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

bool check_char(string key);
void cipher_function(string plaintext, string argv);

int main(int argc, string argv[])
{
    if (argc != 2)
    {
        printf("Usage: ./substitution key\n");
        return 1;
    }
    if (!check_char(argv[1]))
    {
        printf("Key must contain 26 unique characters.\n");
        return 1;
    }

    // Get plaintext from user
    string plaintext = get_string("plaintext: ");

    // Function to generate and print ciphertext based on plaintext and key
    cipher_function(plaintext, argv[1]);

}

bool check_char(string key)
{
    int length;
    length = strlen(key);

    if (length != 26)
    {
        return false;
    }
    for (int i = 0; i < length; i++)
    {
        key[i] = toupper(key[i]);
    }
    for (int i = 0; i < length; i++)
    {
        if (!isalpha(key[i]))
        {
            return false;
        }
        for (int j = i + 1; j < length; j++)
        {
            if (key[i] == key[j])
            {
                return false;
            }
        }
    }
    return true;
}

void cipher_function(string plaintext, string argv)
{
    int length = strlen(plaintext);
    int index;
    char ciphertext[length + 1];

    for (int i = 0; i < length; i++)
    {
        if (islower(plaintext[i])) // If plaintext character is lowercase
        {
            index = plaintext[i] - 97;
            ciphertext[i] = argv[index];
            if (isupper(ciphertext[i])) // If the ciphertext char for corresponding plaintext is upper, then convert to lower
            {
                ciphertext[i] += 32;
            }
        }
        else if (isupper(plaintext[i])) // If plaintext character is uppercase
        {
            index = plaintext[i] - 65;
            ciphertext[i] = argv[index];
            if (islower(ciphertext[i]))
            {
                ciphertext[i] -= 32;   // If the ciphertext char for corresponding plaintext is lower, then convert to upper
            }
        }
        else
        {
            ciphertext[i] = plaintext[i]; // To print out non-alpha characters in the plaintext like spaces, numbers etc.
        }
    }
    ciphertext[length] = '\0';
    printf("ciphertext: %s\n", ciphertext);
}

字符串
因此，我尝试不存储/0（第92行），只将密文数组的长度设置为等于明文数组（第65行）。这导致一些输入在输出的末尾显示一个带有问号的奇怪符号。
我在某个地方读到，我们需要存储/0来向编译器显示字符串的结束位置。我知道这适用于字符串，但它究竟如何适用于数组？