passing dynamicaly allocated 2d arrays

s_siouris · 05-25-2008, 03:14 PM

Hi all,

I'm writing a program where I am allocating a 2d array dynamically (ie using malloc), and then I want to pass this array to a function, but I'm not being successful...

Here are the relevant parts of the program:

Code:

int main(void)
{
        double **data;                        // 2d array to hold input file values
        const unsigned int ncols=29;   // number of columns to read
        unsigned int nrows=0;             // number of rows in input file
  
        /* allocate data array */
        **data=malloc(nrows*sizeof(*data));
        if (data==NULL) {
                printf("Could not allocate data array of size %i\n",nrows*sizeof(*data));
                return 1;
        }
        for (i=0; i<nrows; i++) {
                data[i]=malloc(ncols*sizeof(*data[i]));
                if (data[i]==NULL) {
                        printf("Could not allocate array data[%i] of size %i\n",i, ncols*sizeof(*data[i]));
                        return 1;
                }
        }


        /* read data into data array */
        read_data(nrows, ncols, data);
        .
        .
        .
}

and the function header for read_data is:

Code:

void read_data(unsigned int rows, const unsigned int cols, double *data[][cols]);

gcc (-Wall -pedantic) says:

Code:

lsis.least.sq.c:81: error: incompatible type for argument 3 of `read_data'

I've tried combinations of *data[][ncols], **data, etc to use as my argument for read_data, but it doesn't work. I know that when passing multidimensional array, you need to define the last dimension, but I'm not sure why it doesn't work the ways I've been trying.

Any sugggestions?

Thanks
Spiros

anon · 05-25-2008, 03:30 PM

Have you tried:

Code:

void read_data(unsigned int rows, const unsigned int cols, double **data);

That is, the same type that data is.

root4 · 05-25-2008, 03:32 PM

Code:

**data=malloc(nrows*sizeof(*data));

Why are you dereferencing the pointer here?
this affectation is incorrect (drop **).

Concerning the function:

Code:

...,double *data[][cols]);

You cannot do that, 1st this is certainly not the same type than double** and the size you have to precise is the size of the 1st dimension. But the simplest solution here is to declare the third argument as a double** as you did for the variable.

boyfarrell · 05-25-2008, 03:53 PM

Damn! People in this board are just too damn efficient! Nobody had replied 5 mins ago when I first looked.

Yes, all the above points...! The following doesn't return any errors with -Wall -pedantic

Code:

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

void read_data(unsigned int rows, const unsigned int cols, double **data)
{
	;
}

int main(void)
{
      double **data;                       /* 2d array to hold input file values */
      const unsigned long ncols=29;   	   /*  number of columns to read         */
      unsigned long nrows=0;               /*  number of rows in input file      */
	  unsigned i;

      /* allocate data array */
      data = malloc(nrows*sizeof(double*));
      if (data==NULL) 
        {
          printf("Could not allocate data array of size %lu\n",nrows*sizeof(*data));
          return 1;
        }

      for (i=0; i<nrows; i++) 
        {
           data[i]=malloc(ncols*sizeof(*data[i]));
              
	       if (data[i]==NULL) 
             {
                printf("Could not allocate array data[%u] of size %lu\n",i, ncols*sizeof(*data[i]));
                return 1;
             }
         }


      /* read data into data array */
      read_data(nrows, ncols, data);

      return 0;
}

s_siouris · 05-25-2008, 03:59 PM

thank you for your replies, it works now.

Can you please explain to me why do I need to call read_data using "data", wheras in the declaration of read_data it's "**data"?

boyfarrell · 05-25-2008, 04:17 PM

Hi,

In the first line of the program you specify the type of data,

Code:

double **data;

data is already a pointer to an array of pointers to doubles... which is what read_data requires it to be.

The name is not '**data', the name is 'data' the type is 'double **'.

Dan

s_siouris · 05-25-2008, 04:19 PM

ah yes, it's very clear now

thank you!

05-25-2008, 03:14 PM	#1
s_siouris Registered User Join Date: Nov 2004 Posts: 52	passing dynamicaly allocated 2d arrays Hi all, I'm writing a program where I am allocating a 2d array dynamically (ie using malloc), and then I want to pass this array to a function, but I'm not being successful... Here are the relevant parts of the program: Code: int main(void) { double *data; // 2d array to hold input file values const unsigned int ncols=29; // number of columns to read unsigned int nrows=0; // number of rows in input file / allocate data array / data=malloc(nrowssizeof(data)); if (data==NULL) { printf("Could not allocate data array of size %i\n",nrowssizeof(data)); return 1; } for (i=0; i<nrows; i++) { data[i]=malloc(ncolssizeof(data[i])); if (data[i]==NULL) { printf("Could not allocate array data[%i] of size %i\n",i, ncolssizeof(data[i])); return 1; } } / read data into data array / read_data(nrows, ncols, data); . . . } and the function header for read_data is: Code: void read_data(unsigned int rows, const unsigned int cols, double data[][cols]); gcc (-Wall -pedantic) says: Code: lsis.least.sq.c:81: error: incompatible type for argument 3 of `read_data' I've tried combinations of data[][ncols], *data, etc to use as my argument for read_data, but it doesn't work. I know that when passing multidimensional array, you need to define the last dimension, but I'm not sure why it doesn't work the ways I've been trying. Any sugggestions? Thanks Spiros
s_siouris is offline

05-25-2008, 03:32 PM	#3
root4 Registered User Join Date: Apr 2008 Posts: 282	Code: *data=malloc(nrowssizeof(data)); Why are you dereferencing the pointer here? this affectation is incorrect (drop ). Concerning the function: Code: ...,double data[][cols]); You cannot do that, 1st this is certainly not the same type than double and the size you have to precise is the size of the 1st dimension. But the simplest solution here is to declare the third argument as a double as you did for the variable.
root4 is offline

05-25-2008, 03:59 PM	#5
s_siouris Registered User Join Date: Nov 2004 Posts: 52	thank you for your replies, it works now. Can you please explain to me why do I need to call read_data using "data", wheras in the declaration of read_data it's "**data"?
s_siouris is offline

05-25-2008, 04:17 PM	#6
boyfarrell Registered User Join Date: Jul 2005 Posts: 31	Hi, In the first line of the program you specify the type of data, Code: double data; data is already a pointer to an array of pointers to doubles... which is what read_data requires it to be. The name is not 'data', the name is 'data' the type is 'double *'. Dan Last edited by boyfarrell; 05-25-2008 at 04:19 PM. Reason: Further explaination*
boyfarrell is offline

05-25-2008, 04:19 PM	#7
s_siouris Registered User Join Date: Nov 2004 Posts: 52	ah yes, it's very clear now thank you!
s_siouris is offline

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