substr() in C++

In C++, the string substr() function is used to extract a substring from the given string. It generates a new string with its value initialized to a copy of a sub-string of the given string.

Example:

#include <iostream> #include <string> using namespace std; int main() {    // Take any string  string s = "Geeks";  // Extract two characters of s1 (starting  // from index 3)  string sub = s.substr(3, 2);  cout << sub;  return 0; } 

ks

In the above example, we have extracted the last two characters from the string s and stored them in sub string using substr() function.

Syntax of String substr()

The std::string::substr() is the member function of string class defined inside <string> header file.

substr(pos, len); 

Parameters:

• pos: Index of the first character to be copied.
• len: Length of the sub-string.

Return Value:

• It returns a string object.

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 happens, there are no changes in the string.
4. If the requested sub-string len is greater than the size of a string, then returned sub-string is [pos, size()).
5. If len is not passed as a parameter, then returned sub-string is [pos, size()).

Examples of substr()

The following examples illustrates the use of substr() function in C++:

Get a Sub-String after a Character

In this example, a string and a character are given and you have to print the complete sub-string followed by the given character.

#include <iostream> #include <string> using namespace std; int main() {  string s = "dog:cat";  // Find position of ':' using find()  int pos = s.find(":");  // Extract substring after pos  string sub = s.substr(pos + 1);  cout << "Substring is: " << sub;  return 0; } 

String is: cat

Get a SubString Before a Character

This example is a follow-up to the previous one, where we example, we print the complete substring up to a specific character.

#include <iostream> #include <string> using namespace std; int main() {  string s = "dog:cat";  // Find position of ':' using find()  int pos = s.find(":");  // Copy substring before pos  // Extract everything before the ":" in the string  // "dog:cat".  string sub = s.substr(0, pos);  cout << "Substring is: " << sub;  return 0; } 

String is: dog

Print all Sub-Strings of a Given String

This example is a very famous interview question also. We need to write a program that will print all non-empty substrings of that given string.

#include <bits/stdc++.h> using namespace std; int main() {    string s = "abcd";  // subString(s, s.length());  int n = s.length();    // Logic to print all substring  // using substr()  for (int i = 0; i < n; i++)  for (int len = 1; len <= n - i; len++)  cout << s.substr(i, len) << endl;  return 0; } 

a ab abc abcd b bc bcd c cd d 

Print Sum of all Substrings of a String Representing a Number

Given an integer represented as a string, we need to get the sum of all possible substrings of this string.

#include <bits/stdc++.h> using namespace std; int main() {  string s = "1234";  int res = 0;  int n = s.length();  for (int i = 0; i < n; i++) {  for (int len = 1; len <= n - i; len++) {  string sub = (s.substr(i, len));    // Convert the substring into   // integer number  int x = stoi(sub);  res += x;  }  }  cout << res;  return 0; } 

1670