Why is my vigenere.c not working? - c

Question

I keep making changes to the looping part of this code and my check50 always fails. I don't know what's going on. Below is my code:
#include <stdio.h>
#include <ctype.h>
#include <cs50.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, string argv[])
{
// declare variables
int cipherText;
if (argc != 2)
{
printf("Usage: ./vigenere keyword");
printf("\n");
return 1;
}
// keyword is the second command line argument
string key = argv[1];
int keylen = strlen(argv[1]);
// iterate through keyword to check if alphabetical
for (int i = 0, n = strlen(argv[1]); i < n; i++)
{
if ((key[i] >= '0') && (key[i] <= '9'))
{
printf("Keyword must consist only of letters.");
return 1;
}
}
// get the plaintext
string plainText = GetString();
// encypher - iterate over the characters in string, print each one encrypted
for (int i = 0, j = 0, n = strlen(plainText); i < n; i++, j++)
{
// start the key again if key shorter than plainText
if (j >= strlen(key))
{
j = 0;
}
// skip key[j] if plainText[i] is not an alpha character
if (!isalpha(plainText[i]))
{
j = (j-1);
}
// makes Aa = 0, Zz = 25 for the uppercase letters
if (isupper(key[j]))
{
key[j] = (key[j] - 'A');
}
// makes Aa = 0, Zz = 25 for lowercase letters
else if (islower(key[j]))
{
key[j] = (key[j] - 'a');
}
if (isupper(plainText[i]))
{
cipherText = (plainText[i] - 'A');
cipherText = ((cipherText + key[j%keylen])%26) + 'A';
printf("%c", cipherText);
}
else if (islower(plainText[i]))
{
cipherText = (plainText[i] - 'a');
cipherText = ((cipherText + key[j%keylen])%26 + 'a');
printf("%c", cipherText);
}
else
{
printf("%c", plainText[i]);
}
}
printf("\n");
return 0;
}
Some answered this: "The first for loop has a problem. The condition is checking for i > keylen when it should be checking for i < keylen".
Also when computing the next output value, the steps should be
(p[i]-65) results in a number between 0 and 25
adding (key[i % keylen]) results in a number between 0 and 50
apply modulo 26 so the number is between 0 and 25 (this is the missing step)
then add 65 to get the output"
and it's what I tried to do.

Answer 1

Given this code:
int keylen = strlen(argv[1]);
// iterate through keyword to check if alphabetical
for (int i = 0, n = strlen(argv[1]); i < n; i++)
{
if ((key[i] >= '0') && (key[i] <= '9'))
{
printf("Keyword must consist only of letters.");
return 1;
}
}
Your test inside the loop identifies digits as 'not a letter' (which is valid), but ignores punctuation, spaces and so on. You should probably be using if (!isalpha(key[i])) for the test (and it is courteous to print the erroneous character in the error message, which should be printed on standard error, not standard output, and should end with a newline:
fprintf(stderr, "Keyword must consist only of letters (%c found at %d)\n",
key[i], i+1);
You could refine that so it doesn't try printing non-printable characters with %c, but this is a huge step in the right direction.
You really don't need to set n in the loop; you just set keylen before the loop, so you could have written:
for (int i = 0; i < keylen; i++)
However, that is mostly cosmetic. Your real problem lies here:
// start the key again if key shorter than plainText
if (j >= strlen(key))
{
j = 0;
}
// makes Aa = 0, Zz = 25 for the uppercase letters
if (isupper(key[j]))
{
key[j] = (key[j] - 'A');
}
// makes Aa = 0, Zz = 25 for lowercase letters
else if (islower(key[j]))
{
key[j] = (key[j] - 'a');
}
You modify the key string on each iteration through the key. Unfortunately, though, if any of the letters in the key is a or A, you've converted that to '\0', which means that strlen(key) returns a different answer from before. So, you should use keylen in place of strlen(). AFAICS, if there isn't an a or A, that part of the code is OK.
Later, you have:
if (isupper(plainText[i]))
{
cipherText = (plainText[i] - 'A');
cipherText = ((cipherText + key[j%keylen])%26) + 'A';
printf("%c", cipherText);
}
The j % keylen is superfluous; j is already limited to 0 .. keylen-1. Similarly with the code for lower-case text.
Putting these changes together, and dummying up a GetString() function using fgets(), I get:
#include <stdio.h>
#include <ctype.h>
// #include <cs50.h>
#include <stdlib.h>
#include <string.h>
typedef char *string;
static char *GetString(void)
{
static char buffer[4096];
if (fgets(buffer, sizeof(buffer), stdin) == 0)
{
fprintf(stderr, "EOF detected in GetString()\n");
exit(EXIT_SUCCESS);
}
buffer[strlen(buffer) - 1] = '\0';
return buffer;
}
int main(int argc, string argv[])
{
// declare variables
int cipherText;
if (argc != 2)
{
printf("Usage: ./vigenere keyword");
printf("\n");
return 1;
}
// keyword is the second command line argument
string key = argv[1];
int keylen = strlen(argv[1]);
// iterate through keyword to check if alphabetical
for (int i = 0; i < keylen; i++)
{
if (!isalpha(key[i]))
{
printf("Keyword must consist only of letters (%c at %d)\n",
key[i], i+1);
return 1;
}
}
// get the plaintext
string plainText = GetString();
// encypher - iterate over the characters in string, print each one encrypted
for (int i = 0, j = 0, n = strlen(plainText); i < n; i++, j++)
{
// start the key again if key shorter than plainText
if (j >= keylen)
{
j = 0;
}
// skip key[j] if plainText[i] is not an alpha character
if (!isalpha(plainText[i]))
{
j = (j - 1);
}
// makes Aa = 0, Zz = 25 for the uppercase letters
if (isupper(key[j]))
{
key[j] = (key[j] - 'A');
}
// makes Aa = 0, Zz = 25 for lowercase letters
else if (islower(key[j]))
{
key[j] = (key[j] - 'a');
}
if (isupper(plainText[i]))
{
cipherText = (plainText[i] - 'A');
cipherText = ((cipherText + key[j]) % 26) + 'A';
printf("%c", cipherText);
}
else if (islower(plainText[i]))
{
cipherText = (plainText[i] - 'a');
cipherText = ((cipherText + key[j]) % 26 + 'a');
printf("%c", cipherText);
}
else
{
printf("%c", plainText[i]);
}
}
printf("\n");
return 0;
}
Sample run:
$ ./vigenere bakedalaska
What a wonderful world! The news is good, and the Vigenere cipher is solved.
Xhkx d wznvorguv arrwd! Lre oegw ls rogn, aod dlh Vtgwxese mmshpr ac splfig.
$