Vigenere

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

  int main(int argc, char* argv[])
{

  int i=0,n=0,k=0,j;

    if(argc!=2)

    {
      printf("Bad I/p\n");
      return 1;
    }


    char* key=argv[1];

     n=strlen(key);

         for(i=0;i<n;i++)

            {

            if(isalpha(key[i]))

            }

             else
              printf("\nBad key");

              return 1;

              char* msg=GetString();


            int m=strlen(msg);

         for(j=0;j<m;j++)
           {

           k=0;

           if(isalpha(msg[j]))
             {
               if(isupper(msg[j]))
                   {

                   if(isupper(key[k%n]))
                       {

                           msg[j]=(((msg[j]-65)+key[k%n]-65)%26)+65;    // k%n = k% strlen(keyword) cipher formula for upper case

                           printf("%c",msg[j]);
                       }
                    else

                    msg[j]=(((msg[j]-65)+key[k%n]-97)%26)+65;    // k%n = k% strlen(keyword) cipher formula for upper case

                           printf("%c",msg[j]);

                    }

                    if(islower(msg[j]))

                      {

                       if (isupper(key[k%n]))
                          {

                            msg[j]=(((msg[j]-97)+key[k%n]-65)%26)+97;    // k%n = k% strlen(keyword) cipher formula for upper case

                           printf("%c",msg[j]);

                          }
                        else if(islower(key[k%n]))

                          {
                           msg[j]=(((msg[j]-97)+key[k%n]-97)%26)+97;    // k%n = k% strlen(keyword) cipher formula for upper case

                           printf("%c",msg[j]);
                          }

                      }

                 k++;
                    }

           }
    else
    printf("%c\n",msg[j]);


}