Variables – Part 1

Data Types and Naming Conventions

A variable is simply a name you give to an area of memory in which a data value used by your program is stored.  When you need to retrieve that data, or modify its value, you can refer to the memory location by the variable's name.

The VB variable types, as well as the range or types of values they can store, are listed below:

Variable Type	Description
Boolean	2-bytes, can contain the special values True or False
Byte	1-byte, unsigned integer 0 to 255
Currency	8-byte fixed-point. Stores 4 digits after the decimal point. Stores approx. +/- 922 trillion.
Date	8-byte date/time value from 1/1/100 to 12/31/9999. Additional Info: Dates are stored internally as eight-byte Double Precision floating point variables which can represent dates from January 1, 100 A.D. up to  December 31, 9999. The integer portion holds the date with zero being December 30, 1899. Dates prior to this are stored as negative values; those after as positive. The February 12, 1997, for example, is stored as 35,473. The fraction portion holds time information. Midday, for example, is represented as 0.5.
Double	8-byte floating point number (-1.79E+308 to 1.79E+308)
Integer	2-byte integer, stores approx. +/-32K
Long	4-byte integer, stores approx. +/-2 billion
Object	A 32-bit (4-byte) address that refers to the location of an object.
Single	4-byte floating point number (-3.40E38 to 3.40E+38)
String	Variable-length: 10 bytes + string length, holds up to 2 billion chars Fixed-length: holds up to 64K chars
Variant	Can hold any native data type, object reference, and the special values Error, Empty and Null.  Numeric variants require 16 extra bytes, string variants require an extra 22.

The rules for forming a valid VB variable name are as follows:

(1)        The first character must be a letter A through Z (uppercase or lowercase letters may be used).   Succeeding characters can be letters, digits, or the underscore (_) character (no spaces or other characters allowed).

(2)        The final character can be a "type-declaration character".  Only some of the variable types can use them, as shown below:

Data Type	Type Declaration Character
String	$
Integer	%
Long	&
Single	!
Double	#
Currency	@

Use of type-declaration characters in VB is not encouraged; the modern style is to use the "As" clause in a data declaration statement (described below).

(3) The name can contain a maximum of 255 characters.

 (4)       The name cannot be a reserved word (VB keyword).

VB is not case-sensitive: PAY, Pay and pay all refer to the same variable.  If you type a variable on a line in a case other than the case you used when you declared the variable, VB will change the case to match that of the declared variable when you leave that line.

Variable Naming Conventions

As long as you stick to the rules above, VB is "happy"; however, many programmers follow stylistic naming conventions in an effort to improve the readability of their code.  The naming conventions involve using a one-to-four character lower-case prefix to start off the name.  The purpose of the prefix is to identify the data type of the variable (like "int" for integers and "str" for strings).  These naming conventions are sometimes called "Hungarian naming conventions" (after Charles Simonyi, an early Microsoft programmer – and Hungarian native – who proposed the conventions).  Two other prominent programmers, Leszynski and Reddick, also have led the charge for naming conventions.

The chart below shows each variable type, a recommended three-character prefix, and a sample variable name:

Variable Type	Recommended Prefix	Sample Variable Name
Boolean	bln	blnDataIsValid
Byte	byt	bytSmallNum
Currency	cur	curBigMoney
Date	dtm (for "Date/Time")	dtmEmpBirthDate
Double	dbl	dblAnnSalary
Integer	int	intCounter
Long	lng	lngBigIntValue
Object	obj	objExcelApp
Single	sng	sngHrlyRate
String	str	strMyWord
Variant	vnt  (or "var")	vntWhatever

Note the style of using a lower-case, three-character prefix, followed by a descriptive name in mixed case.  This is the style of naming variables found in most VB documentation. 

It is also recommended that you indicate the scope of the variable with an additional one-character prefix in front of the variable name. My recommendations are:

• None for a local variable (i.e., a variable that is declared in a Sub or Function). Thus, variable names like the ones in the sample names above would be used for local variables.
• The letter m for module-level variables (variables that are declared with Private scope in the General Declarations section of a code module). Such variables can be "seen" in any Sub or Function of that module. For example, a String variable declared at the module level might be named mstrEmpName.
• The letter g for global  (project-level) variables (variables that are declared with Public scope in the General Declarations section of a code module). Such variables can be "seen" in any Sub or Function of any module in the project. For example, a Boolean variable declared at the project level might be named gblnFirstTime.
• The letter p for a variable that is passed as an argument to a Sub or Function procedure. Such a variable would have local scope within that procedure, but it is defined within the procedure's header. For example, an Integer variable passed to a Sub might be named pintCurrCode when defined in the Sub's header.

In addition, it is recommended that the letter a be used to indicate an array name. In an array name, the letter "a" would appear before the variable type indicator, but after the scope indicator. For example, an array of Boolean switches might be named as follows, depending on where it is declared:

ablnSwitches               local level

mablnSwitches            module level

gablnSwitches             global (project) level

pablnSwitches             passed parameter