substitution.c

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

int main(int argc, char *argv[])
{
    if (argc != 2)
    {
        printf("Usage: ./substitution key\n");
        return 1;
    }
    
    // argv is a list of strings (ie. char **)
    // argv is an array of strings, or say, an array of char *. So the type of argv[1] is char *, and the type of argv[1][0] is char.
    // https://stackoverflow.com/questions/21136735/store-argv1-to-an-char-variable
    for (int k = 0; k < strlen(argv[1]); k++)
    {
        if (!isalpha(argv[1][k]))
        {
            printf("Usage: ./substitution key\n");
            return 1;
        }
    }
    
    if (strlen(argv[1]) != 26)
    {
        printf("Key must contain 26 characters.\n");
        return 1;
    }
    
    // Check all characters are different in a string.
    for (int i = 0; i < 26; i++)
    {
        for (int j = i + 1; j < 26; j++)
        {
            if (tolower(argv[1][i]) == tolower(argv[1][j]))
            {
                printf("has same characters.\n");
                return 1;
            }
        }
    }
    
    char *p = get_string("plaintext: ");
    
    int x = 0;
    // Why? https://www.reddit.com/r/cs50/comments/gryswz/please_help_i_get_a_segmentation_fault_in/
    char *y = malloc(sizeof(p) * sizeof(char) + 1);
    for (int a = 0, n = strlen(p); a < n; a++)
    {
        if (isalpha(p[a]))
        {
            x = tolower(p[a]) - 'a';
            if (islower(p[a]))
            {
                y[a] = tolower(argv[1][x]);
            }
            if (isupper(p[a]))
            {
                y[a] = toupper(argv[1][x]);
            }
        }
        else
        {
            y[a] = p[a];
        }
    }
    printf("ciphertext: %s\n", y);
    free(y);
    return 0;
}