1. From C-strings to C++ strings: Abstraction at Work
Chapter 5
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

2. Agenda Review: C++ I/O and <string> classes
Where it began: C-strings and their problems
Burying the problem: a simple MString class
(M is for MPC!)
Finding the Algorithm for string operations
Syntax forms for MString operators
Using MString in a typical application
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

3. Chapter 5 Contents— Read it, it’s good!
5.1 The C++ Standard I/O Classes
5.2 The C++ String Types
5.3 Case Study: Text Editing
5.4 Introduction to Pattern Matching (optional)
5.5 Introduction to Data Encryption (optional)
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

4. REVIEW: The C++ Standard I/O Classes
Input viewed as a stream of characters
Flowing from some source into an executing program
Output viewed as a stream of characters
Flowing from program to output device
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

5. The C++ Standard I/O Classes
C++ has <iostream> library
istream for input
ostream for output
These are classes that model data streams
Therefore, objects of these classes
can be passed to functions
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

6. cin is a standard istream object
The istream Class
Models flow of characters from input device to program
Characters enter an istream object
Object transmits characters from device to program
Input operator >>
Extraction operator istreamObject >> variable;
Stream states accessed with functions
.good(), .bad(), .fail(), .eof()
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

7. The ostream Class
Models flow of characters from executing program to output device
Characters enter ostream object
Transmitted to specified output device
Output operator <<
Insertion operator ostreamObject << expression;
Note Table 5-3 in text for Output Stream Operations and Methods
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

8. The ostream Class Standard ostream objects cout for normal output
cerr and clog for error and diagnostic messages
Buffered streams (cout and clog)
Held in the stream until it is flushed
Contrast cerr which is sent to the output device immediately
Can cause confusion when debugging
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

9. File I/O: ifstream, ofstream Classes
FileStream object must be constructed for purpose of receiving/sending I/O to files
Called opening a stream to the file
Stream activities
Declaring ifstream inStream;
Opening ifStream.open(file_name);
Closing ifStream.close();
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

10. Output File Stream Example
#include<fstream>
#include<iostream>
int main() Create a new file stream newfile {
ofstream newfile;
Connect newfile to the file on disk
newfile.open("students.txt");
newfile<<“this is written to the file”;
cout<<“this is written to the monitor”;
newfile.close( );
return 0; Disconnect newfile stream }

11. Input from file of unknown size
ifstream infile(“accounts.txt”);
infile>>acntnum>>balance;
while(infile) {
// process last data
// get next data }
infile.close();
Check stream status after
each read-if “true” process

12. A more compact version Process indefinite list in a file:
while(infile>>acntnum>>balance) {
// process data }
Read data and Check status after each read

13. The I/O Class Hierarchy
Characteristics of OOP hold
Encapsulation
Inheritance
Polymorphism
Note demo of file I/O, Fig 5-1
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

14. Passing Streams to Functions
It may seem odd to pass cin,cout or a filestream to a function
Needed to generalize moving of data
Fairly unique to C++
Always pass by reference (use &) (so we don’t make a copy)
Example: In some Student class…
void Student::input(istream& in);
void Student::output(ostream& out);
in, out are placeholders
for any input or output stream arguments
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

15. Flexibility gained…
Student s1(333, "Rebold", 3.4), s2; ofstream fout("data.txt"); ifstream fin("enrollments.txt"); s1.output(cout);  send to screen s2.input(cin);  read from keyboard s1.output(fout);  send to file “data.txt” s2.input(fin);  input from “enrollments.txt”
But the syntax looks clumsy, so we overload the >> and << operators
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

16. REVIEW: The C++ <string> Class
It’s a class…like you developed last week
Variety of constructors provided for defining strings
Define an empty string String s;
Define a string initialized with another string String s("some other string");
Note further options in text, Table 5-7 String Constructors
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

17. C++ String Member Functions
Editing operations provided such as
appending
inserting
erasing
replacing
swapping
Note table 5-10 in text, String Editing Operations
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

18. Other C++ String Operations
Copiers – make copies of part or all of a string
Operators = and +=
Accessing Individual Characters
Use overloaded subscript operator [ ]
String element accessors
Functions such as find(), find_first_of()
See Table 5-12 in text
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

19. Example <string> Operations
Code string a="the"; a = a + " truth"; a.insert(" whole",3); a.erase(4,1); a.replace("alf",5); Also a.length(); returns a.find("half") returns a.find("t",2) returns
Contents of a
the
the truth
the whole truth
the hole truth
the half truth 13 4 9
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

