关于CS50 pset2 vigenere

Question

Why my code would not skip the space and it led to wrong encryption sequence?

When I check with an example "Hello, World!", my code counted the space also and converted to "Iekmo, Wnslc!" instead of "Iekmo, Vprke!" using the key "baz"

Can someone explain the logic behind? Many thanks!

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

//getting user encryption key


int main(int argc, string argv[])
{

    if (argc != 2)
    {
        printf("Usage: ./vigenere keyword\n");
        return 1;
    }
    //check if all are alphabeticals
    else 
    {
           for (int i = 1; i < argc; i++)
        {
            for (int j = 0; j < strlen(argv[i]); j++)
            {
                if (isalpha(argv[i][j]) == false)
                {                
                    printf("Usage: ./vigenere keyword\n");
                    return 1;
                }
            }
        }
    }  


    //getting plaintext divide it into each character
    string pt = get_string("plaintext: ");

    printf("ciphertext: ");
    //convert to ciphertext
    //C = (P + k) % 26
    for (int r = 0; r < strlen(pt); r++)
    {                           
        if (isupper(pt[r]))
        {
            //making loop with j group corresponding to keyword
            int j = r % strlen(argv[1]);
            int key = tolower(argv[1][j]) - 97;
            printf("%c", (pt[r] - 65 + key) % 26 + 65);
        }
        else if (islower(pt[r]))
        {
            //making loop with j group corresponding to keyword
            int j = r % strlen(argv[1]);
            int key = tolower(argv[1][j]) - 97;
            printf("%c", (pt[r] - 97 + key) % 26 + 97);
        }
        else
        {
            printf("%c", pt[r]);
        }

     }            
       printf("\n");       

}

Answer 1

The problem is with the calculation of j here int j = r % strlen(argv[1]); . The index of the key is not related to r (the index of the message). Program needs to iterate through the key based solely on (length of) the key. You need to increment it each time you "use" a key index, and "wrap" it so it doesn't go past the end. You might consider declaring j before the r loop; increment j whenever you use a key index (hint: j++ ); and "wrap" j with modulo operator (hint: j % strlen(argv[1] ). I leave the actual code to you.

Answer 2

It's a lot easier to show than explain:

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

int main(int argc, char **argv)
{
    if (argc != 2)
    {
        fprintf(stderr, "Usage: %s keyword\n", argv[0]);
        return 1;
    }

    for (int i = 1; i < argc; i++)
    {
        for (int j = 0; argv[i][j] != '\0'; j++)
        {
            if (isalpha(argv[i][j]) == false)
            {                
                fprintf(stderr, "%s: non-alphabetic character '%c' (%d) in key\n",
                        argv[0], argv[i][j], argv[i][j]);
                return 1;
            }
        }
    }

    string pt = get_string("plaintext:  ");

    printf("ciphertext: ");
    int k = 0;
    int keylen = strlen(argv[1]);
    for (int r = 0; pt[r] != '\0'; r++)
    {                           
        if (isupper(pt[r]))
        {
            int j = k++ % keylen;
            int key = tolower(argv[1][j]) - 'a';
            printf("%c", (pt[r] - 'A' + key) % 26 + 'A');
        }
        else if (islower(pt[r]))
        {
            int j = k++ % keylen;
            int key = tolower(argv[1][j]) - 'a';
            printf("%c", (pt[r] - 'a' + key) % 26 + 'a');
        }
        else
        {
            printf("%c", pt[r]);
        }

     }            
     printf("\n");       
     return 0;
}

Example run:

$ ./vig89 baz
plaintext:  Hello, World!
ciphertext: Iekmo, Vprke!
$

As I noted in a comment, you need to separate 'position in string, r ' from 'encrypted character number'. You need an extra variable that you only increment when the character is alphabetic.

In the code above, k is the extra variable ( keylen is another, but it simply records the length of the key rather than repeatedly calling strlen() ). The value in k is incremented when it is known that the character is a letter and not otherwise.

I observe that it might be sensible to process argv[1] so that you don't have to do the tolower() conversion each time; you could do that while validating the keyword.

I've also reported errors on standard error, and not used strlen() in the condition part of the loops. While the cost is not prohibitive if you're dealing with 3-letter keys, if you start calculating the length of a string that's 20 KiB long on each iteration, you might start to spot the overhead (unless the compiler manages to optimize it away — it might, it might not). I've also aligned the plain text and the cipher text in the I/O.

There are quite a lot of other changes that could/should be made. For example, the first for (int i = 1; …) loop is not needed; you only have one argument so you only need the inner for (int j = 0; …) loop. It is also more idiomatic to use if (!isalpha(argv[i][j])) than to compare the result with false , especially since the isalpha macro is not guaranteed to return 0 or 1 (it returns zero or non-zero) – so changing if (isalpha(argv[i][j]) == false) to if (isalpha(argv[i][j] != true) would not be reliable. I'd probably create a simple variable char *key = argv[1]; (or string key = argv[1]; in the context of CS50, though I'm not convinced that the CS50 typedef char *string; is a good idea) and use that in the program.