daily.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

#define MAX_MESSAGE_LENGTH 1024

int main(int argc, char* argv[]) {

	if (argc != 2) {
		printf("Please select a key.");
		return 1;
	}

	int key = atoi(argv[1]);

	puts("What message would you like to encrypt?\n");

	char message[MAX_MESSAGE_LENGTH];
	char cipher[MAX_MESSAGE_LENGTH];

	fgets(message, MAX_MESSAGE_LENGTH, stdin);

	int i = 0;
	int length = strlen(message);

	// trim the last char (newline)
	length--;
	message[length] = '\0';

	for(; i < length; i++) {

		if(isalpha(message[i])) {
			if(isupper(message[i])) {
				cipher[i] = (message[i] - 65 + key) % 26 + 65;
			} else if(islower(message[i])) {
				cipher[i] = (message[i] - 97 + key) % 26 + 97;
			}
		}

		printf("input char:%c cipher char:%c (ascii value: %d)\n", message[i], cipher[i], cipher[i]);

	}

	return 0;

}