Getting Caesar Cipher to wrap around - c

Question

I can get it to print the plaintext and and shift by the key value, but
i'm a bit confused on how to get the letters to wrap around, and how to implement it into my code.
Any suggestions would be appreciated.
Thank you.
#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>
//Gets number of user arguments and the key.
int main (int argc, string argv[]) {
if(argc != 2) {
printf("try again\n");
}
//Converts string to int.
int key = atoi(argv[1]);
//Will store the chars + key.
int result;
printf("Please enter what you would like to encrypt: ");
//Gets plaintext from user.
string plainText = get_string();
//Iterates over the user's input, checking for the case of each char.
for (int i = 0; i <= strlen(plainText); i++) {
if (toupper(plainText[i]) || tolower(plainText[i])) {
result = plainText[i];
}
//Checks if i'th char is a letter and shifts it.
if (isalpha(plainText[i])) {
result = plainText[i + key];
}
}
printf("%c", result);
}

Answer 1

One of the neatest tricks to do this is to use the modulo % operator.
Now talking about your code,
for (int i = 0; i <= strlen(plainText); i++) {
if (toupper(plainText[i]) || tolower(plainText[i])) {
result = plainText[i];
}
//Checks if i'th char is a letter and shifts it.
if (isalpha(plainText[i])) {
result = plainText[i + key];
}
}
printf("%c", result);
This code makes no sense to me.
Your first if condition is I guess to distinguish of the not alphabetical characters, so the if condition could be something like if (! isalpha(plainText[i]) ,
Then your second condition is to add the key to the character if it is an alphabet. It should be something like
if (isalpha (plainText[i])) {
if (islower(plainText[i])
result = ((plainText[i] - 'a') + key) % 26 + 'a';
else
result = ((plainText[i] - 'A') + key) % 26 + 'A';
}
Explanation of above logic:: First you check weather the letter is lowercase or uppercase, so that you can make it in range of 0 to 26,
Then you add the key with the modulo of key, so that it can circle back to 0, then you again convert that it to ascii by adding the value of 'a' their.
e.g. if plainText[i] = 'x' (ascii value 120) and key = 5, then
plainText[i] = 120
plaintext[i] - 'a' = 23
(plaintext[i] - 'a') + key = 28 // Out of 0-25 alphabet range
((plaintext[i] - 'a') + key) % 26 = 2 // Looped back
(((plaintext[i] - 'a') + key) % 26) + 'a' = 99 (ascii value for 'c')
So as you can see we got c after adding 5 to x
And finally the position of your print should be inside loop, otherwise it's gonna only print the last input, which is not correct.
I hope I did everything to help you, keeping in mind the CS50's Honor Code. And also I would suggest you to ask these questions in their forums, because they are a more knoledgeble community to use <cs50.h>
Also, enjoy CS50, it is one of the best CS courses to get you started ;)