i++ or ++i?
Using i++ is fine here, but also read this discussion:
http://stackoverflow.com/questions/4261708/i-or-i-in-for-loops
fgets(message, 128, stdin);
How about fget(message, sizeof(message), stdin) -- makes it easier to change the size of the message buffer.
int main(int argc, char * keyWord[])
Most people are used to seeing:
int main(int argc, char* argv[])
int alphaCheck(char * argv[])
How about:
int alphaCheck(char *str) {
for ( ;*str; ++str) {
if (!isalpha(*str)) {
...
}
}
return 0; // You don't have this return.
}
First of all, this makes alphaCheck usable on any string, not just argv[1]. Also you avoid computing the length of the string. Moreover, this is the idiomatic way to iterate over a null-terminated string in C.
Finally - make sure you always return a value since you've declared this function returns an int.
for (i = 0, j = strlen(keyWord[1]); i < j; i++) {
You are modifying cipher[i] but are you checking that i < 64? What if keyWord[1] has length > 64?
keyWord[1]
You reference this array element many times in the program. Perhaps you should assign a meaningful variable name to it, e.g.:
char* secret = keyWord[1];
It will make you program more readable.
printf("%c\n", message[i]);
Do you want to always print a \n here? I would think you only want to print an extra \n at the end of the message.
the main loop
You main loop has this structure:
for each character c in the message:
let e = encrypt c using the secret key
printf("%c", e)
advance pointers, counters, etc.
printf("\n")
so it would be nice if the code could match this as much as possible.
How about:
int i;
for (char *p = message, i = 0; *p; ++p, ++i) {
char e = ...encrypt (*p) using secret, strlen(secret) and i...
printf("%c", e);
}
printf("\n");
Now there is only one place where the encrypted character is printed out - instead of three difference places.
You can implement this via a helper function:
char encrypt(char* secret, int secret_len, char c, int i) {
// return how c is encrypted when at position i in the message.
if (isupper(c)) {
return ...
} else if (islower(c)) {
return ...
} else {
return c;
}
}
Hint: The expression i % secret_len might come in handy here.