Structures and undifined array dimensions...

Hello,

I want to define a structure that has 5 elements in it: two integers, two arrays of type int and one multydimensional array of type float.

The thing is that the dimensions of my arrays depend on my first two intingers...

Something like this:

Code:

---

struct DATA
{
      int num_columns;
      int num_rows;
      int temp_air[num_columns];
      int speed_wind[num_rows];
      float mat_tempspeed[num_columns][num_rows];
};

---

Now, when i try to compile, 4 errors show up telling me that num_columns and num_rows are undeclared here (not in a function)...

I know the errors are caused by my arrays but i dont know how to fix this problem.

You're not allowed to define the arrays that way based upon integer variables like that. Use constants for the array size or use a dynamic memory approach with a function that creates and initializes an instance of that data type.

Here is a program i wrote...

Code:

---

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

int main()
{
    int y, x;
    int tab[x];
   
    x = 5;
    y = x;
    for(x = 0; x < y; x++)
          {
          tab[x] = x;
          printf("&#37;d", tab[x]);
          }
   
   
      system("PAUSE");
      return 0;
}

---

Youre telling me I cant do this when i put my array in a structure?

Your array must be a fixed size:

Code:

---

#define num_columns 10
#define num_rows 10
struct DATA
{
      int num_columns;
      int num_rows;
      int temp_air[num_columns];
      int speed_wind[num_rows];
      float mat_tempspeed[num_columns][num_rows];
};

---

Change the defines to whatever size you want.
If you need a dynamic size, you need to use malloc.

Quote:

---

Originally Posted by Tyrant

Youre telling me I cant do this when i put my array in a structure?

---

If this is C99, yes you can do that. But still it might not be a good idea. You'll ever know how big
tab will become ( x is uninitialized ).
Kurt

I do need a dynamique size..
I havent learned dynamic memmory allocation yet but I did read about it in my c programing book so I could mannage...

The thing is I dont see how it could help because I dont really understand it....
Could someone point me in the right direction...

never mind
Ill read on t first....again

Pointers + malloc or calloc is your friend.

Whats the diffrence beetween calloc and malloc...?

This is cool thanx for the help everybody...I think I understand now....

Quote:

---

Originally Posted by Elysia

Code:

---

#define num_columns 10
#define num_rows 10
struct DATA
{
      int num_columns;
      int num_rows;
      int temp_air[num_columns];
      int speed_wind[num_rows];
      float mat_tempspeed[num_columns][num_rows];
};

---

---

Doing that would cause an error, since it would expand into:

Code:

---

struct DATA
{
        int 10;
        int 10;
        ... etc
};

---

If num_rows and num_columns really were constant, there'd be no need to have them as struct members anyway.

Eh, I didn't check the names of the member variables, so you're right...
Then again, that structure is really silly since there's no way you can initialize num_columns and num_rows before the rest of the members are created.

the rest of the members shall now be pointers to arrays in the heap....

refroidPtr->temp_airPtr = (int *) calloc(refroidPtr->num_columns, sizeof (int));

this is my call to calloc, it creates an array of dimensions refroidPtr->num_columns in the heap where the adresse of the first cell is temp_airPtr.

now I want to create a matrix in the heap, how do I do this?

Hello,

This is something I have to do all the time, for my data processing and simulation code (scientific programming).

Define you structures with a typedef like this (notice I have changed you index to be an unsigned int),

Code:

---

//this goes in DATA.h file
typedef struct _DATA
{
      unsigned num_columns;
      unsigned num_rows;
      unsigned *temp_air;                        //size of num_columns
      unsigned *speed_wind;                //size of num_rows
      float **mat_tempspeed;                //size of num_columns*num_rows
} DATA;

DATA * DataAlloc (unsigned columns, unsigned rows);
DataDealloc (DATA*);

---

Now, we will use malloc to create a chunk of memory at run time, this means we will also need to 'free' the memory when we are done. Otherwise, it will never get released and you will have 'memory leak'. Therefore, we need to make alloc and dealloc functions,

Code:

---

//this goes in DATA.c file

#include "DATA.h"

DATA * DataAlloc (unsigned columns, unsigned rows)
{
  //make the dynamic memory for the arrays
        DATA * data = (DATA*)malloc(sizeof(DATA));
        data->num_columns = columns;
        data->num_rows = rows;
        data->temp_air  =  (unsigned*)malloc(sizeof(unsigned) * columns);
        data->speed_wind =  (unsigned*)malloc(sizeof(unsigned) * rows);
        data->mat_tempspeed = (float**)malloc(sizeof(float*) * rows * columns);
  return data;
}

void DataDealloc (DATA * data)
{
        free(data->temp_air);
        free(data->speed_wind);
        free(data->mat_tempspeed);
        free(data);
}

---

Here our alloc function returns a pointer to the structure, this is a good habit to get into.

So here is how you use this structure in your main file.

Code:

---

//this goes in main.c file

#include "DATA.h"

int main(int argc, char *argv[])
{
  DATA * data = DataAlloc(3,3);
       
        //access the structure elements like this
        data->temp_air[0] = 0;
        data->temp_air[1] = 1;
        //etc...
       
        //first element
        data->mat_tempspeed[0][0] = 3.14;
        //last element
        data->mat_tempspeed[2][2] = 1.23;
       
        //don't forget to free the memory allocated!
        DataDealloc(data);
 
  return 0;

}

---

Haven't tested this code, so let me know if you have any problems.

Cheers,

DAN