CS50 - Pset2 - substitution help me

Question

I can't see what my code for pset2 substitution is missing. When I use check50 to test the program it returns this result:

:) substitution.c exists

:) substitution.c compiles

:( encrypts "A" as "Z" using ZYXWVUTSRQPONMLKJIHGFEDCBA as key
    expected "ciphertext: Z\...", not "ciphertext: Z\..."

:( encrypts "a" as "z" using ZYXWVUTSRQPONMLKJIHGFEDCBA as key
    expected "ciphertext: z\...", not "ciphertext: z\..."

:( encrypts "ABC" as "NJQ" using NJQSUYBRXMOPFTHZVAWCGILKED as key
    expected "ciphertext: NJ...", not "ciphertext: NJ..."

:( encrypts "XyZ" as "KeD" using NJQSUYBRXMOPFTHZVAWCGILKED as key
    expected "ciphertext: Ke...", not "ciphertext: Ke..."

:) encrypts "This is CS50" as "Cbah ah KH50" using YUKFRNLBAVMWZTEOGXHCIPJSQD as key

:) encrypts "This is CS50" as "Cbah ah KH50" using yukfrnlbavmwzteogxhcipjsqd as key

:) encrypts "This is CS50" as "Cbah ah KH50" using YUKFRNLBAVMWZteogxhcipjsqd as key

:( encrypts all alphabetic characters using DWUSXNPQKEGCZFJBTLYROHIAVM as key
expected "ciphertext: Rq...", not "ciphertext: Rq..."
:) handles lack of key

:) handles invalid key length

:) handles invalid characters in key

:) handles duplicate characters in key

:) handles multiple duplicate characters in key

my code is :

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

int main(int argc, string argv[])
{
    // VALIDATE THE KEY
    // for argc
    if (argc != 2)
    {
        printf("Usage: ./substitution key\n");
        return 1;
    }
    
    // var n for nubers of argv
    int n = 26;
    // for alphabit
    for (int i = 0; i < n; i++)
    {
        if (isalpha(argv[1][i]) == 0)
        {
            printf("Usage: ./substitution key\n");
            return 1;
        }
    }
    // for length of  argv
    if (n != 26)
    {
        printf("Key must contain 26 characters.\n");
        return 1;
    }
    // repeated charcters
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            // step over for the character
            if (i == j)
            {
                j += 1;
            }
            // error message for repeted character
            if (argv[1][i] == argv[1][j])
            {
                printf("Key must not contain repeated characters.\n");
                return 1;
            }
        }
    }
    // GET PLAINTEXT
    string plaintext = get_string("plaintext: ");
    int npi = strlen(plaintext);
    // ENCIPHER AND PRINT CIPHERTEXT
    // var for ciphertext
    char ci[npi];
    // print ciphertext
    printf("ciphertext: ");
    // encipher
    for (int i = 0; i < npi;)
    {
        for (int j = 0; j < n; j++)
        {
            // for not alphabits
            if (isalpha(plaintext[i]) == 0)
            {
                printf("%c", plaintext[i]);
                i++;
                j = 26;
            }
            // for upper case
            if (isupper(plaintext[i]) && ('A' + j) == plaintext[i])
            {
                ci[i] = toupper(argv[1][j]);
                printf("%c", ci[i]);
                i++;
                j = 0;
            }
            // for lower case
            else if (islower(plaintext[i]) && ('a' + j) == plaintext[i])
            {
                ci[i] = tolower(argv[1][j]);
                printf("%c", ci[i]);
                i++;
                j = 0;
            }
        }
    }
    printf("\n");
    return 0;
}

Answer 1

Program outputs non printable characters, which is why the check50 output is confusing, it is not "printing" them.

Try this at the command line and enter a as the plaintext to see result:

./substitution ZYXWVUTSRQPONMLKJIHGFEDCBA | cat --show-nonprinting

The fundamental flaw in the program is changing the loop variable values within the loops (i and j). In the plaintext "a" example, i gets greater than npi. Walk it through on paper.