 15  True 
<=  Less or equal  9 <= 15  True 
<>  Not equal  9 <> 9  False 
AND  Logical AND  (9 = 9) AND (7 = 6)  False 
OR  Logical OR  (9 = 9) OR (7 = 6)  True 


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Control Structures

If...Then...Else

If condition1 Then
     statements1
Else
     statements2
End If

If condition1 is True, then statements1 block is executed; Else, condition1 is not True, therefore statements2 block gets executed. The structure must be terminated with the End If statement.

The Else clause is optional. In a simple comparison, statements1 get executed or not.

If condition1 Then
     statements1
End If



Select Case

Can be used as an alternative to the If...Then...Else structure, especially when many comparisons are involved.

Select Case ShirtSize
     Case 1
          SizeName.Caption = "Small"
     Case 2
          SizeName.Caption = "Medium"
     Case 3
          SizeName.Caption = "Large"
     Case 4
          SizeName.Caption = "Extra Large"
     Case Else
          SizeName.Caption = "Unknown size"
End Select



Do...Loop

Used to execute a block of statements an unspecified number of times.

Do While condition
     statements
Loop

First, the condition is tested; if condition is True, then the statements are executed. When it gets to the Loop it goes back to the Do and tests condition again. If condition is False on the first pass, the statements are never executed.




For...Next

When the number of iterations of the loop is known, it is better to use the For...Next rather than the Do...Loop.

For counter = start To end
     statements
Next

1) The counter is set to the value of start.
2) Counter is checked to see if it is greater than end; if yes, control passes to the statement after the Next; if not the statements are executed.
3)At Next, counter is incremented and goes back to step 2).


More will be covered on Control strucures as it becomes necessary in upcoming lessons. Meanwhile,if you want to know more, consult the VB Language Reference.



For additional resources you might find this site useful:
Free Visual Basic 6 tutorials and source code examples .



Assignment

To practise your coding and editing skills, try modifying the Football example by adapting it for different sports. For example, in American football, which is similar to rugby, there are 4 different ways to score, as shown here:
Touchdown  6 points 
Field goal  3 points 
2-point Convert or Safety  2 points 
Convert   1 point 




Look at "Lesson 11 - Downloads" for the solution.




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