Strings, Characters, and Regular Expressions

1
Strings, Characters, andRegular Expressions
2
String Theory
Assignment
3
Fundamentals of Characters Strings

• A character constant is a character that is
  represented as an integer
• value and this value is known as a character
  code.
• z corresponds to the integer value 122. \n
  corresponds
• to integer value 10.
• The unicode character set is an international
  character set that extends ASCII.
• See Appendix G in your textbook.
• A string is a series of characters treated as a
  single unit.
• A string is an object of class String in the
  System namespace.
• String constants are sequences of characters in
  double quotation marks
• Gabe Lujan
• (972) 599 1698
• Weve used string color blue
• string file C\\Inetpub\\wwwroot\\Login\\login
  .mdb
• string file _at_C\Inetpub\wwwroor\Login\login.m
  db

4
Fundamentals of Characters Strings

• Class String provides eight constructors for
  initializing strings.
• Code Example for String Constructors

Note ArgumentOutOfRangeException
5
String Constructors

• Examples
• string mystr, string1, string2, string3,
  string4, output
• char characterArray a, n, n,i,
  v,e,r,s,a,r,y
• mystr This is my string.
• string1 mystr
• string2 new string( characterArray )
• string3 new string( characterArray 6, 3 )
• string4 new string( C, 5)
• output string1 string2

6
String Indexer, Length Property, CopyTo Method

• This code example illustrates the use of the
  string indexer. It
• facilitates the retrieval of any character in
  the string.
• The Length property returns the length of a
  string.
• The CopyTo method copies a specified number of
  characters from a string into a character array.

7
Comparing Strings

• In C, one string can be greater than or less
  than another string. The
• alphabetic symboloy of a character string
  ultimately maps to an integer
• representation for computer representation.
• Class string provides several ways to compare
  strings.
• Equals
• CompareTo
• Equals uses a lexicographical comparison
• a Unicode character by Unicode character
• comparison.
• StringCompare Code Example
• Code Example shows how to test
• if a string starts with or ends with
• a given string.

8
GetHashCode

• Often it is necessary to store strings and other
  data types in a manner that
• enables the information to be found quickly.
• Hash tables make information look-up easy. A hash
  table stores an
• object by performing a special calculation on
  that object, which produces a hash code.
• The object then is stored at a location in the
  hash table determined by the calculated hash
  code.
• When a program needs to retrieve the information,
  the same calculation is performed, generating the
  same hash code.
• Class Object defines method GetHashCode. Class
  String overrides this method to provide a
  hashcode distribution based on the contents of a
  string.
• GetHashCode Example

9
Locating Characters and Substrings in Strings

• Code Example
• The input string is abcdefghijklmabcdefghijklm
• What are the outputs from the following methods
• IndexOf
• IndexOfAny
• LastIndexOf
• LastIndexOfAny

In the LastIndexOf method, the search moves from
the end of the string toward the start of the
string.
10
Extracting Substrings

• Class String provides two Substring methods,
  which are used to
• create a new string by copying part of an
  existing string.
• 2. Code Example showing two uses of Substring()
  method.

Input text string is abcdefghijklmabcdefghijklm
11
Concatenating Strings

• The operator is one way of concatenating
  strings.
• The static method Concat of class String
  concatenates two
• strings and returns a new string. The input
  strings are not
• modified.
• Code Sample

12
Miscellaneous String Methods

• Note the usages of the following String methods
• Replace
• ToLower
• ToUpper
• Trim
• ToString
• Code Sample

13
Class StringBuilder

• The String class provides many capabilities for
  processing strings.
• However, a strings contents can never change.
  Operations that
• concatenate strings are really assigning string
  references to
• newly created strings. For instance, creates
  a new string and
• assigns the initial string reference to the
  newly created string.
• Class StringBuilder comes from System.Text
  namespace.
• Used to create mutable strings.
• StringBuilder can store a fixed amount of
  characters, but their capacities can also expand
  to accommodate additional characters.
• Concatenation can be achieved using methods
  Append() and AppendFormat().
• Code Sample Showing StringBuilder Constructors

14
StringBuilder Indexer, Length Capacity
Properties, and EnsureCapacity Method

• StringBuilder provides Length and Capacity
  properties to return
• the number of characters in a StringBuilder and
  the number
• of characters that a StringBuilder can store
  without allocating
• more memory. These properties can also increase
  or decrease
• the capacity of the StringBuilder.
• Method EnsureCapacity allows programmers to
  guarantee that a StringBuilder has a capacity
  that reduces the number of times the capacity
  must be increased. It doubles the StringBuilder
  instances current capacity. If the doubled value
  is greater than the value that the programmer
  wishes to ensure, it becomes the new capacity.
  Otherwise, EnsureCapacity alters the capacity to
  make it one more than the requested number,
• Example If the current capacity is 17 and we
  wish to make it 40, 17 x 2 lt 40, so the call
  will result in a new capacity of 41. If the
  current capacity is 23 and we wish to make it 40,
  23 will be multiplied by 2 to result in a new
  capacity of 46.
• Code Example

