LESSON 2 - Writing code

Writing code:

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.

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 doe:s 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 ormodule-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.