Language Fundamental of VB.NET Part 1. Heng Sovannarith

Transcription

1 Language Fundamental of VB.NET Part 1 Heng Sovannarith heng_sovannarith@yahoo.com

2 Variables Declaring Variables Variables are named storage areas inside computer memory where a program places data during processing. A program sets aside these storage areas by declaring variables, that is, by assigning them a name and indicagng what types of data will occupy these areas during processing.

3 Variables (cont.) Within a Visual Basic program, variables are declared using the Dim statement whose general format is shown below. Dim Variable as Type Naming Variables Programmer- supplied variable names are also referred to as idengfiers, and they must conform to Visual Basic naming convengons and a variable name must Be composed only of alphabegc, underscore, and numeric characters. Begin with an numeric or alphabegc character, or the underscore (_) character. Not contain any embedded blank spaces. Not be the same as a Visual Basic keyword, words the comprise of the Visual Basic language itself.

4 Variable (cont.) Variable names are not case sensigve, so the name expressed in upper- case characters refers to the same storage area as a name expressed in lower- case characters and however, for programming consistency, and to avoid unnecessary errors, it is best to express a variable as it is originally declared. Thus, valid variable names include X, My_Variable, MyVariable, My_Data_Variable, and other names using meaningful and simple idengfiers.

5 Data Types When declaring a variable an indicagon can be given about what type of data will be stored and there are many data types permissible under Visual Basic. In computer processing, the following are common for declaring the usual types of data involved: Short. (2 bytes )A non- decimal (whole) number with a value in the range of the - 32,768 to +32,767. Integer. (4 bytes) A non- decimal (whole) number with a value in the range of the - 2,147,483,648 to +2,147,483,647. Long. (8 bytes) A non- decimal (whole) number with a value in the range of the - 9,223,372,036,854,775,808 to +9,223,372,036,854,775,807.

6 Data Types (cont.) Single. (4 bytes) A floagng- point (decimal) posigve or negagve number with a value in the range of up to the E38 (sciengfic notagon). Double (8 bytes) A floagng- point (decimal) posigve or negagve number with a value in the range of up to the E308. Decimal. (16 bytes) A decimal number with a value in the range of up to +79,228,162,514,264,337,593,950,335. without decimal precision and up to the with 28 places of decimal precision. String. Any number of special, numerical,alphabegc and special characters. Date. (8 bytes) Dates in the range between January 1, 0001 to December 31, 9999 and Gmes in the range between 0:00:00 to the 23:59:59. Boolean. (1 bytes) The value of True or False.

7 Data Types (cont.) Example Dim My_Counter As Integer Dim My_Salary As Decimal Dim My_Great-Big-Number As Double Dim My_Name As String Dim My_Birthday As Date Dim My_Current_Time As Date Dim My_Decision As Boolean

8 Assigning Values to Variables With the Visual Basic assignment statement, variables are assigned values whose general format is shown below. Variable = value Dim The_Answer As Decimal Dim Var_A As Decimal = 2.5 Dim Var_B As Decimal = 3.5 The_Answer = 5 * (Var_A + Var_B)

9 ArithmeGc Operators Much of informagon processing involves applying arithmegc to numeric data types and values are added, subtracted, mulgplied, and divided to generate results, either final results or intermediate values that are involved in further arithmegc operagons. For these purposes Visual Basic supplies a set of seven arithmegc operators shown in the following table.

10 ArithmeGc Operators (cont.) Operator Use + Addition. Adds the values appearing on the two sides of the operator. - * / \ Mod ^ Subtraction. Subtracts the value on the right of the operator from the value on the left of the operator. Multiplication. Multiplies the values appearing on the two sides of the operator. Division. Divides the value on the left of the operator (dividend) by the value on the right of the operator (divisor). Integer Division. Divides the value on the left of the operator by the value on the right of the operator, giving an Integer result. Modulo Division. Returns the remainder of the division of the value on the left of the operator by the value on the right of the operator. Exponentiation. Raises the value on the left of the operator to the power on the right of the operator.

11 ArithmeGc Operators (cont.) Example Dim Answer1, Answer2 As Single Answer1 = 1 + (2 / 3) Answer2 = (1 + 2) / 3)

12 String Operators Visual Basic supplies the concatenagon operator (&) to put together individual data strings to create a longer string of those combinagons and the Like operator looks for matching characters in a string. Operator & or + Like Use ConcatenaGon. Appends the string value appearing on the right of the operator to the string value on the lee of the operator. Determines whether a string matches a given pafern.

13 RelaGonal Operators Visual Basic supplies a set of six relagonal operators to compare the magnitudes of two data values and comparisons can be made with numeric or string values, with a Boolean value returned from the comparison. Operator = Tests whether two values are equal. Use <> Tests whether two values are not equal. < Tests whether the first value is less than the second value. > Tests whether the first value is greater than the second value. <= Tests whether the first value is less than or equal to the second value. >= Tests whether the first value is greater than or equal to the second value.