15
StringBuilder Append AppendFormat Methods

• Class StringBuilder provides 19 overloaded Append
  methods that
• allow various data-type values to be added to
  the end of a
• StringBuilder.
• C provides versions for each of the primitive
  data types and for character arrays.
• Each of the methods takes an argument, converts
  it to a string, and appends it to the
  StringBuilder.
• Code Example
• AppendFormat

16
StringBuilder Insert, Remove, Replace Methods

• StringBuilder provides 18 overloaded Insert
  methods to allow various datatype values
• to be inserted at any position in a
  StringBuilder. Versions are provided for the
• primitive data types, character arrays, strings
  and objects.
• Each method takes its second argument, converts
  it to a string, and inserts the string
• into the StringBuilder in front of the index
  specified by the first argument. If the index is
  wrong, the program will throw an
  ArgumentOutOfRangeException.
• Class StringBuilder provides method Remove for
  deleting any portion of a StringBuilder. The two
  arguments include the index at which to start the
  remove and the number of characters to delete.
• Insert and Remove Code Example
• The Replace method searches for a specified
• string or character and substitutes another
• string or character in its place.
• 6. Code Example for Replace

17
Char Methods

• C provides a data type called a structure that
  is similar to a class. Structures
• are a value type and they include methods and
  properties.
• 2. Structures are created using the keyword
  struct.
• The primitive datatypes such as Int, Char, and
  Float are aliases for structures from class
  ValueType
• Some Char methods
• IsDigit()
• IsLetter()
• IsLetterOrDigit()
• IsLower()
• IsPunctuation()
• IsSymbol()
• Code Example

18
Card Shuffling Dealing Simulation

• This program example uses random number
  generation to develop a program
• that simulates the shuffling and dealing of
  cards.
• What to look for
• a) The Card constructor
• b) The deck data type how each of the 52 cards
  is constructed.
• c) The shuffle method
• d) The deal method.
• Card Code
• Deck of Cards

19
Regular Expressions

• Regular expressions are specially formatted
  strings used to find patterns in text
• and can be useful during information validation.
• Example Zip code must contain 5 digits, last
  name must start with a
• capital letter.
• Class Regex (Syste.Text.RegularExpressions
  namespace) represents an immutable expression. It
  contains static methods so no class
  instantiation is needed.
• Class Regex provides a method Match which returns
  an object of class Match that represents a single
  regular expression match.
• Method Matches finds all matches of a regular
  expression in an arbitrary string and returns a
  MatchCollection object.
• A word-character is any alphanumeric character or
  underscore. A whitespace character is a space,
  tab, carriage return, newline, or form feed. A
  digit is any numeric character.

20
Regular Expressions

• Regular expression code example
• This code example takes birthdays and tries to
  match them to a regular expression. The
  expression matches only birthdays that do not
  occur in April and that belong to people whose
  names begin with J.

• What to look for
• The . character matches any single character
  except a newline character.
• . matches any number of unspecified
  characters.
• will match one or more occurances. A will
  match A and and empty string.
• aeiou can be used to match any vowel.
• 0-35-9 matches only digits in the ranges
  specified by the pattern. This pattern matches
  any digit other than 4.
• - the expression accepts any character other
  than the ones indicated. 4 is not the same
  as 0-35-9 Why? The former also matches any
  non-digit.

21
Regular Expressions

• 7. (?) matches zero or one occurrences of the
  expression that it quantifies.
• 8. (n) matches exactly n occurrences of the
  expression it quantifies
• 9. (n,) matches at least n occurrences.
• 10. (n,m) matches between n and m occurrences.
• 11. Code Example
• What to Look For
• The program checks to see if an entire string
• conforms to a regular expression. and
• characters match the positions at the beginning
• and end of a string. This forces the regular
• expression to match the entire string and not
• return a match if a substring matches
  successfully.
• \s matches any white space
• \d5 matches any five digits.
• matches the expression to its left or to its
  right.
• Hi (John Jane) matches Hi John and Hi Jane

22
Regular Expressions

• Sometimes it is necessary to replace parts of a
  string with another or split a string according
  to a regular expression.
• Regex provides static instance methods of Replace
  and Split.
• Code Example

23
Assignment

• Write a C GUI program using a menu that replaces
  all of the
• occurrences of a word in a file with another
  word. Display
• the file before and after using Notepad. The
  program
• should prompt the user for the input file name
  and
• the string to be substituted for.