Identifiers An identifier is a string used to identify an area in memory. Typically, variable names need to be declared so the operating system can allocate sufficient space. The amount of memory is determined by the type of data that it will store.
Some VB Data Types Boolean2 bytesTrue or False (1 or 0) Char2 bytes0 to 65,535 representing the Unicode character set Date8 bytes0:00:00 on 1/1/0001 to 23:59:59 on 31/12/9999 Decimal16 bytes +/- 79,228,162,514,264,337,593,543,950,335 +/ Double8 bytes +/ X to +/ X Integer4 bytes-2,147,483,648 to 2,147,483,647 Long8 bytes -9,223,382,036,854,775,808 to 9,223,382,036,854,775,807 Object4 bytesany type Short2 bytes-32,768 to 32,767 Single4 bytes+/ X to +/ X String0 to approximately 2 billion characters
Identifiers There are two types of identifiers for scalar data: Constants Variables
Constants A constant is a name for a fixed value. The value of a constant cannot be changed. They must be declared and initialised in the same step: Const g As Decimal = 9.8
Constants Note that the type declaration can be omitted for brevity. Const g = 9.8 VB will use the data to determine type.
Identifier Names in VB Must begin with a letter –Can include letters and numerals –Cannot include spaces Use names that are descriptive Capitalising convention –InterestRate, InitialCapital
The Scope of Variables Local (procedure-level) –Declared within a subprogram –Dim varName As dataType Global (module-level) –Declared at the top of the code listing –Private varName As dataType
Exploring Integer Data Types
Short type (low-order byte) =255
Short type (high-order byte) = The largest Unsigned value that can be stored in 16 bits. How many patterns are there?
Short Type To store integers, half the combinations are used to represent negative values. The range for Integer type variables is: -32,768 to The MSB is used to represent the sign. Which value of the sign bit (0 or 1) will represent a negative number?
Integer Storage (4 bytes) High order bit (MSB) is worth 2 31 The number of different combinations =2 32 =4,294,967,296 Half are used for negative values, so the range is –2,147,483,648 to + 2,147,483,647
Long Integers In 8 bytes there are 64 bits! High order bit (MSB) is worth The number of different combinations =2 64 =18,446,744,073,709,650,616 Ranging from -9,223,382,036,854,775,808 to 9,223,382,036,854,775,807