The **`substr()` function** is defined in the `string.h` header and is used for string slicing in C++. It can be used to extract a part of a string, i.e., get a substring.

A **substring** is a sequence of consecutive characters from a string. For example, "with Educative" is a substring of "Learn C++ with Educative".

The function will return a substring from the character of the `pos` index (inclusive) up to the character with the (`pos`+`len` - 1) index. The original string will not be changed.

# Prototype
```py
string substr (size_t pos = 0, size_t len = npos) const;
```

# Parameters
1. **`pos`**: The position of the character from which the substring will start.
2. **`len`**: The number of characters to include in the substring.

# Return value
A string object is returned.

# Important points
1. In C++, the index starts from 0, not 1.
2. If `len` is greater than the size of the string, then `len` is taken to be equivalent to the size of the string.
3. If `pos` is greater than the size of the string, then the `out_of_range` exception is thrown. No change will be made to the string.

#include <iostream>
#include<string.h>
using namespace std;

int main() {
  string str = "This is a really long sentence with a lot of words.";

  string substrTest;
  // we can see if we start indexing from 1,
  // the first letter is skipped and the substring
  // starts from 'h' not 'T'
  substrTest = str.substr(1,4);
  cout << substrTest << endl;

  // this substring will be from 'T' which has index 0
  // to 's' which has index 3
  substrTest = str.substr(0,4);
  cout << substrTest << endl;

  // this will give the last word of the sentence as it is 5 letters
  // and 'w' has index str.size() - 6
  substrTest = str.substr(str.size() - 6, 5);
  cout << substrTest << endl;

  // this will give substring from 'l' which has index 17
  // till 'e' in 'sentence' which has index 29 (17+13-1)
  substrTest = str.substr(17, 13);
  cout << substrTest << endl;

  // here the len is obviously larger than the size
  // of the string, so all the characters after and 
  // including index 8 are extracted
  substrTest = str.substr(8, 100);  
  cout << substrTest << endl;

  // since pos == str.size(), empty sting is returned
  substrTest = str.substr(str.size(), 4);
  cout << substrTest << endl;

  // this will throw an error as pos > str.size()
  substrTest = str.substr(str.size() + 1, 4);
  
  return 0;
}

How to use std::string::substr() in C++

The substr() function in C++ extracts a substring from a given string without altering the original string.

How to use std::string::substr() in C++

Prototype

Parameters

Return value

Important points

Code