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Dr. Ahmad R. Hadaegh

A.R. Hadaegh National Universi ty (NU ) Page 1

The C++String Class

The contents of this particular lecture prepared by the instructors at the

University of Manitoba in Canada and modified by Dr. Ahmad Reza Hadaegh

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What is a Character String?

- This is exactly what the C++ string class does

for us - lets us declare variables as STRINGS,

and not care how they are implemented

- they are in fact not implemented in precisely this

manner in C++, but the beauty of it is, that doesn't

affect us!

- Strings are an Abstract Data Type. We will have

general operations that will do specific things

- And we COULD implement those operations

ourselves with some scheme; we just don't have to!

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The C++ String Class

- To declare a variable of type string:

#include // declaring 3 variables of type string

string play, author, mainCharacter;

- We can assign to strings:

play = Hamlet;

author = W. Shakespeare;mainCharacter = play;// assign to string

-Notice: we dont worry about the length!

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String Class I/O

- Do I/O on a string in a natural way:

string name = Jim Anderson;

cout

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7/40Dr. Ahmad R. HadaeghA.R. Hadaegh National Universi ty (NU ) Page 7

String Class I/O

- String class input:

cin >> name;// reads first word from input stream

cout > skips over whitespace characters, reads next

word (until next whitespace character)

- Notice the difference

> read in the next (whitespace delimited) word

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String Class I/O- A simple program to copy one file to another

#include #include

int main()

{

string line;

ifstream infile; ofstream outfile;

infile.open("infile.txt");

outfile.open("outfile.txt");

getline(infile,line);

while (!infile.eof()) {

outfile

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String Element Access

- String bounds are checked:

- access a string at an index beyond the last

character (or

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String Element Access

- Example of "size" function:

string name=Adam Smith";

cout

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Objects

- An object is an abstract grouping of dataand operations

- Imagine a CAR object - all the facts about

your car- It's green, it's a 4 door, it's rusty,

- COMBINED with all the operations

- you can DRIVE my car

- you can STOP my car

- you an TURN- LEFT or TURN- RIGHT

- We will talk about how we build these later!

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String Element Access- Example: Reversing a string

int main(){

string line;

char tmp;

int i;

cout

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Strings as Parameters

- Strings behave "normally" when used as

parameters

- can be passed by value, or reference (with &)

- unlike C-style strings, or arrays of chars for

that matter, which are always passed by

reference

- can still use size() inside a function - the whole

object gets passed, all members are accessible

- can be returned from functions- unlike C-style strings which can't be easily

returned from functions

- Essentially, strings behave like ints when

used as parameters/return values

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Strings as Value Parameters

- Call this function as follows:

string line1, line2;

line1= This is Adam Smith";

line2= reverse( line1);

cout

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Strings as Reference Parameters

- Try out reverse2:

string line1, line2;

line1="This is Mr. Brown";

cout

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String Operators: Concatenation

- Another operator provided by the string classlibrary is concatenation using +:

string word1, word2, word3, line="I ";

word1="am "; word2=an "; word3="instructor in this college" ;

line= line+ word1 + word2 + word3;

cout

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Dr. Ahmad R. HadaeghA.R. Hadaegh National Universi ty (NU ) Page 25

String Operators: Concatenation

- This is an example of operator overloading

- operator + is used for ints, floats, and strings

- has a different meaning in each case

- operator + for strings is actually not part of C++

- it is part of the C++ class library

- you can (in the future) define your own

meanings for various operators

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Overloading Shortcuts

- The reason I bring this up is that += isoverloaded for strings as well

- you can write:

line += word1+ word2+ word3;

- same meaning asline = line+ word1+ word2+ word3;

- The others don't make sense for strings, but

you may see them overloaded for othertypes

- and as already stated possible to overload

them yourself

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String Member Functions- The string class provides a number of useful

member functions

- Object-Oriented-Programming (OOP)

- recall that member functions of a class are

functions which operate on objects of that class

(really part of the object!)

- sometimes called "Methods- notation is:

VariableName.FunctionName(...)

- the function FunctionName operates on the

object (string) VariableName

- may have some other parameters as well

(e.g. fin.open() for file streams)

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String Member Functions

- Substrings:

- the string class provides the substr memberfunction to extract substrings from a string

- Specification:

string str1, str2;str2= str1.substr( first, numchars);

- str2 is assigned the substring of str1 starting

at position first and containing numchars

characters

- if str1 does not have numchars chars after

first, str2 just copies up to end of str1

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- Substrings (continued)

- Example:string line="This course is not that hard guys";string line2, line3;line2 = line.substr(5,6);line3 = line.substr(15,8);cout

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- Substring replacement- The substr function does not modify its object

- If you want to change a substring, use the

replace function

- Specification:

string str1, str2;

str1.replace( first, numchars, str2);

- Action:- replace chars in str1, starting at index first

and extending numchars positions, with the

string str2.

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- Example

string line="Let us now replace some text.";

line.replace( 4, 10, "it snow. P");

cout

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- Can replace substrings with shorter orlonger substrings

- the length of the string will change

- Example:

string line="Let us now replace some text.";

cout

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- Erase:

- Specification:

string line;

line.erase( start, numchars);

- delete numchars chars starting at index start

- functionally same as:

line.replace( start, numchars, "");

- "" is the null string (length 0)

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- Examples:

string line="Walking on the sun is hot";

line.erase(15,4);

cout

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String Search Functions

- The string class also offers a very useful

searching function: find

- Specification:

string line, pattern;int pos, x;

...

x = line.find( pattern, pos);

- x will contain the first location of pattern within

line starting at or after position pos or line.npos

(a special constant) if pattern not found in line

at or after position pos

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String Search Functions- Example:

string line=" This and that and more and less";int p;

p= line.find( "and" ,0); // search, start at pos'n 0

while (p != line.npos)

{

cout

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- Makes Sure to Study all the examples for this lecture.

- Specially, examples 10 and 11are very important.Similar questions may appear in the labs, assignments

and exams

01c string

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