Vigenere program is not working correctly!

Question

I think the keyword is not repeating itself. Kindly help!

string p = GetString();
        int a;
        for (int i=0, n=strlen(p); i<n; i++)
        {
            if (isupper(p[i]))
            {
                    if (isupper(k[i]))
                        a = k[i]-65;
                    else
                        a = k[i]-97;
                int z = a%n;
                int b = p[i]-65;
                int c = (z+b)%26;
                int d = c+65;
                printf ("%c", d);
            }
            else if (islower(p[i]))
            {
                    if (isupper(k[i]))
                        a = k[i]-65;
                    else
                        a = k[i]-97;
                int z = a%n;
                int b = p[i]-97;
                int c = (z+b)%26;
                int d = c+97;
                printf ("%c", d);
            }
            else 
            {
                printf ("%c", p[i]);
            }
        }
        printf ("\n");
        return 0;