c-如何使用指针遍历结构中的内存块

Question

目标：遍历Heap结构中的vertices内存块。

错误：我在尝试访问vertices时得到expected unqualified-id before ‘(’ token。此外，‘vertices’ was not declared in this scope。

标题：

typedef struct Heap *PriorityQueue_t;
typedef struct HeapItem *PriorityItem_t;
PriorityQueue_t Init(int max_capacity, int source_vertex);

实施：

#include "heap.h"
#include <stdlib.h>
#include <limits.h>
struct Heap
{
  int size;
  PriorityItem_t vertices;
};
struct HeapItem
{
  int id;
  int distance;
};
PriorityQueue_t Init(int max_capacity, int source_vertex)
{
  PriorityQueue_t Q;
  Q = (PriorityQueue_t)malloc(sizeof(PriorityQueue_t));
  Q->vertices = (PriorityItem_t)malloc(sizeof(PriorityItem_t)*max_capacity);
  Q->size = max_capacity;
  int i;
  for(i = 0; i < Q->size; i++)
  {
    Q->(vertices + i)->id = i; //Errors on this line.
    Q->(vertices + i)->distance = INT_MAX;
  }
  Q->(vertices + source_vertex)->distance = 0;
  return Q;
}

Answer 1

您将parens放错了位置。试试这个：

    (Q->vertices + i)->id = i; //Errors on this line.
    (Q->vertices + i)->distance = INT_MAX;
  }
  (Q->vertices + source_vertex)->distance = 0;

Answer 2

Q->(vertices + i)->id = i; //Errors on this line.
Q->(vertices + i)->distance = INT_MAX;
Q->(vertices + source_vertex)->distance = 0;

你不能那样做。。。

Q->vertices[i].id = i;
(Q->vertices + i)->distance = INT_MAX;
(*(Q->vertices + source_vertex)).distance = 0;  // Not sensible, but legal

Answer 3

您可以通过使用普通索引和删除强制转换来避免使用许多括号：

PriorityQueue_t Init(int max_capacity, int source_vertex)
{
  PriorityQueue_t Q;
  Q = malloc(sizeof *Q);
  Q->vertices = malloc(max_capacity * sizeof *Q->vertices);
  Q->size = max_capacity;
  int i;
  for(i = 0; i < Q->size; i++)
  {
    Q->vertices[i].id = i;
    Q->vertices[i].distance = INT_MAX;
  }
  // maybe add next line: 
  // assert (source_vertex >=0 && source_vertex < Q->size);
  Q->vertices[source_vertex].distance = 0;
  return Q;
}