20. C++ <string> Comparisions
Comparisons
Overloaded operators for <, >, ==, etc.
string a="the", b="whole";
(a<b) is true
(a==b) is false
(a=="the") is true
note that “the” is a string literal
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

21. Why are these useful? Essential for editing/processing documents
Find/replace operations
Count the number of occurrences of a word
Moving files on a website, changing URLs
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

22. Example: Managing a Website
When files are relocated on a server, all webpages linking them have to be changed or links are broken
becomes:
for mpc01c01 to mpc01c20…needs automation!
Insert
Insert
Append
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

23. C-strings It wasn’t always this easy!
Before C++ and object oriented programming, strings in C were represented by cumbersome character arrays:
char name[10];
char name[10] = "Sax Winderhaven"
OOPS! Too big
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

24. C-string Problems Memory management Lack of Features
You always had to count letters to make sure the array was big enough
Lack of Features
Assignment, concatenation (append): yes
Insert, Erase: tough luck!
Comparision: yes, but confusing
Frequent reading of documentation to clarify
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

25. Solving the C-string Problem
C++ with object oriented features allowed for the creation of a new class that works more logically and naturally.
This class, <string>, is built on top of (using) the clumsy C-string operations
This is another example of Abstraction
Building an ideal/logical type that hides the messy details of an inferior, less effective set of tools.
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

26. Today’s Lab Practice working with C-strings
Develop a very simple MString class (i.e. MPC String) using C-string operations
Develop general insert/erase operations (fairly complex array manipulations)
Code comparison and other operators
Use MString to solve text processing applications
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

27. More on C-strings
C-strings use a \0 or “Null character” to indicate end of string (literally a binary 0)
char name[10]="Felix“
When sizing the array, you must leave one extra cell for the null character
Max number of letters in name[] is 9 F e l i x \0
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

28. C-string Operations-- #include<cstring>
strcpy(a,b); copies string b into string a
this is like a=b;
strcat(a,b); appends string b to end of a
this is like a=a+b; OR a+=b;
strlen(s); returns number of letters in s
does not count null character
strcmp(a,b); returns code for <, > or ==
Negative number: a < b is true
Positive number: a > b is true
Zero a == b is true
Note: any capital letter comes before any lowercase letter
i.e.: Z < a = true!
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

29. C-string I/O
Luckily, the << and >> operators are defined for C-strings:
char str[10];
cout<<"Enter a c-string"<<endl;
cin>>str;
cout<<"you entered: "<<str<<endl;
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

30. Hands On Time Practice using C-strings Download and expand week5.zip
Open Lab5Strings folder
run Project.sln (currently with CStringDemo.cpp)
Read and interpret output
Add code where indicated
Experiment with making the array sizes too small
Watch for errors in output
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

31. The basic MString Class (M=MPC)
MString: a new class built using C-string
Constructors: default and explicit
Input and output
read() and print() member functions
insertion << and extraction >> operators
Wrappers around read and print
Private data: a char array (C-string)
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

32. The class declaration This is also a C-string
class MString { public: constructors: MString(); default MString(char *newStr); explicit void print(ostream& out) const; void read(istream& in); private: char myText[100]; a c-string };
This is also a C-string
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

33. Examples using MString members
MString text; default cnstr MString a("hello"); explicit cnstr a.print(cout); prints a to cout cout<<"Enter a string"<<endl; text.read(cin); reads text from cin cout<<"text = "<<text<<endl; operator<< cout<<"enter a new value"<<endl; cin>>text; operator>>
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

34. MString Class Definition
const means print( ) will not change myText
MString Class Definition
MString::MString() { myText[0]=0; } MString::MString(char *newStr) strcpy(myText, newStr);
void MString::print(ostream& out) const { out<<myText; } void MString::read(istream& in) in>>myText;
Empty string
Copy newStr
to myText
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

35. The stream operators declared
These are “free functions” not part of MString
Output:
ostream& operator<<(ostream& out, MString& s);
Input:
istream& operator>>(istream& in, MString& s);
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

36. Logic behind the operators
C++ treats operators as functions:
The expression a+b is really operator+(a,b)
This transformation is handled by the compiler
Expression cout<<a is operator<<(cout,a)
cout is passed to ostream& out
a is passed to MString& s
This explains the red part--
ostream& operator<<(ostream& out, MString& s);
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

37. What about the return type?
When operations are chained: a + b + c
Only one operator is processed at a time:
a + b + c  operator+(a, b)
operator+( , c)
Or, think of it as operator+(operator+(a, b), c)
operator+ must return same data type as a,b
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

