3. The Visual Basic .NET Language Learning to Program
Overview � The Common Language Runtime � Variables, data types, constants and literals � Identifiers � Expressions, operators � Statements and blocks � Structured variables and enumerations
VB .NET and The CLR � .NET programs execute on a virtual machine called the .NET CLR � CLR = Common Language Runtime � This is so that a .NET program can be run on any computer provided a version of CLR is implemented for it � Also allows programs to be written for an ‘ideal’ computer that was created by programmers for programmers � CLR supports many programming languages – not just Visual Basic
VB Running on a .NET Platform A Visual Basic program The .NET Common Language Runtime environment Computer hardware and Operating System
CLR Features � CLR has a number of useful features for programmers � Common Type System � A range of variable types that represent all of the common types of computer data � Numbers, dates, times, characters, text, currency etc. � Since all .NET languages use it, all are directly compatible with each other � Can use a .NET class in a C# program etc. � Data types are independent of the computer or operating system used
Variables in Programs � Programs need to store and use data � Values input by the user � Intermediate results in calculations � Data read from disk, CD-ROM, other programs or the Internet � Information indicating the current state of a program � How far through a list of values it has got � The name of a data file etc. � Variables are elements of computer memory made accessible by a programming language � Storage for a data value is given a name, or Identifier � The Identifier is used in program statements to access (read or write) the contents of a variable � The values stored in variables can change over time (that’s why they are called Variables) � Program statements can update a value as necessary
Example use of variables Sub Main() Length Dim length, width, area As Single length = 12.5 12.5 width = 8.75 Width area = length * width Console.WriteLine("Area is {0}", area) 18.5 End Sub Area � Variables can be used to 231.25 represent physical values � e.g. The Length, Width and Area of a rectangular item � Until a value is assigned, a variable will usually contain 0
Data Types � The CTS provides a wide range of types � Numbers � Integers, real (floating point) numbers � Various precisions � Characters and Text � Dates and Times � True or False/Yes or No � This range allows almost any physical or logical value to be stored and manipulated
Types in Detail Type Name Range Description -2,147,483,648 to 2,147,483,647 A 32 bit signed number with Integer no fractional part -9,223,372,036,854,775,808 to A 64 bit signed number with Long 9,223,372,036,854,775,807 no fractional part -3.402823E+38 to -1.401298E-45 for negative values; A single-precision floating- Single 1.401298E-45 to 3.402823E+38 for positive values point number stored in an efficient binary representation (7 digits of precision) -1.79769313486231E+308 to A double-precision floating Double -4.94065645841247E-324 for negative values; point number stored in 4.94065645841247E-324 to an efficient binary 1.79769313486231E+308 for positive values representation (15 digits of precision) +/-79,228,162,514,264,337,593,543,950,335 with no A high precision number Decimal decimal point; stored in a direct +/-7.9228162514264337593543950335 with 28 decimal representation places to the right of the decimal; smallest non-zero to improve the accuracy number is of calculations (29 digits +/-0.0000000000000000000000000001 of precision)
Types in Detail II Type Name Range Description January 1, 0001 to December 31, 9999 Dates, times and Date combinations of both. A single date variable stores a point in time (Date and Time) True or False Used to store simple Boolean true/false, on/off, up/down or other 2-state values 0 to 65535 Stores a single alphanumeric Char character (e.g. the letter ‘x’, or a digit or punctuation symbol Up to 2 billion characters Sequences of text String characters, e.g. names, addresses, paragraphs of a document, whole documents etc.
