C Program for Naive algorithm for Pattern Searching

Last Updated : 24 Oct, 2023

Write a C program for a given text string with length n and a pattern with length m, the task is to print all occurrences of the pattern in text.
Note: You may assume that n > m.

Examples:

Input: text = “THIS IS A TEST TEXT”, pattern = “TEST”
Output: Pattern found at index 10

Input: text = “AABAACAADAABAABA”, pattern = “AABA”
Output: Pattern found at index 0, Pattern found at index 9, Pattern found at index 12

Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

C Program for Naive Pattern Searching Algorithm:

Slide the pattern over text one by one and check for a match. If a match is found, then slide by 1 again to check for subsequent matches.

// C program for Naive Pattern Searching algorithm
#include <stdio.h>
#include <string.h>

void search(char* pat, char* txt)
{
    int M = strlen(pat);
    int N = strlen(txt);

    /* A loop to slide pat[] one by one */
    for (int i = 0; i <= N - M; i++) {
        int j;

        /* For current index i, check for pattern match */
        for (j = 0; j < M; j++)
            if (txt[i + j] != pat[j])
                break;

        if (j
            == M) // if pat[0...M-1] = txt[i, i+1, ...i+M-1]
            printf("Pattern found at index %d \n", i);
    }
}

// Driver's code
int main()
{
    char txt[] = "AABAACAADAABAAABAA";
    char pat[] = "AABA";

    // Function call
    search(pat, txt);
    return 0;
}

Output

Pattern found at index 0
Pattern found at index 9
Pattern found at index 13

Time Complexity: O(N²)
Auxiliary Space: O(1)

Complexity Analysis of Naive algorithm for Pattern Searching:

Best Case: O(n)

When the pattern is found at the very beginning of the text (or very early on).
The algorithm will perform a constant number of comparisons, typically on the order of O(n) comparisons, where n is the length of the pattern.

Worst Case: O(n²)

When the pattern doesn’t appear in the text at all or appears only at the very end.
The algorithm will perform O((n-m+1)*m) comparisons, where n is the length of the text and m is the length of the pattern.
In the worst case, for each position in the text, the algorithm may need to compare the entire pattern against the text.
Naive algorithm for Pattern Searching