Intro to Classes via the C String Class

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Intro to Classes via the C String Class

• November 18, 2002
• CSE103 - Penn State University
• Online at http//www.personal.psu.edu/djh300/cse10
  3/string.ppt
• Prepared by Doug Hogan

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Announcements

• Late drop deadline is Wednesday, 11/20
• Review scores
• If you do not earn a C or better, youll need to
  retake the class for the major.
• 4-credit D GPA impact?
• Dr. Quick has normal office hours this week.
• HW5.dat was modified on Wednesday, 11/13
  (leading 0 in an ID)
• HW5 due next Monday

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Overview

• String class
• Headers
• Creating strings
• Manipulating and comparing strings
• Motivation for Object Oriented Programming
• Strings as objects
• Terminology and theory
• Problems

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The string type

• Alternative to character arrays
• Hides many details
• Easier to manipulate
• Required headers
• include ltstringgt
• using namespace std
• string is part of the C standard library

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Declaring strings

• Uninitialized
• Like primitive data types
• e.g. string myString
• Can then use assignment operator
• e.g. myString this is a string
• Initialized
• Use string keyword, name, and initial value in
  parentheses
• e.g. string myString(a string)

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Input/Output

• cin and cout
• cin stops at whitespace
• getline can be used for reading in strings with
  spaces included
• getline(stream, receivingString)
• example
• cout ltlt Enter a stringgetline(cin, str1)

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Manipulating Characters

• Exactly the same as with arrays of characters!
• Use an index in brackets to get or manipulate
  that character.
• string myString(a string)
• cout ltlt myString0
• prints a
• myString0 A
• changes myString to A string

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Exercises

• Create a string called testString that is
  initially Its 8 a.m. and hes expecting me to
  think
• string testString(Its 8 a.m. and hes expecting
  me to think)
• Change the 8 to a 9.
• testString5 9
• Output the string
• cout ltlt testString

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Operators and strings

• The string class lets you use the following
  operators
• Assignment
• Comparison gt, gt, lt, lt
• Equality , !
• Concatenation
• Example
• if(string1 lt string2) cout ltlt string1 ltlt
  is before ltlt string2 ltlt endl

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Problem (should be familiar)

• Suppose you have these declarations
• string str1 I love
• string str2 computer programming!
• Create a string called str3 from these two
  strings that reads I love computer programming!
• string str3 str1 str2

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A bit of terminology before the fun part

• Well call string variables objects.
• We can operate on strings with functions
• use dot notation
• e.g. objectName.operation()
• said to be sending a message to the string object

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length() message

• length( ) returns the length of the string its
  called on
• ex
• string hello(Hello)
• cout ltlt hello.length() ltlt endl
• prints 5
• Dont forget the parentheses!!
• Must give the string object, then the dot
  operator!!

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Practice

• Given
• string noun
• cin gtgt noun
• Output the length of noun.
• cout ltlt noun.length()

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find() message

• find( ) takes a string as an argument
• returns the index where the argument is found in
  the object its called on
• ex
• string hello(Hello)
• cout ltlt hello.find(ll) ltlt endl
• prints 2
• if the string isnt found, find( ) returns -1

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Substrings substr() message

• Takes two integer arguments
• first is starting character
• second is length
• returns a substring of the given length
• string hello(Hello World)
• cout ltlt hello.substr(6, 5) ltlt endl
• prints World
• goes up to strings length if 2nd argument is too
  short

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Problems

• Given string s1(abcdefghi)
• string s2(s1.substr(4, 3)) What is stored in
  s2?
• Answer efg
• Write a line of code to store the location of the
  letter d from s1 in the following int
• int d
• Answer d s1.find(d)

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Given string s(Any string)Give the result of
each message or what is wrong with it.

• length(s)
• s.length
• s(length)
• s.length()
• find(Any)

• s.find( )
• s.substr(2)
• s.substr(2, 5)
• s.substr(tri)
• s.find(tri)

Modified Self-Check 4-8 from Mercer, Rick.
Computing Fundamentals with C. Wilsonville, OR
Franklin, 1999.
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Given string s(Any string)Give the result of
each message or what is wrong with it.

• length(s)
• no dot notation
• length takes no argument
• s.length
• no parentheses
• s(length)
• parentheses misplaced
• s.length()
• 10
• find(Any)
• no object

• s.find( )
• 3
• s.substr(2)
• not enough arguments
• s.substr(2, 5)
• y str
• s.substr(tri)
• wrong arguments
• s.find(tri)
• 5

Modified Self-Check 4-8 from Mercer, Rick.
Computing Fundamentals with C. Wilsonville, OR
Franklin, 1999.
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More string Messages

• The ones weve discussed are the most useful
• See page 593 of your book for more

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Motivation for classes

• Object-Oriented Programming (OOP)
• Package together a set of related data and
  operations (encapsulation)
• Define a class (abstract data type), or a new
  data type with its operations
• One instance of a class is called an object
• The data and operations of a class are called its
  members.
• string is an example of a class

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Access rights in OOP

• Classes are similar to structs
• Add the notion of access rights
• class member data and operations can be
• public accessible to anyone
• private accessible only to the object
• usually
• data are private
• operations are public

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Information Hiding

• Client (user) has only the information needed to
  use the software.
• Implementer has only the information needed to
  implement the software.
• Communication through pre/post.

Graphic from Headington, Mark A. and David
Riley. Data Abstraction and Structures using C.
Lexington, MAHeath, 1994.
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Information Hiding Applied

• Client can access the public methods of an object
• Sending a message
• Methods can access private data

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An example of a class bankAccount

• Data
• name
• balance
• Operations
• create an account
• withdraw
• deposit
• check balance

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Summary

• string class
• created with string keyword
• operators , gt, lt, defined
• messages length(), find(str), substr(start,
  length) defined
• Object Oriented Programming
• classes and objects
• information hiding

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Homework for next time

• Another motivation for classes is the notion of
  abstraction.
• Find out what the word means.
• Come up with an example of how we use abstraction
  in everyday life.
• Write down an example of a bankAccount object.

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Homework for next time

• Implement the following nonmember function
• string returnedExpression(string inputString)
• // PRE inputString is a line of valid C code
  // (lt80 chars) containing the
  return// keyword and ending with a
  semicolon
• // POST FCTVAL the expression that follows
  // the return keyword, not
  including the// semicolon
• Hint youll need to (and should) use all of the
  string member functions we discussed today.
• Ex returnedExpression( return
  afoo(a-1))
• returns afoo(a-1)
• This PowerPoint is online at http//www.personal.p
  su.edu/djh300/cse103/string.ppt