第二个“if”语句是什么意思（凯撒密码）

Question

I recently took an interest in encryption, then I came across this piece code, but I don't understand the meaning of the second "IF" statement.

#include<stdio.h>

int main()
{
    char message[100], ch;
    int i, key;

    printf("Enter a message to encrypt: ");
    gets(message);
    printf("Enter key: ");
    scanf("%d", &key);

    for (i = 0; message[i] != '\0'; ++i) {
        ch = message[i];

        if (ch >= 'A' && ch <= 'Z') {
            ch = ch + key;

            if (ch > 'Z') {
                ch = ch - 'Z' + 'A' - 1;
            }
            
            message[i] = ch;
        }
    }

    printf("Encrypted message: %s", message);

    return 0;
}

Answer 1

One of the fundamental things to understand for that code to make sense is that char represents both a character and a number at the same time.

That's because to a C program (and effectively all programs on all computers) characters are just numbers anyway.

When you say 'A' you could equivalently use 65 or 0x41 ^* .

You already make use of that property by adding key to ch to "shift up" the characters (ie if key is 1 then 'A' becomes 'B' ).

However, if you add 1 to 'Z' then the result is no longer an upper-case character (it will be [ in ASCII and compatible encodings).

The second if is responsible to wrap those results back down to 'A' by first subtracting 'Z' (which means the value is now "how many steps above Z were we) and then adding 'A' - 1 (which means that 1 step above Z will result in 'A ').

^{* Assuming this is using ASCII or a compatible encoding, which will hold true on most modern operating systems}

Answer 2

Let's rewrite the calculation that is performed if the expression of the if statement is true :

ch = ch - 'Z' + 'A' - 1;

to

ch = ch - ('Z' - 'A' + 1);

This should already make more sense. Now the part between parentheses of course never changes value, so lets introduce a constant:

#define APHABET_SIZE = ('Z' - 'A' + 1)
...
ch = ch - ALPHABET_SIZE;

Ah, so if the value of the character is higher than Z, the last letter of the alphabet, then subtract the alphabet size.

---

This is the kind of programming that you should not perform yourself; it performs all the functions at once instead of breaking them down, and trades briefness for readability.

The Caesar cipher performs modular addition to shift the characters by index. You should instead have a function int charToIndex(char) , then perform the modular addition (or subtraction for decryption) using the % modulo operator and ALPHABET_SIZE , and then have a function char indexToChar(int) to convert it back.