结构与另一个结构

Question

How can I correctly access the price member from the category structure?

This is the code:

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

#define SMAX 128

typedef struct {
  int price;
  char favCharacter[SMAX], edition[SMAX];
} category;

typedef struct {
  char title[SMAX], fname[SMAX], lname[SMAX];
  category *cat;
} book;

int main(int argc, char **argv) {

  book *data = malloc(100 * sizeof(book));
  (data->cat)->price = 10; //1
  printf("\n\n(*(data->cat)).price is: %d%s", (data->cat)->price, "\n\n");

  return 0;

}

My first attempt failed (//1) What is the right way to solve this? (I mean to have a structure with another structure) .

Answer 1

You are allocating memory for book , but not for the cat s inside the books. Here's an example:

/* allocate a hundred books */
book *data = malloc(100 * sizeof *data);
/* allocate one cat per book */
for (i = 0; i < 100; ++i)
    data[i].cat = malloc(sizeof *data[i].cat);

/* now you can use it */
for (i = 0; i < 100; ++i)
    data[i].cat->price = 50;

Note: you need to add checks to make sure malloc doesn't fail before continuing using the returned memory. Also, it's best to avoid magic numbers such as 100 above. Furthermore, don't forget to free the memory later.

Answer 2

You are allocating an array of books in your line:

book *data = malloc(100 * sizeof(book));

So you should access an array element before accessing its contents:

data[0].cat->price = 10; // lets access element 0 for example.

But, first, you must also alloc data for the cat variable inside each array element:

for ( int i = 0; i < 100; i++ ) {
    data[i].cat = malloc(sizeof(category));
}

Now you can access your elements correctly.

Don't forget to free the allocated memory.

Answer 3

Change

book *data = malloc(100 * sizeof(book));
(data->cat)->price = 10; //1
printf("\n\n(*(data->cat)).price is: %d%s", (data->cat)->price, "\n\n");

to

// 1. need explicit conversion
book *data = (book *) malloc(100 * sizeof(book));

// 2. allocate category for book *data
for (int i=0; i<100; i++) 
    data[i].cat = (category *) malloc(sizeof(category));

// 3. setting
(data[0].cat)->price = 10;

// 4. read
printf("\n\n(*(data->cat)).price is: %d%s", (data[0].cat)->price, "\n\n");