Substring in C++

In C++, std::substr() is a predefined function used for string handling. string.h is the header file required for string functions.

This function takes two values pos and len as an argument and returns a newly constructed string object with its value initialized to a copy of a sub-string of this object. Copying of string start from pos and done till pos+len means [pos, pos+len).

Important points:

1. The index of the first character is 0 (not 1).
2. If pos is equal to the string length, the function returns an empty string.
3. If pos is greater than the string length, it throws out_of_range. If this happen, there are no changes in the string.
4. If for the requested sub-string len is greater than size of string, then returned sub-string is [pos, size()).

Syntax:

string substr (size_t pos, size_t len) const;
Parameters:
pos: Position of the first character to be copied.
len: Length of the sub-string.
size_t: It is an unsigned integral type.

Return value: It returns a string object.

// CPP program to illustrate substr()
#include <string.h>
#include <iostream>
using namespace std;

int main()
{
    // Take any string
    string s1 = "Geeks";

    // Copy three characters of s1 (starting 
    // from position 1)
    string r = s1.substr(1, 3);

    // prints the result
    cout << "String is: " << r;

    return 0;
}

Output:

String is: eek

Applications :

1. How to get sub-string after a character?
   In this a string and a character is given and you have to print the sub-string followed by the given character.
   Extract everything after the “:” in the string “dog:cat”.

   // CPP program to illustrate substr()
   #include <string.h>
   #include <iostream>
   using namespace std;
   
   int main()
   {
       // Take any string
       string s = "dog:cat";
   
       // Find position of ':' using find()
       int pos = s.find(":");
   
       // Copy substring after pos
       string sub = s.substr(pos + 1);
   
       // prints the result
       cout << "String is: " << sub;
   
       return 0;
   }
   

   Output:

   String is: cat
   

2. Print all substrings of a given string
3. Sum of all substrings of a string representing a number

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