Question

I want to encode a string in C using a key string.
The encodeMSG function returns int array, so that:
intArr[i] = the int value of msg[i] + the int value of key[i].
If the length of the key string is shorter the the msg string, it should go back to the beginning (cycle).

I’m not sure how I should do this although it doesn’t seem too complicated.
I also wasn’t sure whether I should use atoi(msg + i) or a simple cast like (int)(*(msg + i)).

int *encodeMSG(char *msg, char *key)
{
   int i, msgLen;
   int *encodedArr = (int *)malloc(strlen(msg) * sizeof(int));
   char *keyBackup = key;
   msgLen = (strlen(msg));

   for (i = 0; i < msgLen; ++i)
   {
       if (*(key + i) == '\0')
           key = keyBackup;

       *(encodedArr + i) = *(msg + i); //creating an integer-represented array of the char array [msg]
       *(encodedArr + i) += *(key + i); //adding the [key] array integer values to the integer-represented array of the message
   }

   return encodedArr;
}

Answer 1

Try this:

int *encodeMSG(char *msg, char *key)
{
    char *key0 = key;
    int *result = (int*) malloc( sizeof(int)*(strlen(msg)+1) );
      // +1 to include the final zero of msg
    int *result0 = result;
    while( *msg ) // i assume it's zero-terminated
    {
        *result++ = ((int)*msg) + ((int)*key);
        ++msg;
        ++key;
        if(!*key)key=key0; // reset it to the beginning of the key
    }
    *result = 0; // the final zero
    return result0;
}

This will use the ascii values directly, atoi is not useful here.
Note that *msg is the actual char msg points to. As msg is zero-terminated, the end of the string is 0 of false. Same applys for *key and when *key == 0 (or !*key) then just reset it to the start of the key phrase.

By the way: It returns an int[] with 1 more int than chars in msg, last int is zero.