Thread: Pointer to a 2d array

Originally Posted by nadroj

if you have a problem implementing it or you're getting an error, post all of your code and the exact and complete error/warning message(s).

<filler>

Header File:

Code:

/* File: matrix.h */
#ifndef MATRIXh
#define MATRIXh
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#define FORMAT "%8.3lf"



typedef struct {
  int  rowdim, coldim;
  double** element;
} matrix;

/* function prototypes */
matrix create_empty(int rdim, int cdim);
matrix create_initval(int rdim, int cdim, double* values);
void destroy(matrix);
void matrix_print(matrix a);
double retrieve(int row, int col, matrix m);  /* retrieve an element from m */
/* void assign(int row, int col, matrix*, T val); /* assign a value to an element of m */
void equate(matrix* m1, matrix* m2);  /* m1 = m2 */
matrix add(matrix, matrix);
matrix subtract(matrix, matrix);
matrix negate(matrix);
matrix multiply(matrix, matrix);
/* matrix scalar_multiply(T scalar, matrix); */
/* remaining function prototypes not shown */

#endif

Function Decelerations:

Code:

#include "matrix.h"

matrix create_empty(int rdim, int cdim)
{
	matrix new_matrix;
	new_matrix.element = malloc(sizeof(matrix));
	new_matrix.rowdim = rdim;
	new_matrix.coldim = cdim;
	new_matrix.element = malloc((rdim*cdim) * sizeof(double));
	return new_matrix;
}

matrix create_initval(int rdim, int cdim, double* values)
{
	int i, j, k;
	matrix new_matrix;
	new_matrix.rowdim = rdim;
	new_matrix.coldim = cdim;
	new_matrix.element = malloc(sizeof(double*) * rdim);
	for (k=0; k<rdim; k++)
		new_matrix.element[k] = malloc(sizeof(double) * cdim);
	for (i=0; i<cdim; i++)
	{
		for(j=0; j<rdim; j++)
			new_matrix.element[i][j] = values[i][j]; // does not work
	{
return new_matrix;
}

void destroy(matrix a)
{
	free(a.element);
}

void matrix_print(matrix a)
{
	int i, j;	
	printf("\n");
	for (i=0; i<a.coldim; i++)
	{
		for(j=0; i<a.rowdim; j++)
		{
			printf(" %f ", a.element[i][j]);
		}
	}
}
}
}

Test File:

Code:

#include "matrix.h"

int main()
{
	double one[2][3] = {
						 {1,2,3},
                                                 {4,5,6}
					   };
							

	matrix m_one;
	m_one = create_initval(2, 3, one);
	matrix_print(m_one);
return 0;
}

Errors:
matrix.c: In function ‘create_initval’:
matrix.c:25: error: subscripted value is neither array nor pointer
test.c: In function ‘main’:
test.c:12: warning: passing argument 3 of ‘create_initval’ from incompatible pointer type

I changed all my code to

Code:

/* File: matrix.h */
#ifndef MATRIXh
#define MATRIXh
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#define FORMAT "%8.3lf"



typedef struct {
  int  rowdim, coldim;
  double** element;
} matrix;

/* function prototypes */
matrix create_empty(int rdim, int cdim);
matrix create_initval(int rdim, int cdim, double values[rdim][cdim]);
void destroy(matrix);
void matrix_print(matrix a);
double retrieve(int row, int col, matrix m);  /* retrieve an element from m */
/* void assign(int row, int col, matrix*, T val); /* assign a value to an element of m */
void equate(matrix* m1, matrix* m2);  /* m1 = m2 */
matrix add(matrix, matrix);
matrix subtract(matrix, matrix);
matrix negate(matrix);
matrix multiply(matrix, matrix);
/* matrix scalar_multiply(T scalar, matrix); */
/* remaining function prototypes not shown */

#endif

Code:

#include "matrix.h"

matrix create_empty(int rdim, int cdim)
{
	matrix new_matrix;
	new_matrix.element = malloc(sizeof(matrix));
	new_matrix.rowdim = rdim;
	new_matrix.coldim = cdim;
	new_matrix.element = malloc((rdim*cdim) * sizeof(double));
		if(new_matrix.element == NULL)
		{
		fprintf(stderr, "out of memory\n");
		exit(0);
		}
	return new_matrix;
}

matrix create_initval(int rdim, int cdim, double values[rdim][cdim])
{
	int i, j, k;
	matrix new_matrix;
	new_matrix.rowdim = rdim;
	new_matrix.coldim = cdim;
	new_matrix.element = malloc(sizeof(double*) * rdim);
	if(new_matrix.element == NULL)
		{
		fprintf(stderr, "out of memory\n");
		exit(0);
		}

	for (k=0; k<rdim; k++)
		new_matrix.element[k] = malloc(sizeof(double) * cdim);
		if(new_matrix.element[k] == NULL)
		{
		fprintf(stderr, "out of memory\n");
		exit(0);
		}

	for (i=0; i<cdim; i++)
	{
		for(j=0; j<rdim; j++)
			new_matrix.element[i][j] = values[i][j]; // does not work
	{
return new_matrix;
}

void destroy(matrix a)
{
	free(a.element);
}

void matrix_print(matrix a)
{
	int i, j;	
	printf("\n");
	for (i=0; i<a.coldim; i++)
	{
		for(j=0; i<a.rowdim; j++)
		{
			printf(" %f ", a.element[i][j]);
		}
	}
}
}
}

Code:

#include "matrix.h"

int main()
{
	double one[2][3] = {
									     {1,2,3},
                                {4,5,6}
										};
							

	matrix m_one;
	m_one = create_initval(2, 3, one);
	matrix_print(m_one);
return 0;
}

now the array seems to be passed in fine, but theres a problem with the print statement. when i compile both .c files, i get this:
/tmp/cce6gQuE.o: In function `main':
test.c.text+0x7d): undefined reference to `matrix_print'
collect2: ld returned 1 exit status

In "create_initval"

Code:

	for (i=0; i<cdim; i++)
	{
		for(j=0; j<rdim; j++)
			new_matrix.element[i][j] = values[i][j]; // does not work
	{

The brace there should probably be a close one, not an open one. After fixing that, if you get other errors, make sure all of your brackets line up.

i fixed the bracket, and it compiled fine, but when i ran it,
a null must have been returned because it displayed "out of memory"
now how do i fix that?

Alright, thanks so much for all your help. I really appreciate it.