Unaccepted output with caesar

Question

I am not getting the desired output when I want cipher the plaintext with key 13 with long sentence with commas and space. Please help, thank you.

    //Program to encrypt the message by using Caesar's cipher
#include <cs50.h>
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>

int main(int argc, string argv[])
{
    //make sure the program was run with command-line argument
    if (argc == 2)
    {
       //validate the key
       int key = atoi(argv[1]);
       int len = strlen(argv[1]);
        // iterate over the key 
        for (int i = 0; i < len; i++)
        {
            if (!isdigit(argv[1][i]))
            {
                printf("Usage: ./caesar key\n");
                return 1;
            }
            else if (isalpha(argv[1][i]))
            {
                printf("Usage: ./caesar key");
                return 1;
            }
        }

        // prompt user for plaintext
        string plaintext = get_string("plaintext: ");
        printf("ciphertext: ");

        // iterate over the plaintext
        int length = strlen(plaintext);
        for (int i = 0; i < length; i++)
        {
            if (islower(plaintext[i]))
            {
                printf("%c",plaintext[i] + 'a' + key % 26 - 'a');
            }
            else if (isupper(plaintext[i]))
            {
                printf("%c", plaintext[i] +'A' + key  % 26  - 'A');
            }
            else
            {
                printf("%c", plaintext[i]);
            }
        }
        printf("\n");
    }
    else
    {
        printf("Usage: ./caeasr key\n");
        return 1;
    }
    return 0;
} 

Answer 1

I am a bit confused by the for loop.

for (int i = 0; i < length; i++)
    {
        if (islower(plaintext[i]))
        {
            printf("%c",plaintext[i] + 'a' + key % 26 - 'a');
        }
        else if (isupper(plaintext[i]))
        {
            printf("%c", plaintext[i] +'A' + key  % 26  - 'A');
        }
        else
        {
            printf("%c", plaintext[i]);
        }

What happens when plaintext[i] + key is larger than 90 or 122? You need to implement a code that makes sure the ciphertext stays with in that range. Im sure there are more then one way to do this be you can implement a loop that subtracts 26 if the value of plaintext[i] + key equal more then 90 or 122.

So apart from that, I can see there are a couple of odd things in your code.

for example

else if (isalpha(argv[1][i]))
        {
            printf("Usage: ./caesar key");
            return 1;
        }

What if the key is neither alpha or digit ? like "<" or "?"

Also you are reducing the efficiency of the program by making it more complicated than it needs to be.

If you turn the else if to an else you will fix that issue.

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