Accessing a struct within array within a struct

Question

What's the correct way of accessing (with a pointer) a variable within a struct within an array within a struct?

I's like to get to variables x and y within position2D with a pointer from function()? Note that I'm traversing the nodes (and the points) in function(), and was hoping to write something like:

draw_point(p->vertices[i]->x, p->vertices[i]->y);

but that doesn't seem to work.

typedef struct Position2D{
  uint8_t x;
  uint8_t y;
} position2D;

typedef struct Node{
  int num;
  position2D vertices[4];
  struct Node *next;
} node;

/* initialisation: */

node *next1 = NULL; //should be empty
node node1 = {1, {{0,0}, {5,0}, {5,5}, {0,5}}, &next1};
node *next0 = &node1;
node node0 = {0, {{0,10}, {10,10}, {10,15}, {0,15}}, &next0};
node *start = &node0;

/*traverse all nodes and their inner vertices arrays: */
void function(void){
node *p;
for(p = start; p != NULL; p = p->next){
  int i;
  for (i=0; i<4; i++){ //traverse their four points
    //How to get to the x and y at this line?
  }
}

Think about types and which operator is resulting in what type, and which operator can be applied to this type. — Eugene Sh.
– Eugene Sh., Commented Feb 22, 2018 at 15:45
From a quick glimpse: In draw_point(p->vertices[i]->x, p->vertices[i]->y); I think you need to replace the arrows -> after [i] with . so it becomes draw_point(p->vertices[i].x, p->vertices[i].y); — Morten Jensen
– Morten Jensen, Commented Feb 22, 2018 at 15:46
@BjornA. Although compiled, didn't run properly, embedded environment. ;) I've tried with draw_pixel(p->vertices[i].x, p->vertices[i].y); which also compiles, but now one of my for loops seem to go on forever... :/ — saner
– saner, Commented Feb 22, 2018 at 15:55

Achal · Accepted Answer · 2018-02-22 17:47:50Z

1

vertices is a normal structure variable, not of struct pointer type. While accessing x and y use dot . operator instead of -> operator

Replace below statement

draw_point(p->vertices[i]->x, p->vertices[i]->y);

with

 draw_point(p->vertices[i].x, p->vertices[i].y);

EDIT : Another problem in your code while assigning next field.

node node1 = {1, {{0,0}, {5,0}, {5,5}, {0,5}}, &next1};

should be

node node1 = {1, {{0,0}, {5,0}, {5,5}, {0,5}}, (struct Node*)next1};

Here is the working code

#include<stdio.h>
typedef struct Position2D{
        int x;
        int y;
} position2D;

typedef struct Node{
        int num;
        position2D vertices[4];
        struct Node *next;
} node;
/*traverse all nodes and their inner vertices arrays: */
void function(void){
/* initialisation: */
        node *next1 = NULL;
        node node1 = {1, {{0,0}, {5,0}, {5,5}, {0,5}}, (struct Node*)next1};
        node *next0 = &node1;
        node node0 = {0, {{0,10}, {10,10}, {10,15}, {0,15}}, (struct Node*)next0};
        node *start = &node0;
        node *p = NULL ;
        int i=0;
        for(p=start;p!=NULL;p=p->next) {
                for (i=0; i<4; i++){ //traverse their four points
                        printf("%d %d \n",p->vertices[i].x, p->vertices[i].y);
                }
        }

}
int main() {
        function();
        return 0;
}

edited Feb 22, 2018 at 17:47

answered Feb 22, 2018 at 15:46

Achal

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4 Comments

saner Over a year ago

Thanks, it compiles. (But hangs inside the function(), perhaps my linked list isn't set up properly...)

saner Over a year ago

I've narrowed it down to my outer for loop, it seems to go on forever.

Achal Over a year ago

Hey @eriksan have you got where mistake was there ? If not let me know I can explain.

saner Over a year ago

Thanks for spotting the error with assigning the next filed, I'll try it out now!

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