Strange char appears in the output - c

Question

I'm trying to obfuscate word which is stored in string and my code sometimes works and sometimes doesn't. Here is my code:
// main function
int main(int argc, string argv[])
{
string k, plaintext;
int size, i = 0, key = 0;
k = argv[1];
size = strlen(k);
if (argc < 2 || !isNummeric(k, size) || k < 0)
return 1;
else
plaintext = GetString();
size = strlen(plaintext);
char ciphertext[size];
key = atoi(k);
while(i < size)
{
if (isalpha(plaintext[i]))
{
encipher(key, i, &ciphertext[i], plaintext);
}
else
{
ciphertext[i] = plaintext[i];
}
i++;
}
printf("%s\n",ciphertext);
}
A key is received from the user to shift each letter and I need to check whether the key is numeric value or not so I made isNummeric function to do that
bool isNummeric(string k, int size)
{
int c=0;
for(int i=0; i<size; i++)
{
if(!isdigit(k[i]))
c++;
}
if(c==0)
return true;
return false;
}
Now to encipher I made function to shift each letter:
void encipher(int k, int i, char *pt, string plaintext)
{
int p, c;
if(islower(plaintext[i]))
{
p=plaintext[i]-'a';
c=(p+k)%26;
*pt=c+97;
}
else
{
p=plaintext[i]-'A';
c=(p+k)%26;
*pt=c+65;
}
}

Answer 1

Assembling multiple comments from the question into fixed code yields the following code which seems to work:
#include <cs50.h>
#include <ctype.h>
#include <stdio.h>
#include <string.h>
static bool isNumeric(string k, int size)
{
for (int i = 0; i < size; i++)
{
if (!isdigit((unsigned char)k[i]))
return false;
}
return true;
}
/* There are too many arguments to this function - but it works */
static void encipher(int k, int i, char *pt, string plaintext)
{
int p, c;
if (islower((unsigned char)plaintext[i]))
{
p = plaintext[i] - 'a';
c = (p + k) % 26;
*pt = c + 'a';
}
else
{
p = plaintext[i] - 'A';
c = (p + k) % 26;
*pt = c + 'A';
}
}
int main(int argc, string argv[])
{
string k = argv[1];
if (argc < 2 || !isNumeric(k, strlen(k)))
return 1;
string plaintext = GetString();
int size = strlen(plaintext);
char ciphertext[size + 1];
int key = atoi(k);
for (int i = 0; i < size; i++)
{
if (isalpha(plaintext[i]))
{
encipher(key, i, &ciphertext[i], plaintext);
}
else
{
ciphertext[i] = plaintext[i];
}
}
ciphertext[size] = '\0';
printf("%s\n", ciphertext);
}
The program was called csr13, and gives the following outputs:
$ csr13 4
The Quick Brown Fox Jumped Over The Lazy Dog
Xli Uymgo Fvsar Jsb Nyqtih Sziv Xli Pedc Hsk
$ csr13 22
Xli Uymgo Fvsar Jsb Nyqtih Sziv Xli Pedc Hsk
The Quick Brown Fox Jumped Over The Lazy Dog
$
A better design for the encipher function would pass the single character plus the 'key' offset and would return the encrypted character:
#include <assert.h>
#include <cs50.h>
#include <ctype.h>
#include <stdio.h>
#include <string.h>
static bool isNumeric(string k, int size)
{
for (int i = 0; i < size; i++)
{
if (!isdigit((unsigned char)k[i]))
return false;
}
return true;
}
static int encipher(int k, int c)
{
assert(isalpha(c));
if (islower(c))
return (c - 'a' + k) % 26 + 'a';
else
return (c - 'A' + k) % 26 + 'A';
}
int main(int argc, string argv[])
{
string k = argv[1];
if (argc < 2 || !isNumeric(k, strlen(k)))
return 1;
string plaintext = GetString();
int size = strlen(plaintext);
char ciphertext[size + 1];
int key = atoi(k);
for (int i = 0; i < size; i++)
{
if (isalpha(plaintext[i]))
ciphertext[i] = encipher(key, plaintext[i]);
else
ciphertext[i] = plaintext[i];
}
ciphertext[size] = '\0';
printf("%s\n", ciphertext);
}