C and dynamic structure element access - c

Question

I have this complicated structure thingie:
#include <stdlib.h>
typedef struct {
int x;
int y;
} SUB;
typedef struct {
int a;
SUB *z;
} STRUCT;
#define NUM 5
int main(void)
{
STRUCT *example;
int i;
example = malloc(sizeof(STRUCT));
example->z = malloc(NUM * sizeof(SUB));
for(i = 0; i < NUM; ++i) {
/* how do I access variable in certain struct of array of z's */
}
return 0;
}
example is dynamically allocated structure and z inside the example is dynamically allocated array of SUB structures.
How do I access certain variable in certain element of structure z?
I have been trying something like this: example->z[i].x but it doesnt seem to work.
At the moment I am using this shabby looking workaraound:
SUB *ptr = example->z;
int i;
for(i = 0; i < amount_of_z_structs; ++i) {
/* do something with 'ptr->x' and 'ptr->y' */
ptr += sizeof(SUB);
}

Answer 1

Your problem isn't where you say it is. Your code as posted gives a compile error:
error: request for member ‘z’ in something not a structure or union
at the line
example.z = malloc(sizeof(STRUCT));
because you meant to write example->z, since example is a pointer to STRUCT, not a STRUCT.
From there on, you can access example->z[i].x exactly as you said. That syntax has always been fine.
For example:
/* your declarations here */
example = malloc(sizeof(STRUCT));
example->z = malloc(NUM * sizeof(SUB));
for(i = 0; i < NUM; ++i) {
example->z[i].x = i;
example->z[i].y = -i;
printf("%d %d\n", example->z[i].x, example->z[i].y);
}
/* output:
0 0
1 -1
2 -2
3 -3
4 -4
*/

Answer 2

When you have pointers pointing to pointers you often end up running into precedence issues. I can't recall if this is one, but you might try (example->b)[i].x.

Answer 3

First of all, your second malloc is wrong; example is a pointer so this:
example.z = malloc(NUM * sizeof(SUB));
should be this:
example->z = malloc(NUM * sizeof(SUB));
Then in your loop you can say things like this:
example->z[i].x = i;
example->z[i].y = i;
You'll also want to have this near the top of your file:
#include <stdlib.h>

Answer 4

Try this:
int my_x = example[3].z[2].x;
The above code will first access the example[3] (the fourth element of the example array).
Once you get that particular element, its contents can be automatically access in the same way as you do with normal objects.
You then access z[2] from that element. Note that, example[3] is an element, so you could use a . to access its members; if its an array, you can access it as an array.
So till now, example[3].z[2] is one element of the SUB array inside one element of the example array.
Now you can simply access the member x using the way shown above.
typedef struct {
int x;
int y;
} SUB;
typedef struct {
int a;
SUB *z;
} STRUCT;
STRUCT *example;
int main() {
example = malloc(sizeof(STRUCT)*10); //array of 10;
int i=0,j=0;
for (;i<10;i++){
example[i].a = i;
example[i].z = malloc(sizeof(SUB)*5);
for (j=0; j<5; j++)
example[i].z[j].x = example[i].z[j].y = j;
}
//access example[3] and access z[2] inside it. And finally access 'x'
int my_x = example[3].z[2].x;
printf("%d",my_x);
for (i=0;i<10;i++){
printf("%d |\n",example[i].a);
//example[i].z = malloc(sizeof(SUB)*5);
for (j=0; j<5; j++)
printf("%d %d\n",example[i].z[j].x,example[i].z[j].y);
free(example[i].z);
}
free(example);
}

Answer 5

In the 'shabby workaround', you wrote:
SUB *ptr = example->z;
int i;
for(i = 0; i < amount_of_z_structs; ++i) {
/* do something with 'ptr->x' and 'ptr->y' */
ptr += sizeof(SUB);
}
The problem here is that C scales pointers by the size of the object pointed to, so when you add 1 to a SUB pointer, the value is advanced by sizeof(SUB). So, you simply need:
SUB *ptr = example->z;
int i;
for (i = 0; i < NUM; ++i) {
ptr->x = ptr->y = 0;
ptr++;
}
Of course, as others have said, you can also do (assuming C99):
for (int i = 0; i < NUM; ++i)
example->z[i].x = example->z[i].y = 0;

Answer 6

#include<stdio.h>
#include<stdlib.h>
#include<conio.h>
#define NUM 5
typedef struct
{
int x;
int y;
}SUB;
typedef struct
{
int a;
SUB* z;
}STRUCT;
void main(void)
{
clrscr();
printf("Sample problem..\n\n");
STRUCT* example;
int i;
example = (STRUCT*)malloc(sizeof(STRUCT));
example->z = (SUB*)malloc(NUM * sizeof(SUB));
for(i = 0; i < NUM; i++)
{
example->z[i].x = i +1;
example->z[i].y = (example->z[i].x)+1;
printf("i = %d: x:%d y:%d\n", i, example->z[i].x, example->z[i].y);
}
}