cs50's caesar.c

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

int main (int argc, string argv[]){
    // testing if user has put correct number of arguments in command-line
    if (argc != 2){
        return 1;
    }

    // transforming the string containing the key into an actual integer
    int k = atoi(argv[1]);

    //getting the plain text from the user
    string  plaintext = GetString();

    //declaring the string that will contain the cypher text
    string cipher[strlen(plaintext)];

    int letter;

    for (int i=0, j= strlen(plaintext); i<j; i++)
    {
       // getting letter to be changed
       letter = (int) plaintext[i];
       // checking if character is actually alphabetic
       if (isalpha(plaintext[i])){

           letter = (letter) + (k% 26);
           // checking if wrap round  from Z to A is needed
           if (isupper(plaintext[i])){
               if (letter>90){
                   letter= 65+ letter-91;
               }
           }
           if (islower(plaintext[i])){
           // checking if wrap round  from z to a is needed
               if (letter>122){
                   letter= 97+ letter-123;
               }
           }
           cipher[i]= (string) letter;
           printf("%c", (char) cipher[i]);
       }
       else{
           printf("%c", (plaintext[i]));
       }

    }
    printf("\n");
    return 0;
}