38. Above logic also applies to <<,>>
When output operations are chained: cout<< a << b ;
Only one operator is processed at a time:
operator<<(cout, a)
operator<<( , b)
i.e operator<<(operator<<(cout, a), b)
Operator<< must return cout
(thus… ostream& out , alias cout, is returned)
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

39. Operators <<, >> defined
Operators <<,>> are not MString members
May not access myText
Therefore we use the already defined read,print
ostream& operator<<(ostream& out, MString& s)
{ s.print(out);  use the print function for s
return out;  return the output stream }
istream& operator>>(istream& in, MString& s)
{ s.read(in);
return in;
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

40. Hands On Time #2 Practice using MString Switch source files in Project
In Solution Explorer, R-Click CStringDemo choose remove, remove (not delete)
R-Click SourceFiles > Add > Existing Item
choose StringDemo.cpp, hit + on SourceFile
Then open StringDemo.cpp and run
Add code where indicated to complete <<,>>
Is MString improved over the C-string inside it?
Is MString vulnerable to overflow like C-string?
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

41. The Improved MString class
We want to add some additional features:
length append
erase comparison operators (<,>,==)
insert concatenation op ( + )
We will work on StringDemo2.cpp
See Lab5Strings.doc for instructions
Half of the functions are solved in StringDemoHALF.cpp (use as hints if needed)
More hints follow 
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

42. Append Most functions in this lab come in pairs
One for MString, one for C-string
MString b("yes"), c("hello");
c.append(“, sir"); // the version for C-Strings
b.append(c); // the version for M-Strings
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

43. Prototypes for Append Similar pattern (C-string, MString) for most
These functions just use strcat to do the job
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

44. StringDemo2.cpp layout
Class declaration, definition and main in same file
No need to worry about multi-file compilation
Function definitions provided as shells
Some default definitions so compiler is happy
Just complete the definition of the functions
as you go thru the lab
Do not modify main
until you get to Part 7&8
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

45. Length and EraseOneLetter
Example use of functions
MString s("ABCCDEFGHIJKL");
cout<<"String s = "<<s<<" and it's length = "
<<s.length()<<endl;
s.eraseOneLetter(2);
cout<<"After calling s.eraseOneLetter(2);"<<endl
<<" s = "<<s<<" and is now length "<<s.length()<<endl;
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

46. Prototypes
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

47. Algorithm Development
length() is easy (return strlen(myText);)
What about eraseOneLetter(2) ??
Before, s= ABCCDEFGHIJKL
After, s= ABCDEFGHIJKL
Some kind of shifting required
for loop
If we try to guess it, we’ll most likely fail
Is there a method to developing a correct alg.?
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

48. Algorithm Method Start with a specific function call
With specific data & arguments, i.e.
MString s("ABCCDEFGHIJKL");
s.eraseOneLetter(2);
2. Draw a picture of data structure
Before and after function is called
Required for any assistance from instructor!!
Write out statements without any loop
Generalize into loop that works for all args
Add assert to prevent bad calls
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

49. Before & After: eraseOneLetter()
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

50. Non-looping Specific Solution
For our particular case, index (letter to erase) is 2
Statements to solve our example case:
myText[2] = myText[3];
myText[3] = myText[4];
myText[4] = myText[5]; …
myText[12] = myText[13];  why is this the last copy?
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

51. Formulate the General Soln
We can now see that
2 in the first statement is really index.
13 in the last statement is length of the string
Therefore we can derive:
void MString::eraseOneLetter(int index) {
for (int k=index; k < strlen(myText); k++
myText[k] = myText[k+1]; }
And check desired statements are produced
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

52. Assert – easy debug check
For eraseOneLetter( ) we should verify:
index >= 0
index < strlen(myText)
This becomes first line of function:
assert(index>=0 && index < strlen(myText));
Bad call: s.eraseOneLetter(-1)
Aborts program with “assertion failed” message
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

53. erase() Example use: s.erase(2,3);
cout<<"After erasing 3 letters starting at cell 2,"<<endl
<<" s = "<<s<<" and is now length "<<s.length()<<endl;
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

54. 2 ways to erase()
You can formulate the general solution using just a for loop as above
Fastest execution speed
You can use the eraseOneLetter function repeatedly in a for-loop
Simpler code
Slower to run
3. Either way you need a picture!
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved

55. Continue thru Lab5Strings.doc
Solve for each member function
Remember, hints are in StringDemo2HALF
For 7 and 8, read files essay.txt and webpages.htm in same folder
FYI: MString with Dynamic memory shown in
StringDemo2Dynamic.cpp
Nyhoff, ADTs, Data Structures and Problem Solving with C++, Second Edition, © 2005 Pearson Education, Inc. All rights reserved