C Programming: Check whether a given substring is present in the given string

Last update on October 24 2025 12:11:08 (UTC/GMT +8 hours)

14. Check Substring Presence

Write a C program to check whether a substring is present in a string.

Sample Solution:

C Code:

#include <stdio.h>

int main() {
    char str[80], search[20]; // Declare character arrays for the main string and the substring to search
    int c1 = 0, c2 = 0, i, j, flg; // Declare variables for counting, iteration, and flag for substring check

    printf("\n\nCheck whether a given substring is present in the given string :\n"); // Display information about the task
    printf("-------------------------------------------------------------------\n");

    printf("Input the string : ");
    fgets(str, sizeof str, stdin); // Read a string from the standard input (keyboard)

    printf("Input the substring to be searched : ");
    fgets(search, sizeof search, stdin); // Read the substring to be searched

    // Calculate the length of the main string
    while (str[c1] != '\0') {
        c1++;
    }
    c1--;

    // Calculate the length of the substring
    while (search[c2] != '\0') {
        c2++;
    }
    c2--;

    // Check if the substring exists in the main string
    for (i = 0; i <= c1 - c2; i++) {
        for (j = i; j < i + c2; j++) {
            flg = 1; // Initialize the flag for matching substring
            if (str[j] != search[j - i]) {
                flg = 0; // Set the flag to zero if characters don't match
                break;
            }
        }
        if (flg == 1) {
            break; // Break the loop if substring is found
        }
    }

    if (flg == 1) {
        printf("The substring exists in the string.\n\n"); // Display message if the substring is found
    } else {
        printf("The substring does not exist in the string.\n\n"); // Display message if the substring is not found
		
		return 0; // Return 0 to indicate successful execution of the program
    }
}

Output:

Check whether a given  substring is present in the given string :                                              
-------------------------------------------------------------------                                           
Input the string : This is a test string.                                                                     
Input the substring to be search : search                                                                     
The substring is not exists in the string.

Flowchart:

Flowchart: Check whether a given substring is present in the given string

For more Practice: Solve these Related Problems:

Write a C program to search for a substring within a string using a naive search algorithm.
Write a C program to implement the Knuth-Morris-Pratt algorithm manually for substring search.
Write a C program to find and print the starting index of a substring if it exists in a string.
Write a C program to perform a case-insensitive search for a substring within a string.

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