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c - 测试和学习 C 中的图时出现意外错误

问题描述

我最近开始研究这本关于算法和数据结构的特殊书籍SkienaTheAlgorithmDesignManual.pdf，我不仅在互联网上，而且从我的算法设计老师以及大学时都听到了很多赞美，我最终得到了本书第 153 页（书本身）或 165（pdf 格式）中使用的一些代码存在一些错误。

这是代码：

#include<stdio.h>
#include<stdbool.h>
#include<stdlib.h>
#define MAXV 1000
typedef struct  {
    int y;
    int weight;
    struct edgenode *next;
}edgenode;
typedef struct {
    edgenode *edges[MAXV + 1];
    int degree[MAXV + 1];
    int nvertices;
    int nedges;
    bool directed;
}graph;

void initialize_graph(graph *g, bool directed);
void read_graph(graph *g, bool directed);
void insert_edge(graph *g, int x, int y, bool directed);
void print_graph(graph *g);
void initialize_graph(graph *g, bool directed) {
    int i;

    g->nvertices = 0;
    g->nedges = 0;
    g->directed = directed;
    for (i = 1; i <= MAXV; i++) {
        g->degree[i] = 0;
        g->edges[i] = NULL;
    }

}

void read_graph(graph *g, bool directed) {
    int i;
    int m;
    int x, y;
    initialize_graph(g, directed);
    scanf("%d %d", &(g->nvertices), &m);
    for (i = 1; i <= m; i++) {
        scanf("%d %d", &x, &y);
        insert_edge(g, x, y, directed);
    }
}

void insert_edge(graph *g, int x, int y, bool directed) {
    edgenode *p;
    p = malloc(sizeof(edgenode));
    p->weight = NULL;
    p->y = y;
    p->next = g->edges[x];
    g->edges[x] = p;
    g->degree[x]++;
    if (directed == false)
        insert_edge(g, y, x, true);
    else
        g->nedges++;
}

void print_graph(graph *g) {
    int i;
    edgenode *p;
    for (i = 1; i <= g->nvertices; i++) {
        printf("%d ", i);
        p = g->edges[i];
        while (p != NULL) {
            printf(" %d", p->y);
            p = p->next;
        }
        printf("\n");
    }
}
main() {
    bool directed = true;
    graph *g;
    read_graph(g, directed);
    print_graph(g);
    system("pause");
}

以下是错误：

1>c:\users\dragos\source\repos\learninggraph\learninggraph\main.c(47): 警告 C4047: '=': 'int' 与 'void *' 的间接级别不同 1>c:\users \dragos\source\repos\learninggraph\learninggraph\main.c(49): 警告 C4133: '=': 不兼容的类型 - 从 'edgenode *' 到 'edgenode *' 1>c:\users\dragos\source\repos \learninggraph\learninggraph\main.c(65): 警告 C4133: '=': 不兼容的类型 - 从 'edgenode *' 到 'edgenode *' 1>c:\users\dragos\source\repos\learninggraph\learninggraph\main .c(73): error C4700: uninitialized local variable 'g' used 1>Done building project "LearningGraph.vcxproj" -- 失败。========== 构建：0 成功，1 失败，0 最新，0 跳过 ==========

我认为主要问题是“不兼容的类型”，但我也可能错了。

标签： cdirected-graphadjacency-listundirected-graph

解决方案

在insert_edge

 p->weight = NULL;

无效，因为weight是int但NULL是指针（通常(void*)0）

在insert_edge

 p->next = g->edges[x];

无效，因为next是未定义的类型struct edgenode *，但却edges[x]是edgenode *. 为了解决这个问题，你必须更换

typedef struct {
    int y;
    int weight;
    struct edgenode *next;
}edgenode;

经过

typedef struct edgenode {
    int y;
    int weight;
    struct edgenode *next;
}edgenode;

原因print_graph同上

 p = p->next;

显式设置main的返回类型为int

在main中，您read_graph使用g从不设置/初始化调用，因此当它在行为中被取消引用时read_graph，行为是未定义的，在print_graph. 只需更换

 graph *g;
 read_graph(g, directed);
 print_graph(g);

经过

graph g;
read_graph(&g, directed);
print_graph(&g);

完整修改版：

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

#define MAXV 1000

typedef struct edgenode {
    int y;
    int weight;
    struct edgenode *next;
}edgenode;

typedef struct {
    edgenode *edges[MAXV + 1];
    int degree[MAXV + 1];
    int nvertices;
    int nedges;
    bool directed;
}graph;

void initialize_graph(graph *g, bool directed);
void read_graph(graph *g, bool directed);
void insert_edge(graph *g, int x, int y, bool directed);
void print_graph(graph *g);
void initialize_graph(graph *g, bool directed) {
    int i;

    g->nvertices = 0;
    g->nedges = 0;
    g->directed = directed;
    for (i = 1; i <= MAXV; i++) {
        g->degree[i] = 0;
        g->edges[i] = NULL;
    }

}

void read_graph(graph *g, bool directed) {
    int i;
    int m;
    int x, y;
    initialize_graph(g, directed);
    scanf("%d %d", &(g->nvertices), &m);
    for (i = 1; i <= m; i++) {
        scanf("%d %d", &x, &y);
        insert_edge(g, x, y, directed);
    }
}

void insert_edge(graph *g, int x, int y, bool directed) {
    edgenode *p;
    p = malloc(sizeof(edgenode));
    p->weight = 0;
    p->y = y;
    p->next = g->edges[x];
    g->edges[x] = p;
    g->degree[x]++;
    if (directed == false)
        insert_edge(g, y, x, true);
    else
        g->nedges++;
}

void print_graph(graph *g) {
    int i;
    edgenode *p;
    for (i = 1; i <= g->nvertices; i++) {
        printf("%d ", i);
        p = g->edges[i];
        while (p != NULL) {
            printf(" %d", p->y);
            p = p->next;
        }
        printf("\n");
    }
}

int main() {
    bool directed = true;
    graph g;
    read_graph(&g, directed);
    print_graph(&g);
    system("pause");
}

编译：

pi@raspberrypi:/tmp $ gcc -pedantic -Wall -Wextra g.c
pi@raspberrypi:/tmp $

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