Constants and Variables

Data is categorized as either constants or variables and either numeric or non-numeric.

Constants

Numeric Constant

• May have sign in front of number
• May have decimal point
• No commas, dollar signs, percent signs
• May be used in calculations

 Examples of numeric constants

5         4.5         -7

Nonnumeric Constant

• Anything enclosed in quotation marks is considered a Nonnumeric constant, also called string constants or alphanumeric constants or literals
• Nonnumeric constants SHOULD NOT be used in calculations

Examples of nonnumeric constants

“Hello World”      “5”       “Five”     “”   

Note: Quotes with nothing, not even a space between them are called a zero-length string or empty string.

Variables

A variable is a named storage location (area of memory) that can contain data that can be modified during program execution. A variable can store only 1 item of data at any one time.

Programmers often need to store values for later use. For example, you may have values entered by a user for the number of hours worked and a rate of pay. You plan to multiply the two values and store the result in memory so you can use it later. For this you will need a variable.

Every variable has a Data Type. The data type specifies the kind of data the variable can store. As with constants all programming languages have at least two kinds of data types.

• Nonnumeric data type (called String, Text, Alphanumeric, or Character data in different languages) can be used to store any kind of data.
• Numeric data types are used to store numbers.

Different programming languages may have other data types.

When you declare a variable, memory space (dependant on the data type) is allocated to that variable and the kind of data the variable will hold is specified.

Each variable has a name that is supplied by the programmer. When the programmer references that variable name in the program they are accessing the value stored in the memory location associated with that variable.

The contents of the memory location can change; the name of the memory location will stay the same.

Rules for naming variables in Visual Basic

• Must be unique within scope
• Must begin with a letter or the underscore _ character
• May not contain embedded spaces or many special characters (. , “ - $ # * and others). The underscore _ is a valid character.
• May not be a reserved word

In addition

• Use upper and lower case with purpose. Once a variable is declared you do not have to be concerned with upper/lower case. The editor recognizes words that are the same except for case and makes them all the same for you. (use of upper and lower case differ between languages)
• It is always good programming practice to use names that are descriptive or mnemonic.