14 RelaGonal Operators (cont.) In the following example,the Boolean value True is assigned to Result2 and the Boolean value False is assigned to variable Result1.Value1 is not equal to Value2; rather, Value1 is less than Value2. Dim Value1 As Decimal = 10 Dim Value2 As Decimal = 20 Dim Result1 As Boolean Dim Result2 As Boolean Result1 = Value1 = Value2 Result2 = Value1 < Value2

15 Logical Operators RelaGonal tests can be combined to test mulgple condigons at the same Gme and the logical operators AND, OR, NOT, and Xor are used as conjuncgons between mulgple relagonal tests. In the following example, all of the complex relagonal tests are True and nogce in the third relagonal test that parentheses are used, just as in composing arithmegc expressions, to control the order in which tests are made. Dim Value1 As Decimal = 10 Dim Value2 As Decimal = 20 Dim Value3 As Decimal = -5 Dim Result1 As Boolean Dim Result2 As Boolean Dim Result3 As Boolean Result1 = Value1 < Value2 AND Value1 > Value3 Result2 = Value1 > Value2 OR Value1 > Value3 Result3 = Value1 < Value2 AND (Value1 > Value3 OR Value2 < Value3)

16 MathemaGcal FuncGons In the following table, popular mathemagcal funcgons are summarized. Also note that certain funcgons do not require the Math. prefix.

17 MathemaGcal FuncGons (cont.) FuncEon Use Math.Abs() Returns the absolute value. Math.Abs(- 10) returns 10. Math.Ceiling() Returns an integer that is greater than or equal to a number. Math.Ceiling(5.333) returns 6. Fix() Returns the integer porgon of a number. Fix(5.3333) returns 5. Math.Floor() Returns an integer that is less than or equal to a number. Fix(5.3333) returns 5. Int() Returns the integer porgon of a number. Int(5.3333) returns 5. Math.Max() Returns the larger of two numbers. Math.Max(5,7) returns 7. Math.Min() Returns the smaller of two numbers. Math.Min(5,7) returns 5. Math.Pow() Returns a number raised to a power. Math.Pow(12,2) returns 144. Rnd() Returns a random number between 0 and 1. Used in conjuncgon with Randomize statement to inigalize the random number generator. Math.Round() Rounds a number to a specified number of decimal places. Rounds up on.5. Math.Round( ,5) returns Math.Sign() Returns the sign of a number. Returns - 1 if negagve and 1 if posigve. Math.Sign(- 5) returns - 1. Math.Sqrt() Returns the square root of a posigve number. Math.Sqrt(144) returns 12.

18 String manipulagon funcgons Several built- in string funcgons perform string manipulagons to augment simple concatenagon with the "&" operator and these funcgons are summarized in the following table.

19 Asc() Chr() String manipulagon funcgons (cont.) FuncEon GetChar() InStr() InStrRev() LCase() Lee() Len() LTrim() Mid() Use Returns the character code of the first character of a string. Asc("A") returns 65. Returns the display character of a character code. Chr(65) returns "A". Returns the character at a specified posigon in a string, coungng from 1. GetChar("This is a string", 7) returns "s". Returns the stargng posigon in a string of a substring, coungng from 1. InStr("This is a string", "string") returns 11. Returns the stargng posigon in a string of a substring, searching from the end of the string. InStrRev("This is a string", "string") returns 11. Returns the lower- case conversion of a string. LCase("THIS IS A STRING") returns "this is a string". Returns the lee- most specified number of characters of a string. Lee("This is a string", 4) returns "This". Returns the length of a string. Len("This is a string") returns 16. Removes any leading spaces from a string. LTrim(" This is a string") returns "This is a string". Returns a substring from a string, specified as the stargng posigon (coungng from 1) and the number of characters. Mid("This is a string", 6, 4) returns "is a".

20 String manipulagon funcgons (cont.) Replace() Right() RTrim() Str() Space() StrComp() StrReverse() Trim() UCase() Val() Replaces all occurences of a substring in a string. Replace("This is a string", " s", " longer s") returns "This is a longer string" (replaces an "s" preceded by a blank space). Returns the right- most specified number of characters of a string. Right("This is a string", 6) returns "string". Removes any trailing spaces from a string. RTrim("This is a string ") returns "This is a string". Returns the string equivalent of a number. Str(100) returns "100". Fills a string with a given number of spaces. "This" & Space(5) & "string" returns "This string". Compares two strings. Return values are 0 (strings are equal), 1 (first string has the greater value), or - 1 (second string has the greater value) based on sorgng sequence. StrComp("This is a string", "This string") returns - 1. Reverses the characters in a string. StrReverse("This is a string") returns "gnirts a si siht". Removes any leading and trailing spaces from a string. Trim(" This is a string ") returns "This is a string". Returns the upper- case conversion of a string. UCase("This is a string") returns "THIS IS A STRING". Converts a numeric expression to a number. Val( ( )^2 ) returns 36.