Cs50 Caesar Check50 fail - c

Question

I am doing Caesar exercise from CS50 course, but it fail.
Here is the code:
# include <stdio.h>
# include <cs50.h>
# include <string.h>
# include <stdlib.h>
# include <ctype.h>
int main(int argc, string argv[])
{
if (argc == 2)
{
string key = argv[1];
int l = strlen(key);
for (int i = 0; i < l; i++)
{
// if key[i] is an alphabet character
if (isalpha(key[i]) != 0)
{
printf("Usage: ./caesar key\n");
return 1;
}
}
//change charater to number
int k = atoi(argv[1]);
//print Plaintext
string plaintext = get_string("Plaintext: ");
int n = strlen(plaintext);
char ciphertext[n];
//declare plaintext in ASCII
int nplaintext[n];
//Change plaintext to ASCII
for (int i = 0; i < n; i++)
{
nplaintext[i] = (int)plaintext[i];
}
//Declare ASCII for ciphertext which we name "plusplaintext"
int plusnplaintext[n];
for (int i = 0; i < n; i++)
{
//if Capital
if ((nplaintext[i] < 91) && (nplaintext[i] > 64))
{
plusnplaintext[i] = 65 + ((nplaintext[i] + k) - 65) % 26 ;
}
//if Lowercase
else if ((nplaintext[i] < 123) && (nplaintext[i] > 96))
{
plusnplaintext[i] = 97 + ((nplaintext[i] + k) - 97) % 26 ;
}
//if not character a -> z and A -> Z
else
{
plusnplaintext[i] = nplaintext[i];
}
}
for (int i = 0; i < n; i++)
{
ciphertext[i] = (char)plusnplaintext[i];
}
printf("ciphertext: %s\n", ciphertext);
}
else
{
printf("Usage: ./caesar key\n");
return 1;
}
}
Here is the output from Check50:
:) caesar.c exists.
:) caesar.c compiles.
**:( encrypts "a" as "b" using 1 as key, output not valid ASCII text**
:) encrypts "barfoo" as "yxocll" using 23 as key
:) encrypts "BARFOO" as "EDUIRR" using 3 as key
:) encrypts "BaRFoo" as "FeVJss" using 4 as key
:) encrypts "barfoo" as "onesbb" using 65 as key
**:( encrypts "world, say hello!" as "iadxp, emk tqxxa!" using 12 as key, output not valid ASCII text**
:) handles lack of argv[1]
When I test the code, sometimes it gives right result, sometimes it give the wrong result with some more characters at the end...
How can I correct my code?

Answer 1

The problem is with the logic here
//if Capital
plusnplaintext[i] = 65 + ((nplaintext[i] + k) - 65) % 26 ;
and
//if Lowercase
plusnplaintext[i] = 97 + ((nplaintext[i] + k) - 97) % 26 ;
use this logic instead
//if Capital
((((nplaintext[i] - 90) + 25) + key) % 26) + 65
//if Lowercase
((((nplaintext[i] - 122) + 25) + key) % 26) + 97
and it is also not necessary to assign the letters to a string, instead you can directly print it
#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>
int main(int argc, string argv[])
{
// Error Checking on the command line argument
if (argc != 2)
{
printf("Usage: ./caesar key\n");
return 1;
}
else if (argc == 2)
{
for (int i = 0; i < strlen(argv[1]); i++)
{
if (isdigit(argv[1][i]) == 0)
{
printf("Usage: ./caesar key\n");
return 1;
}
}
}
int key = atoi(argv[1]);
string plain_text = get_string("plaintext: ");
printf("ciphertext: ");
for (int i = 0; i < strlen(plain_text); i++)
{
if (isalpha(plain_text[i]) != 0)
{
// printf("%c", plain_text[i] + key);
if (isupper(plain_text[i]))
{
printf("%c", ((((plain_text[i] - 90) + 25) + key) % 26) + 65);
}
else
{
printf("%c", ((((plain_text[i] - 122) + 25) + key) % 26) + 97);
}
}
else
{
printf("%c", plain_text[i]);
}
}
printf("\n");
}

Answer 2

Instead of using the ascii value of a letter in your code, it could be helpful to keep them as characters for the sake of clarity. I took a snippet of your code and changed the values to demonstrate.
//if Capital
if ((nplaintext[i] <= 'Z') && (nplaintext[i] >= 'A'))
{
plusnplaintext[i] = 'A' + ((nplaintext[i] + k) - 'A') % 26 ;
}
This makes it clear what those values represent, and you don't have to reference an ascii chart. More comments would also help a user, or yourself later on, understand what the purpose is for each part of your code.
You could also create some functions outside of your main function. For example, to check for all digits in command line input, or change the letters by the amount given in the key. If you start working on long programs of code, it could be helpful to have functions defined that you can use as many times as you need. It also cleans up your main for clarity.
This function below takes the character of each letter in the original text (char p), then "moves" the character by k spaces (int k, given by the user as the key in the command line). It works when called in a for loop in main that iterates over each letter in the original given string (text[i]), with i being increased for each time the loop executes.
char rotate(char p, int k)
{
// declare variable for the rotated letter to be stored
char c;
// check if it is a letter
if (isalpha(p))
{
// check for lowercase letter
if (islower(p))
{
// subtract ascii value from p to initialize to 0 - 25 for computations
p -= 'a'; // if p is a, it is now initialized to 0, if b to 1, if c to 3, etc
c = (p + k) % 26; // use Caesar's algorithm to rotate letters, 'wrapping' by using % 26
c += 'a'; // add ascii value back to result to get the rotated letter
}
// the only other option is uppercase since we checked for only letters, do the same for uppercase letters as for lowercase
else
{
p -= 'A';
c = (p + k) % 26;
c += 'A';
}
}
// if it is the nul character '\0' return 0, do not print
else if (p == '\0')
{
return 0;
}
// if not a letter or nul character, return the character as is
else
{
c = p;
}
// print the rotated letter which is now stored in c
printf("%c", c);
// return the value of c
return c;
}