这个初始化有什么问题？ 为什么我尝试使用初始化 function 来初始化图形时无法成功

Question

using VS,C language. I try to initialization a Graph structure by a initialize fuction

#include<stdio.h> #define maxint 1000 #define mvnum 100 typedef struct { int vexs[mvnum]; int edges[mvnum][mvnum]; int n, e; }MGraph; void creatUDN(MGraph *G) { G = (MGraph*)malloc(sizeof(MGraph)); int i, j, k, w; scanf_s("%d", &(G->n)); scanf_s("%d", &(G->e)); for (i = 0; i < G->n; i++) scanf_s("%d", &(G->vexs[i])); for (i = 0; i < G->n; ++i) for (j = 0; j < G->n; ++j) G->edges[i][j] = maxint; for (k = 0; k < G->e; k++) { scanf_s("%d %d %d", &i, &j, &w); G->edges[i-1][j-1] = w; } for (i = 0; i < G->n; ++i) { for (j = 0; j <G->n; ++j) printf("%d\t", G->edges[i][j]); printf("\n"); } }

this is main function

    int main()
{
  MGraph *g;
  g=(MGraph*)malloc(sizeof(MGraph));
  creatUDN(g);
  **printf("%d", g->e);**
}

It shows that g still a Nullpointer after initialize. why?

Answer 1

Remove the assignment to G in creatUDN .

G = (MGraph*)malloc(sizeof(MGraph));

Now your code will work.

You correctly allocated this data structure in main and passed the pointer to creatUDN , but then for some reason you allocated it again in creatUDN so the data structure you allocated in main remains unchanged.

The other way you can do this is to return the allocated data structure from creatUDN . That would look like this

MGraph *creatUDN() {
  G = (MGraph*)malloc(sizeof(MGraph));
  ...
  return G;
}

int main()
{
  MGraph *g = creatUDN();
  printf("%d", g->e);
}

You seem to have written something that's half way between these two methods.