动态分配和GSL（Gnu科学图书馆）

Question

So, I have to use this function from GSL. This one:

gsl_matrix_view_array (double * base, size_t n1, size_t n2)

The first argument ( double * base ) is the matrix I need to pass to it, which is read as input from the user.

I'm dynamically allocating it this way:

double **base;
base = malloc(size*sizeof(double*));

for(i=0;i<size;i++)
base[i] = malloc(size*sizeof(double));

Where size is given by the user. But then, when the code runs, it warns this :

  "passing arg 1 of gsl_matrix_view_array from incompatible pointer type".

What is happening?

Answer 1

The function expects a flat array, eg, double arr[size*size]; .

Here's an example from the documentation that I have slightly modified to use a matrix view:

#include <stdio.h>
#include <stdlib.h>
#include <gsl/gsl_matrix.h>

int main(void) {
  int i, j; 

  double *arr = malloc(10 * 3 * sizeof*arr);
  gsl_matrix_view mv = gsl_matrix_view_array(arr, 10, 3);
  gsl_matrix * m = &(mv.matrix);

  for (i=0; i<10; i++)
    for (j=0; j<3; j++)
      gsl_matrix_set(m, i, j, 0.23 + 100*i + j);

  for (i=0; i<10; i++)
    for (j=0; j<3; j++)
      printf("m(%d,%d) = %g\n", i, j, gsl_matrix_get(m, i, j));

  free(arr);

  return 0;
}

Note that you can also directly allocate memory for the matrix using the provided API.

Here's the original example:

#include <stdio.h>
#include <gsl/gsl_matrix.h>

int main(void)
{
  int i, j; 

  gsl_matrix * m = gsl_matrix_alloc(10, 3);

  for (i=0; i<10; i++)
    for (j=0; j<3; j++)
      gsl_matrix_set(m, i, j, 0.23 + 100*i + j);

  for (i=0; i<10; i++)
    for (j=0; j<3; j++)
      printf("m(%d,%d) = %g\n", i, j, gsl_matrix_get(m, i, j));

  gsl_matrix_free(m);

  return 0;
}

For reference:

• http://www.gnu.org/software/gsl/manual/html_node/Matrix-views.html
• http://www.gnu.org/software/gsl/manual/html_node/Matrix-allocation.html#Matrix-allocation
• http://www.gnu.org/software/gsl/manual/html_node/Accessing-matrix-elements.html#Accessing-matrix-elements

Answer 2

gsl_matrix_view_array expects your matrix as a contiguous single allocation in row-major order. You should be allocating your array like this:

double (*ar)[size] = malloc(sizeof(ar[size]) * size);

Then (after populating it)

gsl_matrix_view_array(ar[0], size, size);

Finally, free your allocation when done with a single call:

free(ar);

Note: Don't try this with C++, as VLA's aren't standard-supported for that language.