Pset 2 vigenere failing one check using Baz as keyword

Question

Took me awhile to figure out the logic behind this project. I thought I had it all worked out but I keep failing one check.

:( encrypts "BaRFoo" as "CaQGon" using "BaZ" as keyword \ expected output, but not "CaQGoh\n"

My code is below. Can someone please help me understand where my mistake is?

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


    int main(int argc, string argv[])

   {


        if (argc != 2)
      {    
           printf("Invalid input!!! Requires a \"String\" as a command line parameter\n");
return 1;
         }

      else

     {
         for (int i = 0, s=strlen(argv[1]); i < s; i++)
     {        
    if (!isalpha(argv[1][i]))
     {        
         printf("You must enter letters only for your key\n");

           return 1;


      }    
       }
        }

    string key = argv[1];

    char* pltxt = GetString();

    int key_index = 0;    
    int j = 0;

    for (int i=0, n =strlen(pltxt); i<n; i++)

        {
            if (isalpha(pltxt[i]))
          {
          {  
             key_index = j % strlen(key);
          }
            if  islower(pltxt[i])
           { 
             int a = ((pltxt[i] -97 +key[key_index] -97 ) %26 + 97);

             printf("%c", a);
          }


             else if isupper(pltxt[i])
          {    
             int b = ((pltxt[i] -65 +key[key_index] - 65 ) %26 + 65); 

             printf("%c", b);
          }

             j++;
        } 
            else 
            {    
                printf("%c", pltxt[i]); 
            }





        }

           printf("\n");
    return 0; 

}

Answer 1

The problem is that the code assumes that the case of the plain text and the key are the same. It doesn't handle the conditions where one is upper case and the other is lower case.

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