1.  2002 Prentice Hall. All rights reserved. 1 Chapter 15 – Strings, Characters and Regular Expressions Outline 15.1Introduction 15.2 Fundamentals of Characters and Strings 15.3 String Constructors 15.4 String Length and Chars Properties and CopyTo Method 15.5 Comparing Strings 15.6 String Method GetHashCode 15.7 Locating Characters and Substrings in String s 15.8 Extracting Substrings from String s 15.9 Concatenating String s 15.10 Miscellaneous String Methods 15.11 Class StringBuilder 15.12 StringBuilder Indexer, Length and Capacity Properties, and EnsureCapacity Method 15.13 StringBuilder Append and AppendFormat Methods 15.14 StringBuilder Insert, Remove and Replace Methods 15.15 Char Methods 15.16 Card Shuffling and Dealing Simulation 15.17 Regular Expressions and Class Regex

2.  2002 Prentice Hall. All rights reserved. 2 15.1Introduction String and character processing –Useful in a variety of applications –String and Char classes ( System ) General string and character processing, storage –StringBuilder class ( System.Text ) Facilitates efficient construction of strings –Regex and Match classes ( System.Text.RegularExpressions ) Powerful pattern matching capabilities

3.  2002 Prentice Hall. All rights reserved. 3 15.2 Fundamentals of Characters and Strings Characters –Fundamental building blocks of source code –Character constants Represented using double quotes and c character All characters correspond to an integer character code –Unicode character set –ChrW function converts Unicode values to characters

4.  2002 Prentice Hall. All rights reserved. 4 15.2 Fundamentals of Characters and Strings Strings –A series of characters treated as a single unit –String literals –Objects of class String Upcoming example: String constructors

5.  2002 Prentice Hall. All rights reserved. Outline 5 StringConstructo r.vb ChrW function Calls to String constructors 1 ' Fig. 15.1: StringConstructor.vb 2 ' Demonstrating String class constructors. 3 4 Imports System.Windows.Forms 5 6 Module modStringConstructor 7 8 Sub Main() 9 Dim characterArray As Char() 10 Dim output As String 11 Dim quotes As String = ChrW(34) 12 Dim originalString, string1, string2, string3, _ 13 string4 As String 14 15 characterArray = New Char() {"b"c, "i"c, "r"c, _ 16 "t"c, "h"c, " "c, "d"c, "a"c, "y"c} 17 18 ' string initialization 19 originalString = "Welcome to VB.NET Programming!" 20 string1 = originalString 21 string2 = New String(characterArray) 22 string3 = New String(characterArray, 6, 3) 23 string4 = New String("C"c, 5) 24 25 output = "string1 = " & quotes & string1 & quotes & _ 26 vbCrLf & "string2 = " & quotes & string2 & quotes & _ 27 vbCrLf & "string3 = " & quotes & string3 & quotes & _ 28 vbCrLf & "string4 = " & quotes & string4 & quotes 29 30 MessageBox.Show(output, "String Class Constructors", _ 31 MessageBoxButtons.OK, MessageBoxIcon.Information) 32 End Sub ' Main 33 34 End Module ' modStringConstructor ChrW converts Unicode value 34 to double quote character, " Creates an array of type Char. Suffix c required when using Option Strict Creates a literal String object string1 and originalString reference same literal String object Creates a new String object containing a copy of characters in characterArray Copies characters from characterArray, starting at the index indicated by second argument and continuing for the number of characters indicated by the third argument Creates a String with length indicated by the second argument, filled with copies of the character passed as the first argument Each instance of variable quotes represents a double quote character, "

