see " Advanced string technique " it C++ tutorial .. btw ..
The C++ standard library provides a class called string. Its been used in several programs so far. To use it you need
#include <string>
using namespace std;
Note the (tiny) difference (there is also the using namespace std;. The components of the C++ standard library belong to a namespace called std. The using statement allows the code to refer to the components without having to write std:: in front, as in std::string x ) to the corresponding statement for C-style strings: for C-style strings you say
#include <string.h>.
You can create a string with
string s = "this is what I want";
You can navigate around a string with the array-indexing notation s[i], with the first character being s[0]. Despite appearances, C++ strings are neither arrays, nor pointers!. The fact you can do this is due to C++ feature known as operator-overloading. Suffice to say that the class is defined in such a way as to support the array indexing operation. There is also a navigation device called an iterator, functionally like a pointer, which we wont be going into.
string is a class, and a function or method m of the class is applied to an object x of the class with the x.m(...) syntax of object oriented programming. So in reading the following tables of functions bear this mind
For finding the size of a string x and searching in it, you need:
int x.size() size of string
int x.find(char c) search for the character c, start at beginning
int x.find(string s) search for string s, start at beginning
int x.find(char c, int pos) search for the character c, start at pos
int x.find(string s, int pos) search for s, start at pos
string::npos value returned when search fails
The return values of the search functions are integers - often called an offset. If the x.find(...) returns offset i, then x[i] is the character where the match begins. Also you can give an offset to specify the start of the search. Where the C-style search functions relied on the NULL pointer for exceptional situations, the C++ search functions rely on the cryptic (no position ?):
string::npos.
Here is an example of searching (compare with ):
#include <string>
using namespace std;
/* to count how often a char occurs */
string s = "aabbaa";
int count = 0;
int offset = 0;
while((offset = s.find('a',offset)) != string::npos) {
count++;
offset++;
}
/* to count how often a string occurs */
string s = "aabbaa";
int count = 0;
int offset = 0;
while((offset = s.find("aa",offset)) != string::npos) {
count++;
offset = offset+2;
}
There are lots of string-creating, and string-updating methods, and the most frequently used are given in the table below. They can be used without any of the qualms or headaches of the corresponding operations on C-style strings. These methods generally have at least 1 and sometimes 2 integer arguments. The first argument will be a start position o for the operation, and the second argument will specify the size l of the operation to be carried out.
string(char *s) create string from char pointer
char * x.c_str(void) make char pointer from string
string x.substr(int o) return substring from o onwards
string x.substr(int o, int l) returns substring starting at o, of length l
x.erase(int o, int l) remove l chars from o onwards
x.insert(int o, string s) insert s at o
s1 + s2 give string representing concatenation of s1 and s2
Note that in the above only erase and insert change the object that they are invoked on. substr returns a string object generated out of the unchanged string object it was invoked on.
I will give a substantial example of string processing in tutorial after vector has been introduced.
A final important facility provided by string is a function to obtain a whole line of input from a stream. Where file is an fstream and line a string, you can say:
getline(file,line)
Recall that the >> mechanism stops at white space. This getline function reads everything up to a new-line character. It does not put the new-line character into the line but it does take it out of the stream. It can be treated as returning false when the end of the file is reached.
Unsolved 
