longestword.c

#include <stdio.h>

int count_letters(char *str, char *word) {
    char c;
    int letters = 0;

    while ((c = *str++) && *word) {
        if (c == *word) {
            letters++;
            word++;
        }
    }

    if (!*word) {
        return letters;
    } else {
        return 0;
    }
}

int main(int argc, char **argv) {
    if (argc < 2) {
        return 1;
    }

    char *word;
    int max_letters = 0;

    for (int i = 2; i < argc; i++) {
        int letters = count_letters(argv[1], argv[i]);
        if (letters > max_letters) {
            word = argv[i];
            max_letters = letters;
        }
    }

    if (max_letters > 0) {
        printf("%s\n", word);
    }

    return 0;
}