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. The substring function is used for handling string operations like strcat(), append(), etc. It generates a new string with its value initialized to a copy of a sub-string of this object.
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 starts from pos and is 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 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()).

Syntax:

string substr (size_t pos, size_t len) const;

Parameters:

1. pos: Position of the first character to be copied.
2. len: Length of the sub-string.
3. size_t: It is an unsigned integral type.

Return value: 

It returns a string object.

Example:

CPP

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

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

    // Copy two characters of s1 (starting
    // from position 3)
    string r = s1.substr(3, 2);

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

    return 0;
}

Output

String is: ks

Time complexity: O(n)

Auxiliary Space: O(n)

Applications:

1. How to get a sub-string after a character? 

In this, a string and a character are given and you have to print the sub-string followed by the given character. 
Extract everything after the “:” in the string “dog:cat“.

Example:

C++

// 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

Time complexity: O(n)

Auxiliary Space: O(n)

2. How to get a sub-string before a character? 

In this, a string and a character are given and you have to print the sub-string followed by the given character. 
Extract everything before the “:” in the string “dog:cat“. 

Example:

CPP

// 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 before pos
    string sub = s.substr(0 , pos);

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

    return 0;
}

Output

String is: dog

Time complexity: O(n)

Auxiliary Space: O(n)

3. Print all substrings of a given string

Given a string as an input. We need to write a program that will print all non-empty substrings of that given string.

Example:

C++

// C++ program to demosntrate all possible
// substrings of a given string
#include <bits/stdc++.h>
using namespace std;

// Function to print all sub strings
void subString(string s, int n)
{
    // Pick starting point in outer loop
    // and lengths of different strings for
    // a given starting point
    for (int i = 0; i < n; i++)
        for (int len = 1; len <= n - i; len++)
            cout << s.substr(i, len) << endl;
}

// Driver program to test above function
int main()
{
    string s = "abcd";
    subString(s, s.length());
    return 0;
}

Output

a
ab
abc
abcd
b
bc
bcd
c
cd
d

Time complexity: O( n³ )

Auxiliary Space: O(1)

4. 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.

Example:

C++

// C++ program to print sum of all substring of
// a number represented as a string
#include <bits/stdc++.h>
using namespace std;

// Utility method to convert character digit to
// integer digit
int toDigit(char ch)
{
    return (ch - '0');
}

// Returns sum of all substring of num
int sumOfSubstrings(string num)
{
    int n = num.length();

    // allocate memory equal to length of string
    int sumofdigit[n];

    // initialize first value with first digit
    sumofdigit[0] = toDigit(num[0]);
    int res = sumofdigit[0];

    // loop over all digits of string
    for (int i = 1; i < n; i++) {
        int numi = toDigit(num[i]);

        // update each sumofdigit from previous value
        sumofdigit[i] = (i + 1) * numi + 10 * sumofdigit[i - 1];

        // add current value to the result
        res += sumofdigit[i];
    }

    return res;
}

// Driver code to test above methods
int main()
{
    string num = "1234";
    cout << sumOfSubstrings(num) << endl;
    return 0;
}

Output

1670

Time Complexity: O(n)

Auxiliary Space: O(n)

5: Maximum value of all substrings of a string representing a number.

Given an integer represented as a string, we need to get the maximum of all possible substrings of the given string which is representing a number.

Example:

C++

// C++ program to demosntrate max. of all possible
// substrings of a given string
#include <bits/stdc++.h>
using namespace std;

void subString(string s, int n)
{
      vector<int> v;
    for (int i = 0; i < n; i++){
        for (int len = 1; len <= n - i; len++){
             string sub =(s.substr(i, len));
             int x=stoi(sub);
             v.push_back(x);
            }
     }
     cout<<*max_element(v.begin(),v.end())<<endl;
}

// Driver program to test above function
int main()
{
    string s = "823";
    subString(s, s.length());
    return 0;
}

Output :

823 
Explanation: all substrings are {8,82,823,2,23,3} and the maximum value substring is 823.

Time complexity: O(n^3)

Auxiliary space: O(n!)

6: Minimum value of all substrings of a string representing a number.

Given an integer represented as a string, we need to get the minimum of all possible substrings of the given string which is representing a number.

Example:

C++

// C++ program to demosntrate minimum of all possible
// substrings of a given string
#include <bits/stdc++.h>
using namespace std;

void subString(string s, int n)
{
      vector<int> v;
    for (int i = 0; i < n; i++){
        for (int len = 1; len <= n - i; len++){
             string sub =(s.substr(i, len));
             int x=stoi(sub);
             v.push_back(x);
            }
     }
     cout<<*min_element(v.begin(),v.end())<<endl;
}

// Driver program to test above function
int main()
{
    string s = "4572";
    subString(s, s.length());
    return 0;
}

Output:

2

Time complexity: O(N^3)

Auxiliary space: O(N!)