凯撒密码程序来加密/解密段落

Question

First off, I am a newcomer to programming, so be gentle. Also I have been working on this assignment to no avail. The assignment is to create a Caesar Cipher program which encrypts or decrypts a paragraph up to 100 characters. It's actually two separate labs. The first lab is to encrypt, then the second lab is to decrypt. Once I figure out how to create the encryption program, the decryption program should be simple as I can just make semantic changes to decrypt instead of encrypt. Anyway here is the code I have so far. It passed 4 of the 5 tests they give us, but for some reason there is one test where the final character is an '@' symbol. This makes no sense to me as it does not happen in any other tests, and I believe that my code places a '\\0' symbol at the of the string, so '@' should not be showing up in this one test.

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

// ---------------------- DO NOT MODIFY THIS SECTION -----------------------    ---------
#define MAX_PGRAPH_LENGTH 100
#define MAX_WORD_LENGHT 20

int main(void) {   
// definitions
char plaintext[MAX_PGRAPH_LENGTH] = "";
char ciphertext[MAX_PGRAPH_LENGTH];
char input[MAX_WORD_LENGHT];    

// read the key
int key;
scanf("Key: %d, ", &key);

// read text
scanf("Input: ");
while (true)
{
    scanf("%s", input);
    if (strlen(plaintext) + strlen(input) + 1 > MAX_PGRAPH_LENGTH)
        break;

    strcat(plaintext, input);
    strcat(plaintext, " ");
}
plaintext[strlen(plaintext) - 1] = '\0';
// ---------------------- --------------------------------------------------    ---------




int i;

for(i = 0; i < strlen(plaintext); ++i) 
{

  if(plaintext[i] >= 'a' && plaintext[i] <= 'z') {
     ciphertext[i] = ((plaintext[i] + (key % 26) - 97) % 26) + 97; 

     if(ciphertext[i] > 'z') {
        ciphertext[i] = ((plaintext[i] + (key % 26) - 97) % 26) + 97 - 26; }
  }


  else if(plaintext[i] >= 'A' && plaintext[i] <= 'Z') {
     ciphertext[i] = ((plaintext[i] + (key % 26) - 'A') % 26) + 'A';

     if(ciphertext[i] > 'Z') {
        ciphertext[i] = ((plaintext[i] + (key % 26) - 'A') % 26) + 'A' - 26; }
  }

  else {
     ciphertext[i] = plaintext[i]; }

}
 for(i = 0; i < strlen(plaintext) && plaintext[i] == '\0'; ++i) {
  ciphertext[i] = '\0'; }



// ---------------------- DO NOT MODIFY THIS SECTION -----------------------    ---------
printf("   Key: %d\n", key);
printf(" Input: %s\n", plaintext); 
printf("Output: %s\n", ciphertext);
// ---------------------- --------------------------------------------------    ---------
}

As you can see, the lab gave us the setup and multiple blocks of code, and I am responsible for the actual Caesar cipher portion of the coding. My question is, am I not properly setting the '\\0' at the last position in C? One thing I noticed as well is that if I type an input such as "The dog ran", when input is printed to the screen, it will print "The dog ran ran ran ran ran.." until it hits 100 characters. Luckily, all of the tests they give us to run are paragraphs which exceed 100 characters, so I actually don't have to worry about passing a test in which I'm encrypting less than 100 characters. But I would still like to know why my plaintext string is repeating the last input word over and over. Sorry for the ridiculously long post, I have tried everything though and do not know where I am going wrong.

Answer 1

It repeats the last word because the you do not exit the infinite loop.

while (true)
{
    scanf("%s", input);
    if (strlen(plaintext) + strlen(input) + 1 > MAX_PGRAPH_LENGTH)
        break;

    strcat(plaintext, input);
    strcat(plaintext, " ");
}

The only way you exit the loop is executing the break statement and this is only possible with total lenghts of input and plaintext being greater than MAX_PGRAPH_LENGTH . The reason why only the last word is being repeated is because most probably scanf() reads the input word by word.

Also, the reason why you did not pass the test case with @ character might be due to the fact that alphabetic characters start with decimal 65 (corresponds to A), and the decimal value of @ is 64. The difference between A and @ being 1 and the code failing the test case with @ is not a coincidence in my opinion. So, I think you should double-check the arithmetic operations and trace the code for a sample input.

Answer 2

The repetition is in the supplied code... not in your code.

Read next word (forever), until the max paragraph length is reached. If there is no next word, the value of input is not updated, and is simply repeated until you reach a total length of MAX_PGRAPH_LENGTH

while (true)
{
    scanf("%s", input);
    if (strlen(plaintext) + strlen(input) + 1 > MAX_PGRAPH_LENGTH)
        break;

    strcat(plaintext, input);
    strcat(plaintext, " ");
}