Pointer to 2D struct array C

Question

I have a certain struct structXand a 2D array which holds these kind of structs.

I want to be able to save a pointer to that 2D struct and iterate over it

in a dynamic way, meaning, the pointer can hold any structX and iterate.

Example in general lines:

struct structX *ptr = NULL;

...

  if(i == OK)
    {
        ptr = General_struct_which_holds_others->ptr1;
    }
    else if(i ==NOT_OK)
    {
        ptr = General_struct_which_holds_others->ptr2;
    }

Now the iteration:

if(ptr[x][y] == OK) <----Error, subscripted value is neither array nor pointer
{
...
}

I hope i'm understood, As i was saying this is very general.

How can the iteration be made? meaning not getting errors?

Thanks!

@storyTeller i is just an integer, nothing special. can be OK = 1 or NOT_OK = 0 — Itzik984
– Itzik984, Commented Mar 4, 2013 at 18:04
So basically... your question is how to use a pointer to a 2d array? — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Mar 4, 2013 at 18:11
@StoryTeller i thought it was pretty well defined in the header... — Itzik984
– Itzik984, Commented Mar 4, 2013 at 18:14
Titles are for mortals. I comment based solely on the content :P — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Mar 4, 2013 at 18:19

Grijesh Chauhan · Accepted Answer · 2013-03-05 10:01:11Z

2

Two problem I can noticce in your code if(ptr[x][y] == OK)

(1):

ptr is pointer to structure (single *) you can't use double indices [][] so error at if(ptr[x][y] == OK)

error, subscripted value is neither array nor pointer because of ptr[][]

(2):

error: used struct type value where scalar is required means if(struct are not allow).

if(should be a scalar value )

scalar value means can be convert into 0/1.

Pointer to 2D struct array C

struct structX matrix2D[ROW][COL];

its pointer

struct structX (*ptr2D)[ROW][COL];

ptr2D = &matrix2D;

ok, access you array structure like this:

struct structX i;
(*ptr2D)[r][c] = i;

If you want to pass in an function do like:

void to(struct structX* ptr2D[][COL]){
   struct structX i;
   ptr2D[][COL] = i;
}
void from(){
  struct structX matrix2D[ROW][COL];
  to(matrix2D);
}

Just to make you sure I written a simple code shows how to work with ptr2D. Hope you find it helpful:

#include<stdio.h>
#define ROW 10
#define COL 5
typedef struct {
 int a;
 char b;
} structX;
void to(structX ptr2D[][COL], int r, int c){
 printf("in to: %d %c\n", ptr2D[r][c].a, ptr2D[r][c].b);
}
int main(){
 structX matrix[ROW][COL];
 structX (*ptr2D)[ROW][COL];
 ptr2D = &matrix;
 structX  i;
 i.a = 5; 
 i.b = 'a';
 int r = 3;
 int c = 2;
 (*ptr2D)[r][c] = i;
 printf("%d %c\n", (*ptr2D)[r][c].a, (*ptr2D)[r][c].b);
 to(matrix, r, c);
}

And its working, Output:

5 a
in to: 5 a

EDIT

I wanted to show two tricks but now I think I should provide a uniform method(as you commented): So here is the code:

#include<stdio.h>
#define ROW 10
#define COL 5
typedef struct {
 int a;
 char b;
} structX;
void to(structX (*ptr2D)[ROW][COL], int r, int c){
 printf("in to: %d %c\n", (*ptr2D)[r][c].a, (*ptr2D)[r][c].b);
}
int main(){
 structX matrix[ROW][COL];
 structX (*ptr2D)[ROW][COL];
 ptr2D = &matrix;
 structX  i;
 i.a = 5; 
 i.b = 'a';
 int r = 3;
 int c = 2;
 (*ptr2D)[r][c] = i;
 printf("%d %c\n", (*ptr2D)[r][c].a, (*ptr2D)[r][c].b);
 to(&matrix, r, c);
}

Output

5 a
in to: 5 a

EDIT:

error: used struct type value where scalar is required means if(struct are not allow).

if(should be a scalar value )

you can't do like if((*ptr2D)[r][c]);

but this is allow:

if((*ptr2D)[r][c].a == 5);

or

if((*ptr2D)[r][c].b == 'a');

or

if((*ptr2D)[r][c].a == 5 && (*ptr2D)[r][c].b == 'a');

or

structX  i;
if((*ptr2D)[r][c] == i);

edited Mar 5, 2013 at 10:01

answered Mar 4, 2013 at 18:09

Grijesh Chauhan

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2 Comments

Itzik984 Over a year ago

for some reason, using (ptr2D)[r][c] = i; without * worked and yours did not. i got a used struct type value where scalar is required any idea? did it in the following matter if((ptr)[x][y])... and it worked

jjm Over a year ago

What should be do when we want to to allocate 2D memory dynamically? ie, if ROW and COL unknown during declaration

Kobo · Accepted Answer · 2013-03-04 19:33:03Z

You might want to ready this article about multidimensional arrays. If you want to iterate over an array, you need to know how big it is (whether it is dynamic or not). If you want it to be dynamic, that means you need to allocate memory for it when it needs to grow and you need to free the old memory. You also have a problem in your question - you declare a single pointer which is null and then try to dereference it but you never allocated memory for it.

If you did allocate memory for it, you could dereference it by saying

ptr[x * ROW_WIDTH + y]

if you set ROW_WIDTH to the maximum value of y. Depending on whether you want to represent a rows major or column major array, you might use y * width instead of x * width.

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