So I am working on a SC50 problem where I need to make a simple cipher and be able to encrypt words or sentences... After 2 full days I actually kindof figured it all out, but my code was really long and after some googling I found a version out there that was much better. And it was real easy and all, except for the part where there is stuff that I don't really understand how it works, and I would really like to find out how... so here is the full code below (unfortunately I can't seem to find the original source of the code right now, but I actually did at least half of it myself, and only the part after "//SUBSTITUTION is copied) :
and also, what I wonder about, are these two rows:
printf("%c", toupper(arg[plaintext[i] - 65])); //calculation to print the encipher text amd make sure it is Uppercase (case doesn't change)
and
printf("%c", tolower(argv[1][plaintext[i] - 97])); ///calculation to print the encipher text amd make sure it is lowercase(case doesn't change)
...I can't wrap my head around, how the calculation "-65" and "-66" are solving the issue... Lets say that in my key, the first letter is a Q, and when I write and A, it should be substituted for a Q...
A = 65 and Q = 81 on the Ascii table, so when I take 65 - 65... mm why would I do that? obviously it needs to be done for this program to work correctly, but I don't understand how it works and what actually happens...
what is the logic behind these calculations? please help!
#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(int argc, string argv[]) {
if (argc != 2) {
printf("Usage: ./substitution key\n");
return 1;
}
string arg = argv[1];
int chars = 0;
for (int i = 0; i < strlen(arg); i++) {
if (isalpha(arg[i])) {
for (int j = i+1; j < strlen(arg); j++) {
if (toupper(arg[j]) == toupper(arg[i])) {
printf("Key must not contain repeated alphabets.\n");
return 1;
}
}
chars += 1;
}
}
if (chars != 26) {
printf("Key must contain 26 characters.\n");
return 1;
}
// SUBSTITUTION
printf("%s\n", arg);
string plaintext = get_string("plaintext: "); //Getting user's input as plaintext
printf("ciphertext: "); //to print the ciphertext
int plaintext_length = strlen(plaintext); //get the strlen of plaintext (user's input)
for (int i = 0; i < plaintext_length; i++) { //iterate over the plaintext_Length
if (isupper(plaintext[i])) { //check if plaintext character is uppercase
printf("%c", toupper(arg[plaintext[i] - 65])); //calculation to print the encipher text amd make sure it is Uppercase (case doesn't change)
}
else if (islower(plaintext[i])) { //check if plaintext character is lowercase
printf("%c", tolower(arg[plaintext[i] - 97])); ///calculation to print the encipher text amd make sure it is lowercase(case doesn't change)
}
else { //if plaintext is anything else, print it like that without changing
printf("%c", plaintext[i]);
}
}
printf("\n"); //print new line
}
In general it is a bad code.
For example instead of using magic numbers 65 or 97
printf("%c", toupper(arg[plaintext[i] - 65]));
printf("%c", tolower(argv[1][plaintext[i] - 97]));
it is better to write
printf("%c", toupper(arg[plaintext[i] - 'A']));
printf("%c", tolower(argv[1][plaintext[i] - 'a']));
argv[1] or arg contains a string of 26 letters as for example
"xyzabcgtidefuvwjklmn0rspqh"
If you have a string as for example "Hello" then 'H' - 'A' gives the value 7. Using the number you can find at position 7 in the array pointed by the string argv[1] or arg the letter t
"xyzabcgtidefuvwjklmn0rspqh"
^
|
'H'
So the letter 'H' in the source string is coded like the letter 'T'. For the second letter 'e' you have 'e' - 'a' is equal to 4. So you have
"xyzabcgtidefuvwjklmn0rspqh"
^
|
'e'
So the first two letter of the string "Hello" becomes "Tbllo". This approach is used for the remaining letters of the source string to encrypt it.