vigenere.c

#include <stdio.h>
#include <cs50.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

int main (int argc, string argv[]) {
    
    // check if you type a single command-line argument
    if (argc != 2) {
        printf ("Type ./vigenere keyword");
        return 1;
    }
    
    else {
        string keyword = argv[1];
        // check if the keyword is composed entirely of alphabetical characters
        int count = 0;
        for (int h = 0; h < strlen(keyword); h++) {
            if (isalpha(keyword[h]) == 0) count++;
        }
        if (count > 0) {
            printf ("Type alphabetical characters.");
            return 1;
        }
        else {
            // now get the plaintext
            string s = GetString();
            
            int j = 0;        
            for (int i = 0; i< strlen(s); i++) {
                
                j = j % strlen(keyword);
                
                // convert keyword[j] to 0-25
                if (65 <= keyword[j] && keyword[j] <= 90) {
                    keyword[j] = keyword[j] - 65;
                }
                else if (97 <= keyword[j] && keyword[j] <= 122) {
                    keyword[j] = keyword[j] - 97;
                }
                
                // now encrypt the plaintext!
                if (65 <= s[i] && s[i] <= 90) {
                    // convert ASCII code to 0-25 (LARGE LETTERS)
                    char letter = s[i] - 65;
                    letter = (letter + keyword[j]) % 26;
                    // reconvert
                    letter = letter + 65;
                    printf ("%c", letter);
                    j++;
                }
                else if (97 <= s[i] && s[i] <= 122) {
                    // convert ASCII code to 0-25 (small letters)
                    char letter = s[i] - 97;
                    letter = (letter + keyword[j]) % 26;
                    // reconvert
                    letter = letter + 97;
                    printf("%c", letter);
                    j++;
                }      
                else {
                    printf ("%c", s[i]);
                }
            }
        printf ("\n");
        return 0;  
        }
    }
}