帮我按这个用c++写一个playfair加密代码

当然可以，以下是你想要的代码：

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

int find_pos(char c, char key[5][5])
{
    int i, j;
    for(i = 0; i < 5; i++)
    {
        for(j = 0; j < 5; j++)
        {
            if(key[i][j] == c)
            {
                return 5 * i + j;
            }
        }
    }
    return -1;
}

void playfair_encipher(char* plaintext, char* ciphertext, char key[5][5])
{
    int i;
    int len = strlen(plaintext);
    for(i = 0; i < len; i += 2)
    {
        char c1 = plaintext[i];
        char c2 = (i + 1 < len) ? plaintext[i+1] : 'x';
        if(c1 == c2) c2 = 'x';

        int pos1 = find_pos(c1, key);
        int pos2 = find_pos(c2, key);

        int row1 = pos1 / 5;
        int col1 = pos1 % 5;
        int row2 = pos2 / 5;
        int col2 = pos2 % 5;

        if(row1 == row2)
        {
            ciphertext[i] = key[row1][(col1+1)%5];
            ciphertext[i+1] = key[row2][(col2+1)%5];
        }
        else if(col1 == col2)
        {
            ciphertext[i] = key[(row1+1)%5][col1];
            ciphertext[i+1] = key[(row2+1)%5][col2];
        }
        else
        {
            ciphertext[i] = key[row1][col2];
            ciphertext[i+1] = key[row2][col1];
        }
    }
    ciphertext[i] = '\0';
}

int main()
{
    char plaintext[] = "hello world";
    char key[5][5] = {
        {'p', 'l', 'a', 'y', 'f'},
        {'i', 'r', 'e', 'x', 'm'},
        {'b', 'c', 'd', 'g', 'h'},
        {'k', 'n', 'o', 'q', 's'},
        {'t', 'u', 'v', 'w', 'z'}
    };
    char ciphertext[100];

    playfair_encipher(plaintext, ciphertext, key);

    printf("plaintext: %s\n", plaintext);
    printf("ciphertext: %s\n", ciphertext);

    return 0;
}