Question

我自己一直在通过Kochan的“C编程”第3版，为明年的研究生做好准备，我第一次陷入困境。我是一个远离指针的章节，但是最近关于字符串的章节末尾的练习有一个问题，我自己的研究似乎只能通过使用指针来解决。

问题与数据结构entry：

有关

struct entry {
    char word[15];
    char definition[50];
};

我们创建了一系列条目：

struct entry dictionary[10] = 
{{"aardvark", "a burrowing African mammal"},
 {"abyss", "a bottomless pit"},
 {"addle", "to become confused"},
 {"aerie", "a high nest"},
 {"ajar", "partially opened"},
 {"acumen", "mentally sharp; keen"},
 {"affix", "to append; attach"},
 {"agar", "a jelly made from seaweed"},
 {"ahoy", "a nautical call of greeting"},
 {"aigrette", "an ornamental cluster of feathers"}};

提示符显示：“编写一个名为dictionary_sort的函数，按照字母顺序对字典（如上所定义）进行排序。”

我知道结构和数组与函数有关的细微之处，以及函数如何将它们作为参数或作为返回值返回。对我来说似乎唯一有用的方法是返回一个结构，或者特别是一个结构数组，但我不认为我在这里正确应用它：

struct entry dictionary_sort(struct entry dictionary)

总的来说，我目前的程序版本如下：

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

struct entry {
    char word[15];
    char definition[50];
};

// Function to compare two character strings

int compare_strings(const char s1[], const char s2[])
{
    int i = 0, answer; 

    while (s1[i] == s2[i] && s1[i] != '\0' && s2[i] != '\0')
        i++;

    if (s1[i] < s2[i])
        answer = -1; // s1 < s2
    else if (s1[i] == s2[i])
        answer = 0; // s1 == s2
    else
        answer = 1; // s1 > s2

    return answer;
}

// Function to sort a dictionary structure

struct entry dictionary_sort(struct entry dictionary[])
{
    int dictionary_length = sizeof(dictionary) / sizeof(dictionary[0]);
    int i, j, minimum;
    struct entry temp;

    for (i = 0; i < dictionary_length; i++) {
        minimum = i;
        for (j = i + 1; j < dictionary_length; j++) {
            if (compare_strings(dictionary[j].word, 
                                dictionary[minimum].definition) == -1)
                minimum = j;
        }
        temp = dictionary[minimum];
        dictionary[minimum] = dictionary[i];
        dictionary[i] = dictionary[minimum];
    }

    return dictionary;
}

int main(void)
{
    struct entry dictionary[10] =
    {{"aardvark", "a burrowing African mammal"},
     {"abyss", "a bottomless pit"},
     {"addle", "to become confused"},
     {"aerie", "a high nest"},
     {"ajar", "partially opened"},
     {"acumen", "mentally sharp; keen"},
     {"affix", "to append; attach"},
     {"agar", "a jelly made from seaweed"},
     {"ahoy", "a nautical call of greeting"},
     {"aigrette", "an ornamental cluster of feathers"}};
    int i, dictionary_length = sizeof(dictionary) / sizeof(dictionary[0]);

    dictionary = dictionary_sort(dictionary);

    for (i = 0; i < dictionary_length; i++)
        printf("%s - %s.\n", dictionary[i].word, dictionary[i].definition);

    return 0;
}

字符串比较函数的行为与预期一致，因为它只返回一个整数。我真的不知道如何在不知道指针的情况下获得所需的功能。有足够的带指针的示例，但我很好奇我在这里缺少什么基本原则，因为我觉得书中的其他内容对我来说非常自然。

提前谢谢！

Answer 1

你根本不需要返回任何内容，甚至不需要显式指针。就地排序，不要重新发明轮子：使用strcmp()和qsort()（live demo here）：

struct entry dictionary[] = {
    { "def", "second entry" },
    { "abc", "first entry" },
    { "ghi", "third entry" },
    { "mno", "fifth entry" },
    { "jkl", "fourth entry" }
};

int compare_entry(const void *l, const void *r)
{
    const struct entry *ll = l;
    const struct entry *rr = r;
    return strcmp(ll->word, rr->word);
}

#define COUNT(x) (sizeof(x) / sizeof(x[0]))

qsort(dictionary, COUNT(dictionary), sizeof(dictionary[0]), compare_entry);

Answer 2

虽然不完美并且仍然需要明确定义指针，但这个答案不仅仅是调用库，而是在问题和本书的范围内。

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

struct entry {
    char word[15];
    char definition[50];
};

// Function to compare two character strings

int compare_strings(const char s1[], const char s2[])
{
    int i = 0, answer; 

    while (s1[i] == s2[i] && s1[i] != '\0' && s2[i] != '\0')
        i++;

    if (s1[i] < s2[i])
        answer = -1; // s1 < s2
    else if (s1[i] == s2[i])
        answer = 0; // s1 == s2
    else
        answer = 1; // s1 > s2

    return answer;
}

// Function to sort a dictionary structure

void dictionary_sort(struct entry *dictionary, int dictionary_length)
{
    int i, j, minimum;
    struct entry temp;

    for (i = 0; i < dictionary_length - 1; i++) {
        minimum = i;
        for (j = i + 1; j < dictionary_length; j++) {
            if (compare_strings(dictionary[j].word, 
                                dictionary[minimum].word) == -1)
                minimum = j;
        }
        temp = dictionary[minimum];
        dictionary[minimum] = dictionary[i];
        dictionary[i] = temp;
    }
}

// Prints the dictionary in its current state

void print_dictionary(struct entry *dictionary, int dictionary_length)
{
    int i;

    for (i = 0; i < dictionary_length; i++) {
        printf("%s - %s.\n", dictionary[i].word, dictionary[i].definition);
    }
}

// Demostrates the dictionary_sort function

int main(void)
{
    struct entry dictionary[10] =
    {{"aardvark", "a burrowing African mammal"},
     {"abyss", "a bottomless pit"},
     {"addle", "to become confused"},
     {"aerie", "a high nest"},
     {"ajar", "partially opened"},
     {"acumen", "mentally sharp; keen"},
     {"affix", "to append; attach"},
     {"agar", "a jelly made from seaweed"},
     {"ahoy", "a nautical call of greeting"},
     {"aigrette", "an ornamental cluster of feathers"}};

    int i, dictionary_length = sizeof(dictionary) / sizeof(dictionary[0]);

    print_dictionary(&dictionary, dictionary_length);
    printf("\nSorting...\n\n");
    dictionary_sort(&dictionary, dictionary_length);
    print_dictionary(&dictionary, dictionary_length);
    printf("\n");

    return 0;
}

Answer 3

使用[]声明函数参数时，您已经使用了指针。功能定义

int compare_strings(const char s1[], const char s2[])

在功能上与

相同

int compare_strings(const char *s1, const char *s2)

与struct entry dictionary_sort(struct entry dictionary[])相同。

由于您获得指向struct entry作为参数dictionary的指针，return dictionary也会将指针返回给struct entry。

您对dictionary所做的所有更改都已在外部可见，因为您修改了数组本身而不是某些本地数组。

Answer 4

像"Returning an array of structs without pointers in C"这样的语句在C中是一个悖论，因为在C中传递或返回数组的唯一方法是通过指针。即使看起来像char *foo(char arr_demo[]){}这样的数组也是如此传递，基本上它是一个指针被传递，因为它减少到char *foo(char *arr_demo){}。

返回C中没有指针的结构数组

4 个答案: