The task is to give a keyword argument when you run the program, which I saved as a string k then the user inputs text and the program will output that inputted text according to the keyword. A = 0, B = 1, so if the keyword is ABABA and the text is hello, it would output hflmo.
If I run the program giving argv[1] as "abc" then plaintext pt as "hello", I should be getting "hfnlp", but instead I get "hnflv", why doesn't the last letter encipher correctly?
string k = argv[1];
int l = strlen(argv[1]);
printf("plaintext: ");
string pt = get_string("");
printf("ciphertext: ");
for (int i = 0, shift = 0; i < strlen(pt); i++)
{
if (!isalpha(pt[i]))
{
printf("%c", pt[i]);
}
else
{
if (isupper(pt[i]))
{
if (isupper(k[shift]))
{
printf("%c", (((pt[i] - 65) + (k[shift % l] - 65)) %26) + 65);
shift++;
}
else
{
printf("%c", (((pt[i] - 65) + (k[shift % l] - 97)) %26) + 65);
shift++;
}
}
else if (islower(pt[i]))
{
if (isupper(k[shift]))
{
printf("%c", (((pt[i] - 97) + (k[shift % l] - 65)) %26) + 97);
shift++;
}
else
{
printf("%c", (((pt[i] - 97) + (k[shift % l] - 97)) %26) + 97);
shift++;
}
}
}
}
printf("\n");
return 0;
}
The lines
if (isupper(k[shift]))
are missing the modulo calculation. The correct version is:
if (isupper(k[shift % l]))
It might be clearer if you put this calculation at some central place in the for-loop:
shift %= l;