A Holistic Look At C++ Substring: All You Need to Know

Substring

In C++, a part of a string is referred to as a substring. substr is a C++ function for obtaining a substring (). There are two parameters in this function: pos and len. The pos parameter defines the substring's start location, while the len indicates the number of characters in the substring.

• The first character's index is 0 (not 1).
• The function returns an empty string if pos is equal to the length of the string.
• It throws out of range if pos is longer than the string length. There are no modifications in the string if this happens.
• If the length of the requested sub-string is greater than the length of the string, the sub-string is returned as [pos, size()).

Basic Syntax: 

std::stringsubstr(size_tpos, size_tlen) const;

Pos-> represents starting index of the substring

Len-> represents length of the substring

Example:

#include <string.h>

#include <iostream>

using namespace std; 

int main()

{

         // Take any string

         string s1 = "Welcome"; 

         // 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:

Explanation:

In the above example, it is mandatory to include the string header file because substring is also one of the string functions. The input string is represented by the variable- s1. The output string is represented by variable r. The output string variable executes the C++ substring function and prints the output value. In substr(1,3), the first argument indicates the starting position. Generally the index starts from 0 therefore the first argument 1 is the second character of the string. Second argument 3 indicates the length of the string. Thus the result will be  3 characters from the input string starting from the second character which is “elc”.

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Sample Code:

#include <iostream>

#include <string.h>

using namespace std;

int main() {

string a = "Hello welcome you all ";

string b = a.substr(11, 3);

string c = a.substr(0, 6);

cout<< "Substring starting at position 11 and length 3 is: " << b <<endl;

cout<< "Substring starting at position 0 and length 6 is: " << c;

return 0;

} 

Output:

In the above program, the first string variable a is declared as “Hello welcome you all”. Then the second string b stores the substring which start from the 11th position and the length becomes 3. So, the resultant value of b becomes “me”. Next, the string c stores the substring which starts from 0 and the length becomes 6, and the resultant value of c becomes “Hello”.

Sample code:

#include <string>

#include <iostream>

int main()

{

std::string a = "abcdefghijklmnopqrstuvwxyz";

// count is npos, returns [pos, size())

std::string s1 = a.substr(10);

std::cout<< s1 << '\n';

// both pos and pos+count are within bounds, returns [pos, pos+count)

std::string s2 = a.substr(5, 3);

std::cout<< s2 << '\n';

// pos is within bounds, pos+count is not, returns [pos, size())

std::string s4 = a.substr(a.size()-3, 50);

// this is effectively equivalent to

// std::string sub4 = a.substr(17, 3);

// since a.size() == 20, pos == a.size()-3 == 17, and a.size()-pos == 3

std::cout<< s4 << '\n'; 

try {

     // pos is out of bounds, throws

std::string sub5 = a.substr(a.size()+3, 50);

std::cout<< sub5 << '\n';

} catch(conststd::out_of_range& e) {

std::cout<< "pos exceeds string size\n";

}

}

Output:

Explanation:

In the above example, the input string variable ‘a’  is “abcdefghijklmnopqrstuvwxyz”. s1,s2,s4 are output string variables. 

 S1 variable holds the output of substr(10) returns string starting from the 10th position as “klmnopqrstuvwxyz”. 

S2 variable holds the output of substr(5,3) returns string starting from the 5th position and number of characters as 3 results “fgh”.

S3 variable holds the output of substr(a.size()-3,50) returns string as xyz.

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Pattern Matching Using C++ Substring

#include<iostream>

#include<cstdlib>

using namespace std;

int main()

{

inti,j,temp;

charstr[100]={"Welcome you all to the learning team"};

charsubstr[20]={"Welcome"};

for(i=0;str[i]!='\0';i++)

{

     j=0;

if(str[i]==substr[j])

     {

temp=i+1;

while(str[i]==substr[j])

         {

i++;

j++;

         }

if(substr[j]=='\0')

         {

cout<<"The substring is present in given string at position "<<temp<<"\n";

exit(0);

         }

else

         {

i=temp;

temp=0;

         }

     }

}

if(temp==0)

cout<<"The substring is not present in given string\n"; 

return 0;

}

Output:

Explanation:

The above example represents the pattern matching scenario. It checks if the given pattern of the string presented in the main string is the input string or not. If it is present, it will return the substring that is present in the given string along with the position. If not, it will return if the given substring is not present.

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Conclusion

In C++, a substring is a segment of a string, and you use the substr() function to extract a substring from a specified string. This substr() function extracts a substring from a string beginning at the specified position and going up to the length specified in characters from the starting position.

The above information will give a detailed explanation of what is substring in C++, basic syntax, basic example, C++ substring example with starting position and length, pattern matching.

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