6.  2002 Prentice Hall. All rights reserved. Outline 6 StringConstructo r.vb

7.  2002 Prentice Hall. All rights reserved. Outline 7 StringMiscellane ous.vb 1 ' Fig. 15.2: StringMiscellaneous.vb 2 ' Using properties Length and Chars 3 ' of class string. 4 5 Imports System.Windows.Forms 6 7 Module modMiscellaneous 8 9 Sub Main() 10 Dim string1, output As String 11 Dim characterArray As Char() 12 Dim i As Integer 13 Dim quotes As String = ChrW(34) 14 15 string1 = "hello there" 16 characterArray = New Char(5) {} 17 18 ' output string 19 output = "string1: " & quotes & string1 & quotes 20 21 ' test Length property 22 output &= vbCrLf & "Length of string1: " & string1.Length 23 24 ' loop through characters in string1 and display 25 ' reversed 26 output &= vbCrLf & "The string reversed is: " 27 28 For i = string1.Length - 1 To 0 Step -1 29 output &= string1.Chars(i) 30 Next 31 32 ' copy characters from string1 into characterArray 33 string1.CopyTo(0, characterArray, 0, 5) 34 output &= vbCrLf & "The character array is: " 35 Length property returns number of characters in string Length property used to loop backwards through characters in string1 Returns the character at the position indicated by the integer argument Copies characters from a string into an array. Respectively, arguments represent: the location at which to begin copying, the destination array, the index into which to place the first character copied and the number of characters to copy

8.  2002 Prentice Hall. All rights reserved. Outline 8 StringMiscellane ous.vb 36 For i = 0 To characterArray.Length - 1 37 output &= characterArray(i) 38 Next 39 40 MessageBox.Show(output, "Demonstrating String" & _ 41 " properties Length and Chars", _ 42 MessageBoxButtons.OK, MessageBoxIcon.Information) 43 End Sub ' Main 44 45 End Module ' modMiscellaneous Displays contents of characterArray

9.  2002 Prentice Hall. All rights reserved. 9 15.5 Comparing Strings Lexicographical comparison –Similar to alphabetization –Each character corresponds to a number –Character codes compared from beginning of string –Methods Equals, CompareTo and = operator Reference comparison –Determines whether two references contain the same object –Is operator –Upcoming example: String test to determine equality

10.  2002 Prentice Hall. All rights reserved. Outline 10 StringCompare.vb Call to Equals = operator 1 ' Fig. 15.3: StringCompare.vb 2 ' Comparing strings. 3 4 Imports System.Windows.Forms 5 6 Module modCompare 7 8 Sub Main() 9 Dim string1 As String = "hello" 10 Dim string2 As String = "good bye" 11 Dim string3 As String = "Happy Birthday" 12 Dim string4 As String = "happy birthday" 13 Dim output As String 14 Dim quotes As String = ChrW(34) 15 16 ' output values of four Strings 17 output = "string1 = " & quotes & string1 & quotes & _ 18 vbCrLf & "string2 = " & quotes & string2 & quotes & _ 19 vbCrLf & "string3 = " & quotes & string3 & quotes & _ 20 vbCrLf & "string4 = " & quotes & string4 & quotes & _ 21 vbCrLf & vbCrLf 22 23 ' test for equality using Equals method 24 If (string1.Equals("hello")) Then 25 output &= "string1 equals " & quotes & "hello" & _ 26 quotes & vbCrLf 27 28 Else 29 output &= "string1 does not equal " & quotes & _ 30 "hello" & quotes & vbCrLf 31 End If 32 33 ' test for equality with = 34 If string1 = "hello" Then 35 output &= "string1 equals " & quotes & "hello" & _ Method Equals performs case- sensitive lexicographical comparison Equality operator produces same results as method Equals

11.  2002 Prentice Hall. All rights reserved. Outline 11 StringCompare.vb Call to Equals Call to CompareTo 36 quotes & vbCrLf 37 Else 38 output &= "string1 does not equal " & quotes & _ 39 "hello" & quotes & vbCrLf 40 End If 41 42 ' test for equality comparing case 43 If (String.Equals(string3, string4)) Then 44 output &= "string3 equals string4" & vbCrLf 45 Else 46 output &= "string3 does not equal string4" & vbCrLf 47 End If 48 49 ' test CompareTo 50 output &= vbCrLf & "string1.CompareTo(string2) is " & _ 51 string1.CompareTo(string2) & vbCrLf & _ 52 "string2.CompareTo(string1) is " & _ 53 string2.CompareTo(string1) & vbCrLf & _ 54 "string1.CompareTo(string1) is " & _ 55 string1.CompareTo(string1) & vbCrLf & _ 56 "string3.CompareTo(string4) is " & _ 57 string3.CompareTo(string4) & vbCrLf & _ 58 "string4.CompareTo(string3) is " & _ 59 string4.CompareTo(string3) & vbCrLf & vbCrLf 60 61 MessageBox.Show(output, "Demonstrating string" & _ 62 " comparisons", MessageBoxButtons.OK, _ 63 MessageBoxIcon.Information) 64 End Sub ' Main 65 66 End Module ' modCompare Shared method Equals compares two String s lexicographically Method CompareTo performs a lexicographical comparison. Returns 0 if String s are equal. Returns –1 if argument String is greater. Returns 1 if calling String is greater

