凯撒密码 C 程序不打印

Question

Basically what the title says. Can anyone tell me why my program won't print at the end? As far as I can tell the for loop should be rotating each letter but at this point, I have no idea.

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

int main(void)
{

    int key; 
    bool keyValid = false;

    while (keyValid == false)
    {
        printf("Enter rotation key: ");
        scanf("%d", &key);

        if (key >= 0 && key <= 26)
        {
            keyValid = true;
        }
        else
        {
            printf("Error - key must be in range 0 and 26. Try again. ");
        }
    }

    // printf("Encrypting message (key = %d): %s \n", key, message);

    
    

    if(keyValid == true) {

        char message[80]; 
        printf("Enter message to encrypt: ");
        scanf(" %80[^\n]*s", message);

        for(int i = 0; i < strlen(message); i++){
            
            if (isupper(message[i])){
                message[i]= (message[i] - 65 + key) & 26 + 65;
            }
           else  if (islower(message[i])){
                message[i]= (message[i] - 97 + key) & 26 + 97;
            }

        }

        printf("%s", message);
    }

    return 0;
}

Answer 1

Your problem is here

message[i]= (message[i] - 65 + key) & 26 + 65; ^ ^

The expression is evaluated as (message[i] - 65 + key) & 91 which most likely results in a unprintable character.

---

You want

message[i] = ((message[i] - 'A' + key) & 26) + 'A';

or, even better, using modulus %

message[i] = ((message[i] - 'A' + key) % 26) + 'A';    // keep the extra parenthesis!
message[i] = (message[i] - 'A' + key) % 26 + 'A';

Answer 2

The calculations being performed in the following lines produce non-printable results.

message[i]= (message[i] - 65 + key) & 26 + 65;
message[i]= (message[i] - 97 + key) & 26 + 97;

Replacing both lines with this should do the trick.

message[i]= (message[i] + key);