凯撒密码算法中 C 中的分段错误

Question

正在尝试解决 CS50 的 Ceasar 解决方案。我遇到分段错误。

更正式地说，凯撒算法（即密码）通过将每个字母“旋转”k 个位置来加密消息。更正式地说，如果 p 是一些明文（即未加密的消息），pi 是 p 中的第 i 个字符，k 是密钥（即非负整数），那么密文 c 中的每个字母 ci 计算为

ci = (pi + k) % 26

这是我的代码：

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

string cipher(string ptext, int k);

int main(int argc, string argv[])
{
    if (argc != 2)
    {
        printf("Usage: ./caesar key\n");
        return 1;
    }
    else
    {
        string ptext = NULL;
        string ctext = NULL;

        for (int i = 0, n = strlen(argv[1]); i < n; i++)
        {
            if (isdigit(argv[1][i]) == 0)
            {
                return 1;
            }
            else
            {
                ptext = get_string("Text:");
                int k = atoi(argv[1]);
                ctext = cipher(ptext, k);
            }
        }

        printf("Plain Text: %s \n Cipher Text: %s", ptext, ctext);
        return 0;
    }
}


string cipher(string ptext, int k)
{
    string ctext = NULL;
    for (int i = 0, n = strlen(ptext); i < n; i++) {
        // check if lower / upper is true if yes then shift chars by k % 26 and append to ctext
        if (islower(ptext[i]))
        {
            ctext[i] = (ptext[i] + k - 96) % 26 + 96;
        }
        else if (isupper(ptext[i]))
        {
            ctext[i] = (ptext[i] + k - 65) % 26 + 65;
        } // else append as it is
        else
        {
            ctext[i] = ptext[i];
        }
    }
    return (ctext);
}

输入 Hi!。我想要一个输出 Ij！我无法弄清楚这个错误。请帮忙

Answer 1

with debug my code is exiting at ctext1[i] = (ptext[i] + k1 - 65) % 26 + 65;

您还没有为您的字符串分配任何内存。这是分段错误的原因，因为 ctext[i] 访问了无效的内存地址。

要解决这个问题，您必须将内存分配给 ctext。

Answer 2

CS50 introduces a type string hiding a char pointer. This creates confusion as this string type is very different from what other languages name as such. Pointers are a fundamental concept that must be mastered to write programs in C.

The statement ptext = get_string("Text:"); sets ptext to the pointer value returned by the function get_string() declared in <cs50.h>. This function allocates memory for an array of char, reads user input into this array, sets a null terminator (a byte with a zero value) at the end of the array and returns a pointer to the first element of this array.

For your program to work, you can either:

modify the array in place, replacing each byte with its cipher value, or
allocate another array and store the cipher values into it, appending a null terminator after them. You chose this approach, but did not allocate the array.

Note also these problems:

the for loop validating the key string should be separate from the code calling the cypher function.
you should use character values such as 'a' and 'A' instead of hard coding their ASCII values. It improves portability and readability.

Here is a modified version:

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

char *cipher(char *ptext, int k);

int main(int argc, char *argv[]) {
    if (argc != 2) {
        printf("Usage: ./caesar key\n");
        return 1;
    } else {
        char *key = argv[1];

        for (int i = 0, n = strlen(key); i < n; i++) {
            if (isdigit(key[i]) == 0) {
                return 1;
            }
        }
        int k = atoi(key);
        char *ptext = get_string("Text:");
        char *ctext = cipher(ptext, k);

        printf("Plain Text: %s\nCipher Text: %s", ptext, ctext);
        free(ctext);
        return 0;
    }
}

char *cipher(char *ptext, int k) {
    int n = strlen(ptext);
    char *ctext = malloc(n + 1);  // allocate memory for the cipher string
    if (ctext == NULL)
        return NULL;
    for (int i = 0; i < n; i++) {
        unsigned char pi = ptext[i];
        // check if lower / upper is true if yes then shift chars by k % 26 and append to ctext
        if (islower(pi))
        {
            // handle lowercase letter
            ctext[i] = (pi - 'a' + k) % 26 + 'a';
        }
        else if (isupper(pi))
        {
            // handle uppercase letter
            ctext[i] = (pi - 'A' + k) % 26 + 'A';
        }
        else
        {
            // else append as it is
            ctext[i] = pi;
        }
    }
    ctext[n] = '[=10=]';  // set the null terminator
    return ctext;
}

凯撒密码算法中 C 中的分段错误

Segmentation fault in C in Caesar cipher algorithm

c

cs50