12.  2002 Prentice Hall. All rights reserved. Outline 12 StringCompare.vb

13.  2002 Prentice Hall. All rights reserved. Outline 13 StringStartEnd.v b Call to StartsWith Call to EndsWith 1 ' Fig. 15.4: StringStartEnd.vb 2 ' Demonstrating StartsWith and EndsWith methods. 3 4 Imports System.Windows.Forms 5 6 Module modStartEnd 7 8 Sub Main() 9 Dim strings As String() 10 Dim output As String = "" 11 Dim i As Integer 12 Dim quotes As String = ChrW(34) 13 14 strings = New String() {"started", "starting", _ 15 "ended", "ending"} 16 17 ' test every string to see if it starts with "st" 18 For i = 0 To strings.GetUpperBound(0) 19 20 If strings(i).StartsWith("st") Then 21 output &= quotes & strings(i) & quotes & _ 22 " starts with " & quotes & "st" & quotes & vbCrLf 23 End If 24 25 Next 26 27 output &= vbCrLf 28 29 ' test every string to see if it ends with "ed" 30 For i = 0 To strings.GetUpperBound(0) 31 32 If strings(i).EndsWith("ed") Then 33 output &= quotes & strings(i) & quotes & _ 34 " ends with " & quotes & "ed" & quotes & vbCrLf Method StartsWith determines whether the beginning of a String matches the String passed as an argument Method EndsWith determines whether the end of a String matches the String passed as an argument

14.  2002 Prentice Hall. All rights reserved. Outline 14 StringStartEnd.v b 35 End If 36 37 Next 38 39 MessageBox.Show(output, "Demonstrating StartsWith and" & _ 40 " EndsWith methods", MessageBoxButtons.OK, _ 41 MessageBoxIcon.Information) 42 End Sub ' Main 43 44 End Module ' modStartEnd

15.  2002 Prentice Hall. All rights reserved. Outline 15 StringIndexMetho ds.vb Calls to IndexOf Calls to LastIndexOf 1 ' Fig. 15.6: StringIndexMethods.vb 2 ' Using String searching methods. 3 4 Imports System.Windows.Forms 5 6 Module modIndexMethods 7 8 Sub Main() 9 Dim letters As String = "abcdefghijklmabcdefghijklm" 10 Dim output As String 11 Dim searchLetters As Char() = New Char() {"c"c, "a"c, "$"c} 12 13 ' test IndexOf to locate a character in a string 14 output &= """c"" is located at index " & _ 15 letters.IndexOf("c"c) 16 17 output &= vbCrLf & """a"" is located at index " & _ 18 letters.IndexOf("a"c, 1) 19 20 output &= vbCrLf & """$"" is located at index " & _ 21 letters.IndexOf("$"c, 3, 5) 22 23 ' test LastIndexOf to find a character in a string 24 output &= vbCrLf & vbCrLf & "Last ""c"" is located at " & _ 25 "index " & letters.LastIndexOf("c"c) 26 27 output &= vbCrLf & "Last ""a"" is located at index " & _ 28 letters.LastIndexOf("a"c, 25) 29 30 output &= vbCrLf & "Last ""$"" is located at index " & _ 31 letters.LastIndexOf("$"c, 15, 5) 32 Alternative to ChrW(34). Two consecutive double quotation marks ( "" ) produces one double quotation mark in the String Finds first occurrence of c in String letters Finds first occurrence of a in letters, starting at position 1 Searches for occurrence of $ in letters starting at position 3 and searching for 5 characters. Returns –1, indicating there is no occurrence Finds last occurrence of c Finds last occurrence of a by searching back from position 25 Finds last occurrence of $ searching back from position 15 for 5 characters. Returns -1