Type Compatibility � Numeric values are interchangeable between different types, but: � There can be a loss of data � E.g. Store a double value 3.14159268 in an Integer variable (result is 3) � General principle � Store contents of lower precision data type in higher precision or wider range variable – no problem � Store contents of higher precision or wider range data type in lower precision variable – loss of data
Constant � A Constant is like a variable with a fixed value (an oxymoron if ever there was one) � Constants are used in programs � to give memorable names to fundamental values � e.g. pi, Gravitational_Constant, cm_per_inch � to provide for values that might change from version to version of a program � e.g. VATRate, max_class_size, database_server_name � to provide for data that may have one value in a test version and another in the final release of the software � e.g. sample_size (could be 10 in tests, 1000 in working s/ware), max_redials (could be 10 in tests, 3 in real life)
Literals � A Literal is a value stated explicitly in program code � e.g. 3.1415927, 100, “Fred Bloggs”, “www.our_site.com/data_server” � i.e. any value that could be applied to a variable � Generally, try to eliminate literals from program code � Define constants to replace them � Not feasible to get rid of them all, but should aim to minimize the need to change statements throughout a program if some assumed value changes
Identifiers � Names given to: � Variables � Constants � Structures and Classes � Class members (member variables, methods) � Code routines � Modules, Namespaces � Forms, controls etc. � The purpose of an identifier is to allow a programmer to manipulate some value in code: � E.g. FullPrice = Price + Price * VATRate � FullPrice and Price are variables, referring to some stored value. Need to use the names to access these values
Rules for Identifiers � Must start with alphabetic or underscore � e.g. Cust_Name, PI, _data_Server � Must not be an existing keyword in the language: � e.g. avoid – Integer, Sub, For, Next, Function etc. � Must contain one or more alphabetics or numerics � e.g. X, A2, _d, _5 � Must be no more than 16383 characters long � Not much of a restriction
Assigning literal values - rules � Numeric assignments � State values directly. e.g. 12.3, 10000, -125 � Char assignments (single characters) � Use double quotes. e.g. “X”, “;” � String assignments (text) � Use double quotes. e.g. “Fred”, “ABCEDFG” � Dates and times � Enclose in double quotes. e.g. “14/02/2003”, “17:00”, “21/08/2003 12:30:00”
Initialising Variables � A variable with no value assigned will contain the type equivalent of zero: � 0 or 0.0 for numbers � “” for strings and characters � “01/01/0001 00:00:00” for dates/times � Typically a variable is declared then assigned � Can lead to a situation where a zero value is in the variable unintentionally for some time � Can lead to subtle errors � Programmer can declare and forget to assign at all � Possible to Declare/Assign in one step � e.g. Dim myName As String = "Alistair" � Prevents problem of unassigned variables
Expressions � An Expression is some combination of: � Variable references � Operators � Constants or literals � Function calls � Legal expressions always reduce to a single value � e.g. 12/3 + 4*2 – 11 (expression) evaluates to 1 (4 + 8 – 11) � e.g. Sqr(16) (expression) evaluates to 4 (sqr root of 16)
Operators � Generally, expressions involve operators � A range of legal operators is available for the range of types � e.g. Numeric (+, -, *, /, ^, +=, *=) � e.g. String (&, &=) � e.g. Dates (+, -, +=, -=)
String Operators � Strings can be added together � Two operators (plus modifications) available for this - + and & � The + operator used with strings joins them together � e.g. “Fred “ + “Bloggs” = “Fred Bloggs” � The & operator used with any data type will produce a string result � e.g. “Catch ” & 22 = “Catch 22” � e.g. "Strange but " & (2 = 2) � Also the ‘increment’ variants of these operators � e.g. s = “Farenheit “ s &= 451 � e.g. name = “Charlie “ name += “Brown”
Statements and sequence � Statements organised in a sequence will operate in that sequence � This is crucial – it provides the mechanism for performing a task in a number of steps Price = 10.00 TaxRate = 0.175 These must come before this for the Price = Price + Price * TaxRate calculation to work. Console.WriteLine(Price)
Blocks � A sequence of statements Sub AddTax() that is not interrupted by control statements (later) Price = 10.00 is a block TaxRate = 0.175 � Can be considered to be a Price = Price + Price * TaxRate unit of code: a block of statements performs a Console.WriteLine(Price) task End Sub � Blocks can be given a name (identifier) by enclosing them as a Sub Sub Main() or Function � This allows us to Call a AddTax() block of code by using its End Sub name as a statement
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