CIS 125, Visual Basic .NET I, Spring 2006 Variable Computer memory locations where programmers can temporarily store data. The data may be entered by the user at the keyboard, or it may be read from a file, or it may be the result of a calculation made by the computer. The memory locations are called variables because the contents of the locations can change as the application is running. Every varialbe has a name, data type, scope, and lifetime. See Figure 3.1 page 72 Boolean True, False Byte 0 to 255 Char One Unicode character A, B, C, ... Date Thu, 01 Jan 1970 00:00:00 GMT Decimal 16 bytes fixed-point number, no rounding 33.3333333 Double 8 bytes Integer 4 bytes 1,2,3,-10 Long 8 bytes 1,2,3,-10 Object Default data type to a variable Short -32,768 to 32,767 Single 11.56, -12.99 String "Hello, World" See Figure 3.2 Start with a 3 character id intScore, intRegionWest m can be used to stand for module scope, mintScore names must begin with a letter names can not contain punctuation names can contain numbers, but the number can not be at beginning, dec2002Sales Syntax accessibility variablename [As datatype][= initialvalue] Dim decPrice As Decimal Private mblnDataOk As Boolena = True ^ (exponentiation, 4^2 = 4 * 4 = 16) - (negation) *, / (multiplication & division) \ (integer division) Mod (modulus arithmetic) +, - Scope Where in the application's code the variable can be used Lifetime How long the variable remains in the computers internal memory. Procedure-Level Variable Has procedure scope becuase only that procedure can use the variable. See Figure 3.10 Module-Level Variable Has module scope because it can be used by all of the procedures in the form. See Figure 3.11 Block-Level Variables Has block scope because it can be used by block of code If...THen...Else or For...Next Syntax Const constantname [As datatype][= expression] Const decPI As Decimal = 3.141593D Const intMAX_HOURS As Integer = 40 Place an apostrophe (') before the text you want to treat as a comment. By default Visual Basic .NET creates undeclared variables for you Option Explicit On Variables must be declared Option Strict On Strings will not implicitly convert to numbers. Numeric data types will be implicity converted to less restrictive types. Implicit conversion to a more restrictive type will cause a syntax error. Use short phrases to describe the steps a procedure needs to take to accomplish its goal See Figure 3.19 page 90 Valid Data Expected Data - ??? Invalid Valid Data Unexpected Data - ??? Debugging Locating errors in a program Syntax Errors Typing Errors Logic Errors You enter instructions that don't give the expected results.