c - 维吉尼亚密码困难

Question

这是 cs50 的 Vigenere 密码。这是我第一次编码，我已经解决这个问题一周了，而且我似乎无法在第一次循环完成后打印第一个字母。

例如:

jharvard@appliance (~/Dropbox): ./viginere abcde You're key is abcdeType your text: aaaaa aaaaaa aaaaaaa aaaaaaaaabcde bcde bcdebc debcde

First a is printed but then it starts on b and in the end it doesn't print every letter. The key is chosen by the user.

I have no idea what I'm doing wrong.

for (int i = 0, j = strlen(plain_text), l = 0; i < j; i++)
{    
    int rotation_1 = (tolower(plain_text[i]) + (key[l] - 97)) % 122;
    int rotation_2 = (plain_text[i] + (key[l] - 97)) % 122;

    //if it is a letter
    if (isalpha(plain_text[i]))
    {   
        l = l % strlen(key);
        //if the it is uppercase
        if (isupper(plain_text[i]))
        { 
            printf("%c", toupper(rotation_1)); 
        }
        //else if it is lowercase
        else
        {  
            printf("%c", rotation_2);   
        }
        l++;        
    }                     
    // if it is not a letter we print it as it is     
    else           
    {
        printf("%c", plain_text[i]);
    }            
}

Answer 1

这是一个更简单的版本，其中逻辑已修复:

for (int i = 0, n = strlen(plain_text), k = 0, klen = strlen(key); i < n; i++) {
    int c = (unsigned char)plain_text[i];

    //if it is a letter
    if (isalpha(c)) {
        if (isupper(c)) {
            c = 'A' + (c - 'A' + key[k] - 'a') % 26;
        } else {
            c = 'a' + (c - 'a' + key[k] - 'a') % 26;
        }
        k = (k + 1) % klen;
    }
    putchar(c);
}                     

投plain_text[i]很重要如(unsigned char)因为isalpha和isupper仅为 unsigned char 的所有值定义和EOF 。 char可能会被您的平台默认签名。

另请注意 putchar(c)比 printf("%c", c); 效率高得多