Variables

• A variable is a location in the computer’s memory where a value can be stored for use by a program. link
• A variable name is any valid identifier.
• Variable names cannot be keywords and must begin with a letter and contain only letters, numbers, and underscores.
• An identifier cannot begin with a digit and cannot contain spaces.
• VB is NOT case-sensitive.
• Begin each identifier with lowercase letters in order to distinguish them from keywords.
• Choose meaningful variable names so that your programs are self-documenting.
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• Keyword Dim explicitly declares variables.
• The clause beginning with As describes the variable’s type.
• Dim sum As Integer declares a variable named sum.
• Variables of the same type can be declared in separate statements or in one statement with variables separated by commas.
• A statement like Dim loanAmount, interestRate As Integer declares both loanAmount  and interestRate as Integers.
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• Variables dimensioned outside of a Sub are called form-level variables and are said to reside in the general declarations, which makes them available to every event procedure.
  • Form-level variables come into existence when the program is started and destroyed when the program terminates.
  • Form-level variables can be used to store information between controls or between multiple clicks of the same button.
  • Form-level variables can cause hard-to-trace program errors.
• Subroutine-level variables are variables declared in a sub and can be referenced only in that sub.
  • Subroutine-level variables are created when they are declared in a sub, and destroyed when execution of that sub terminates.

Initialize all Variables

VB provides default values for all variables.  For example, integer variables default to 0. 

It is better to initialize your variables, because 

• some programming languages do not provide default values and uninitialized variables may have random values
  

• others reading your code do not know whether you wanted to accept the VB defaults or simply forgot to initialize your variables 

Option Explicit

Option Explicit is imposed by default.  You must declare all variables before using them.

Constants

Constants are named memory locations whose values that cannot change during program execution.

Constants are often used to make a program more readable. The program below demonstrates using constants.

Constants are declared using the keyword Const or ReadOnly.  If Const is used the constant must be initialized with a constant expression when they are declared and cannot be modified thereafter.  If ReadOnly is used the constant must be initialized with a constant expression when they are declared (or in a class constructor) and cannot be modified thereafter.

Visual Basic Data Types

Data types describe the information that a variable can store, and also describe how many bytes of memory are required to represent a type.

• A Short can store a value in the range -32,768 to 32,767.

• An Integer can store a value in the range -2147483648 to 2147483647.

• A Long, which is 4 bytes, can store a whole number in the range -9,223,372,036,854,775,808 through 9,223,372,036,854,775,807.

• Boolean represents True/False values.

• The Byte data type stores one byte of information.

• Date stores date and time formats.

• Double represents double-precision floating-point numbers.

• Single represents single-precision floating-point numbers.

• String stores a series of characters.

• Object stores any type of data and is the default type.

• Default values: