Notes

#include <stdio.h>
#include <string.h>
#include <cs50.h>
#include <ctype.h>

char caesar(char token, int key);

int main(int argc, string argv[]) {

// Variable declarations
bool keySuccessful = false;
string keyword = "";
int keylength = 0;
string user_text = "";
int user_text_length;
int key_count = 0;

do
{
// The number of command line args submitted was incorrect.
if(argc != 2)
{
printf("You didn't enter the correct number of keywords.n");
return 1;
}
else if(argv[1])
{
int length = strlen(argv[1]);
for(int i = 0; i < length; i++)
{
if(!isalpha(argv[1][i]))
{
// We accept only letters as input.
printf("Your input contains illegal characters.n");
return 1;
}
else
{
// All good, input can be accepted as key.
keySuccessful = true;
keyword = argv[1];
}
}
}
} while(!keySuccessful);

// We check for the length of the keyword and define an array with that length.
keylength = strlen(keyword);
int keycodes[keylength];

// The letters in the keyword array should be converted to numbers
// starting from A = 0 to Z = 25 ignoring case.
for(int i = 0; i < keylength;i++)
{
keycodes[i] = toupper(keyword[i]) - 65;
}

// Read in user text and calculate its length.
user_text = GetString();
user_text_length = strlen(user_text);

for (int i = 0; i < user_text_length; i++)
{
// If input at given position is not letter, just mirror it.
if(!isalpha(user_text[i]))
{
printf("%c", user_text[i]);
}
// Process input.
else
{
printf("%c", caesar(user_text[i], keycodes[key_count]));

// Increase the position counter for the keycode array.
if(key_count < keylength - 1)
{
key_count++;
}
else
{
key_count = 0;
}
}
}

// Be a nice citizen and exit cleanly.
printf("n");
return 0;
}

/*
This function applies a case-sensitive caeser cipher for a single
character using the supplied key.
Please note that it provides no input checking!
The caller is responsible for this task.
*/
char caesar(char token, int key)
{
if(islower(token))
{
return ((((token - 97)+key)%26)+97);
}
else
{
return ((((token - 65) +key)%26)+65);
}
}