16.  2002 Prentice Hall. All rights reserved. Outline 16 StringIndexMetho ds.vb Calls to IndexOf Calls to LastIndexOf Calls to IndexOfAny 33 ' test IndexOf to locate a substring in a string 34 output &= vbCrLf & vbCrLf & """def"" is located at" & _ 35 " index " & letters.IndexOf("def") 36 37 output &= vbCrLf & """def"" is located at index " & _ 38 letters.IndexOf("def", 7) 39 40 output &= vbCrLf & """hello"" is located at index " & _ 41 letters.IndexOf("hello", 5, 15) 42 43 ' test LastIndexOf to find a substring in a string 44 output &= vbCrLf & vbCrLf & "Last ""def"" is located " & _ 45 "at index " & letters.LastIndexOf("def") 46 47 output &= vbCrLf & "Last ""def"" is located at " & _ 48 letters.LastIndexOf("def", 25) 49 50 output &= vbCrLf & "Last ""hello"" is located at " & _ 51 "index " & letters.LastIndexOf("hello", 20, 15) 52 53 ' test IndexOfAny to find first occurrence of character 54 ' in array 55 output &= vbCrLf & vbCrLf & "First occurrence of ""c""," & _ 56 " ""a"" or ""$"" is located at " & _ 57 letters.IndexOfAny(searchLetters) 58 59 output &= vbCrLf & "First occurrence of ""c"", ""a"" or " & _ 60 """$"" is located at " & _ 61 letters.IndexOfAny(searchLetters, 7) 62 63 output &= vbCrLf & "First occurrence of ""c"", ""a"" or " & _ 64 """$"" is located at " & _ 65 letters.IndexOfAny(searchLetters, 20, 5) 66 Whereas previous calls to IndexOf searched for a character, this call finds the substring "def" Searches for "def" starting at position 7 Searches for "hello" starting at position 5, continuing for 15 characters Searches from end of String to find last occurrence of "def" Searches back from position 25 Searches back from position 20 for 15 characters Searches for first occurrence of any character in searchLetters array Searches for first occurrence of a character in searchLetters, starting at position 7 Searches for 5 characters starting at position 20

17.  2002 Prentice Hall. All rights reserved. Outline 17 StringIndexMetho ds.vb Calls to LastIndexOfAny 67 ' test LastIndexOfAny to find first occurrence of character 68 ' in array 69 output &= vbCrLf & vbCrLf & "Last occurrence of ""c""," & _ 70 " ""a"" or ""$"" is located at " & _ 71 letters.LastIndexOfAny(searchLetters) 72 73 output &= vbCrLf & "Last occurrence of ""c"", ""a"" or " & _ 74 """$"" is located at " & _ 75 letters.LastIndexOfAny(searchLetters, 1) 76 77 output &= vbCrLf & "Last occurrence of ""c"", ""a"" or " & _ 78 """$"" is located at " & _ 79 letters.LastIndexOfAny(searchLetters, 25, 5) 80 81 MessageBox.Show(output, _ 82 "Demonstrating String class index methods") 83 End Sub ' Main 84 85 End Module ' modIndexMethods Searches for last occurrence of any in an array of characters Searches back from position 1 Searches back from position 25 for 5 characters

