测试和学习 C 中的图形时出现意外错误

Question

我最近开始研究这本关于算法和数据结构的书SkienaTheAlgorithmDesignManual.pdf，不仅在互联网上，而且我的算法设计老师以及大学，最后我在第 153 页（在书本身）或 165（pdf 格式）上使用了一些代码时遇到了一些错误。

代码如下：

#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");
}

错误如下：

1>c:\users\dragos\source\repos\learninggraph\learninggraph\main.c(47): warning C4047: '=': 'int' differs in levels of indirection from 'void *' 1>c:\users\dragos\source\repos\learninggraph\learninggraph\main.c(49): warning C4133: '=': incompatible types - from 'edgenode *' to 'edgenode *' 1>c:\users\dragos\source\repos\learninggraph\learninggraph\main.c(65): warning C4133: '=': incompatible types - from 'edgenode *' to '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" -- FAILED. ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

我认为主要问题是"incompatible types"，但我也可能是错的。

Answer 1

在insert_edge

 p->weight = NULL;

无效，因为 weight 是一个 int 而 NULL 是一个指针（通常是 (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 edges[x] 是 edgenode *。要解决这个问题，您必须更换

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

来自

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

print_graph行也是一样的道理

 p = p->next;

显式设置 main 的 return 类型为 int

在 main 中你用 g 调用 read_graph 永远不会 set/initialized 所以当它在 read_graph行为未定义，print_graph中也是如此。只需更换

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

来自

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 $

Answer 2

您从未为 graph *g 分配任何内存。

没必要把它当成一个指针，让它成为一个普通的变量，然后把它的地址传递给函数。

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

测试和学习 C 中的图形时出现意外错误

Unexpected error in testing and learning about graphs in C

c

directed-graph

adjacency-list

undirected-graph