Variables, Constants and Data Types

1. Variables, Constants and Data Types

2. Variables • Three components define a variable: • Name (memory location) • Type • Information (value) • Variable name must: • Start with a letter • No spaces, periods nor punctuation characters • Unique (within its scope) • 255 characters in length • Not a VB reserved word

3. Variables • Variable naming conventions: • Descriptive (code is easy to read) • Short as possible (easy to type) • Use a prefix for type (easy for programmer to know its type) • String s sName • Integer n nAge • (table 8.1, page 161)

4. Types of Variables • Integer 2 bytes -32768 to +32767 • Long 4 +/-2 billion • Single 4 +/-1E-45 to 4E38 • Double 8 +/-5E-324 to 1.8E308 • Currency 8 +/- 9E14 • String 1B/char 65000 fixed, 2 billion dynamic • Byte 1 0 to 255 • Boolean 2 True or False • Date 8 1/1/100 to 12/31/999 • Object 4 • Varient 16B+1B/Char

5. Variable Declaration • If a variable is not declared, it is declared by default by VB as Variant • Waste memory resources • Type might be invalid for use with functions • It is good programming practice to declare variables

6. Explicit Declaration • Explicit declaration uses statements to define the names and types of variables • Dim sFname as String, sLname as String • Private sFname as String, sLname as String • Static sFname as String, sLname as String • Public sFname as String, sLname as String • Multiple variables with multiple types • If type is not indicated, Variant will be used

7. Implicit Declaration • VB defines the variable the first time it is used • Variable name is preceded with a special character: • Integer % • Long & • Single ! • Double # • Currency @ • String $ • Example: • nNumVal% = 0 • sFirstName$ = “Ali”

8. Fixed-Length Strings • Strings in general are variable-length • Fixed length strings are declared like: • Dim sName As String * 25 • If a shorter string is assigned, it is appended with blanks • If a longer string is assigned, it is truncated

9. Variable Array • Array: is a group of variables of the same type, sharing the same name • Example: Dim nScores (1 to 35) As Integer • Use for loop with array For nCounter = 1 to 20 nScores(nCounter) = 0 Next nCounter

10. Scope • By default variables are local to the procedures where they are created in (local variable) • Public variables: can be accessed anywhere at the code • Example: Public sUserName As String (Defined in the General Declarations Section) • Using Public variables: • Hard to debug • Bad use of resources

11. Static Variables • Variables declared local to a procedure are discarded afterwards • To preserve a variable in side a procedure use Static • Static nPages As Integer • Using Static with functions or sub treats all variables inside as Static

12. Option Explicit • Setting “REqire Variable Declaration” option (Tools – Options – Editor) • It places Option Explicit in the general section • has no effect on Forms/Modules created before setting it

13. Constants • Cannot be modified • Easy to remember then it value • Avoid typing long strings • Minimize program modifications • Examples: • Const vbActibeTitleBar = -2147483646 • Const PI = 3.141592654 • Public Const PI = 3.141592654

14. Converting Data Types • VB provides several conversion functions you can use to convert values into a specific data type. • To convert a value to Currency, for example, you use the CCur function: PayPerWeek = CCur(hours * hourlyPay)

15. The Empty Value • Sometimes you need to know if a value has ever been assigned to a created variable. A Variant variable has the Empty value before it is assigned a value. The Empty value is a special value different from 0, a zero-length string (""), or the Null value. You can test for the Empty value with the IsEmpty function: • If IsEmpty(Z) Then Z = 0 • When a Variant contains the Empty value, you can use it in expressions; it is treated as either 0 or a zero-length string, depending on the expression. • The Empty value disappears as soon as any value (including 0, a zero-length string, or Null) is assigned to a Variant variable. You can set a Variant variable back to Empty by assigning the keyword Empty to the Variant.

16. The Null Value • The Variant data type can contain another special value: Null. • Null is commonly used in database applications to indicate unknown or missing data. Because of the way it is used in databases, Null has some unique characteristics: • Expressions involving Null always result in Null. Thus, Null is said to "propagate" through expressions; if any part of the expression evaluates to Null, the entire expression evaluates to Null. • Passing Null, a Variant containing Null, or an expression that evaluates to Null as an argument to most functions causes the function to return Null.

17. The Null Value • You can also assign Null with the Null keyword: • Z = Null • You can use the IsNull function to test if a Variant variable contains Null: If IsNull(X) And IsNull(Y) Then Z = Null Else Z = 0 End If

18. Built-in Constants • Intrinsic Constants • Colors, keycodes, shapes • Described in help • Object Browser