grep

The grep Command

grep searches for a pattern in text and prints every line that contains it:

$ grep "Linux" notes.txt
Learn Linux

Only lines containing "Linux" are printed. Lines without it are discarded.

Your Implementation

Write void my_grep(const char *s, const char *pat) that prints every line in s that contains the substring pat.

You need two sub-problems:

1. Split the input into lines (scan for '\n')
2. Check if a line contains the pattern (substring search)

For substring search, use two pointers — one walking the haystack, one walking the needle:

int contains(const char *hay, const char *needle) {
    for (; *hay; hay++) {
        const char *h = hay, *n = needle;
        while (*n && *h == *n) { h++; n++; }
        if (!*n) return 1;
    }
    return 0;
}

For each position in hay, try to match the full needle starting there. If n reaches the null terminator, the full needle matched.

Then collect each line into a buffer with a pointer, check if it contains the pattern, and print it if so.

Why Nested Loops?

The contains function is O(n × m) where n is the line length and m is the pattern length. For every position in the haystack, it tries to match the full needle. This is the naive algorithm.

Real grep uses algorithms like Boyer-Moore or Knuth-Morris-Pratt that are faster in practice, but for learning purposes, the nested loop is perfectly clear.

Your Task

Implement my_grep that prints lines containing the given pattern.