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,j=0,n=0,k=0;

    if(argc!=2)

    {
      printf("Bad I/p\n");  // checking for valid argv[1]
      return 1;
      exit(0);
    }

    char* key=argv[1];

    for(i=0,n=strlen(key);i<n;i++)  // To check wether each character of key is alphabet

    {

      if (isalpha(key[i]))
      {

         printf("AFTER VALID\n");


      if (isupper(key[i]))

        {
          key[i]=key[i]-65;
        }


      if (islower(key[i]))

        {
          key[i]=key[i]-97;
        }

      }
       else

       printf("Enter valid key\n");
       exit(0);
       return 1;
     }



    printf("B4 input\n"); // why isn't this statement getting executed ?

    string msg=GetString();    // plaintext input from user

    if (isalpha(msg[j]))

    {
      for (int j=0,m=strlen(msg);j<m;j++)

      {


        if (isupper(msg[j]))

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

         }

      if (islower(msg[j]))

         {
             msg[j]=(((msg[j]-97)+key[k%n])%26)+97;   // 97 and 65 are added to convert back to ascii form

         }
      else

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

      k++;

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

       } // j-loop 2


    } //isalpha du
    else
  {
   printf("Enter valid key\n");
      exit(0);
      return 1;

   }

} //main du bracket