Get a Substring in C

Given a string str and pos and len that defines the starting and the length of the subarray. The task is to generate a substring of size len starting from the index pos. 

A substring is a contiguous sequence of characters within a String.

Examples:

Input: Str =”the”, pos=1, len=2
Output:  “th”
Explanation: substrings will be: “”, “t”, “h”, “e”, “th”, “he”, “the”.

Input: Str =”geeks”, pos=3, length=3
Output: “eks” 
Explanation: substrings are: “”, ” g”, “e”, “e”, “k”, “s”, “ge”, “ee”, “ek”, “ks”, “gee”, “eek”, “eks”, “geek”, “eeks”, “geeks”.

Approach: The problem can be solved following the below idea:

Create a character array and put the characters starting from pos to the character array to generate the substring. 

Follow the below steps to implement the idea:

• Create a character array to store the substring.
• Iterate from the given position for the given length to generate the substring required.
  • Then store each character in the character array and print the substring.

Follow the below illustration for a better understanding.

Illustration:

Consider a string str=”abcde” , pos = 2, len = 3.

=> At i = 2 our ans = “c”. 

=> At i = 3, the character is ‘d’.
So add ‘d’ to the answer. 
Our ans = “cd”

=> At i = 4, the character is ‘e’.
So add ‘e’ to the answer.
Uur ans = “cde”.

Below is the implementation of the above approach.

C

// C implementation of code
#include <stdio.h>
#include <string.h>

// Function to get substr in C
void getString(int pos, int len, int c, char string[])
{

    char substring[1000];

    while (c < len) {
        substring = string[pos + c - 1];
        c++;
    }

    substring = '\0';

    // Print the result
    printf(substring);
    printf("\n");
    return 0;
}

// Driver code
int main()
{

    int pos, len, c = 0;

    // Testcase1
    char string[14] = "geeksforgeeks";

    // Initialize pos, len i.e., starting
    // index and len upto which we have to
    // get substring respectively.
    pos = 6;
    len = 5;
    printf("String: %s ", string);
    printf("\nsubstring is: ");

    // Function call
    getString(pos, len, c, string);

    // Testcase2
    char string2[5] = "abcde";
    pos = 1;
    len = 3;
    c = 0;
    printf("\nString: %s ", string2);
    printf("\nsubstring is: ");

    // Function call
    getString(pos, len, c, string2);

    return 0;
}

Output

String: geeksforgeeks 
substring is: forge

String: abcde 
substring is: abc

Time complexity: O(len)
Auxiliary space: O(len)

Using strncpy() function in C

We can also use strncpy() function in C to copy the substring from a given input string. It takes 3 parameters which are the destination string, source string along with starting index and length of the substring which we need to copy.

Syntax:

strncpy(destination_string,input_string+pos,len);

Here pos is the starting index and len is the length of the substring which we want to copy.

Below is the code for the above approach.

C

// C implementation of code
#include <stdio.h>
#include <string.h>

// Driver code
int main()
{
    int pos, len;

    // Testcase1
    char string[14] = "geeksforgeeks";
    char substring[14];

    // Initialize pos, len i.e., starting
    // index and len upto which we have to
    // get substring respectively.
    pos = 6;
    len = 5;
    printf("String: %s ", string);
    printf("\nsubstring is: ");

    // Using strncpy function to 
    // copy the substring
    strncpy(substring,string+(pos-1),len);
    printf(substring);

    // Testcase2
    char string2[5] = "abcde";
    char substring2[5];
    
    pos = 1;
    len = 3;
    printf("\nString: %s ", string2);
    printf("\nsubstring is: ");

    // Using strncpy function to 
    // copy the substring
    strncpy(substring2,string2+(pos-1),len);
    printf(substring2);
    return 0;
}

// This code is contributed by Pushpesh Raj.

Output

String: geeksforgeeks 
substring is: forge
String: abcde 
substring is: abc

Time complexity: O(len)
Auxiliary space: O(len)

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