18.  2002 Prentice Hall. All rights reserved. Outline 18 StringIndexMetho ds.vb

19.  2002 Prentice Hall. All rights reserved. Outline 19 SubString.vb Call to Substring Call to Substring 1 ' Fig. 15.7: SubString.vb 2 ' Demonstrating the String Substring method. 3 4 Imports System.Windows.Forms 5 6 Module modSubString 7 8 Sub Main() 9 Dim letters As String = "abcdefghijklmabcdefghijklm" 10 Dim output As String 11 Dim quotes As String = ChrW(34) 12 13 ' invoke SubString method and pass it one parameter 14 output = "Substring from index 20 to end is " & _ 15 quotes & letters.Substring(20) & quotes & vbCrLf 16 17 ' invoke SubString method and pass it two parameters 18 output &= "Substring from index 0 to 6 is " & _ 19 quotes & letters.Substring(0, 6) & quotes 20 21 MessageBox.Show(output, _ 22 "Demonstrating String method Substring") 23 End Sub ' Main 24 25 End Module ' modSubString Method Substring returns a new String object generated by copying characters from the calling String. One argument version returns characters between position indicated and end of String Two argument version returns substring starting at position indicated by first argument with length indicated by second argument

20.  2002 Prentice Hall. All rights reserved. Outline 20 SubConcatination.vb Call to Concat 1 ' Fig. 15.8: SubConcatination.vb 2 ' Demonstrating String class ConCat method. 3 4 Imports System.Windows.Forms 5 6 Module modSubConcat 7 8 Sub Main() 9 Dim string1 As String = "Happy " 10 Dim string2 As String = "Birthday" 11 Dim output As String 12 13 output = "string1 = """ & string1 & """" & _ 14 vbCrLf & "string2 = """ & string2 & """" 15 16 output &= vbCrLf & vbCrLf & _ 17 "Result of String.Concat(string1, string2) = " & _ 18 String.Concat(string1, string2) 19 20 MessageBox.Show(output, _ 21 "Demonstrating String method Concat") 22 End Sub ' Main 23 24 End Module ' modSubConcat Shared method Concat returns a new String object containing the combined characters from both original String s

21.  2002 Prentice Hall. All rights reserved. Outline 21 StringMiscellane ous.vb Call to Replace Call to ToUpper Call to ToLower 1 ' Fig. 15.9: StringMiscellaneous.vb 2 ' Demonstrating String methods Replace, ToLower, ToUpper, Trim, 3 ' and ToString. 4 5 Imports System.Windows.Forms 6 7 Module modStringMiscellaneous 8 9 Sub Main() 10 Dim string1 As String = "cheers!" 11 Dim string2 As String = "GOOD BYE " 12 Dim string3 As String = " spaces " 13 Dim output As String 14 Dim quotes As String = ChrW(34) 15 Dim i As Integer 16 17 output = "string1 = " & quotes & string1 & quotes & _ 18 vbCrLf & "string2 = " & quotes & string2 & quotes & _ 19 vbCrLf & "string3 = " & quotes & string3 & quotes 20 21 ' call method Replace 22 output &= vbCrLf & vbCrLf & "Replacing " & quotes & "e" & _ 23 quotes & " with " & quotes & "E" & quotes & _ 24 " in string1: " & quotes & string1.Replace("e"c, "E"c) & _ 25 quotes 26 27 ' call ToLower and ToUpper 28 output &= vbCrLf & vbCrLf & "string1.ToUpper() = " & _ 29 quotes & string1.ToUpper() & quotes & vbCrLf & _ 30 "string2.ToLower() = " & quotes & string2.ToLower() & _ 31 quotes 32 Method Replace replaces every instance of the character indicated by the first argument with the second argument Method ToUpper creates a new String with all lowercase characters converted to uppercase Converts all uppercase characters to lowercase

22.  2002 Prentice Hall. All rights reserved. Outline 22 StringMiscellane ous.vb Call to Trim Call to ToString 33 ' call Trim method 34 output &= vbCrLf & vbCrLf & "string3 after trim = " & _ 35 quotes & string3.Trim() & quotes 36 37 ' call ToString method 38 output &= vbCrLf & vbCrLf & "string1 = " & _ 39 quotes & string1.ToString() & quotes 40 41 MessageBox.Show(output, _ 42 "Demonstrating Miscellaneous String Methods") 43 End Sub ' Main 44 45 End Module ' modStringMiscellaneous Method Trim returns a copy of the calling String with leading and trailing whitespace characters removed Method ToString is provided for class String because String is derived from class Object