MATLAB Application Program Interface Reference

Get the jc array of a sparse matrix

#include "matrix.h"
int *mxGetJc(const mxArray *array_ptr);

Arguments

array_ptr

Pointer to a sparse mxArray.

Returns

A pointer to the first element in the jc array, if successful; otherwise, returns NULL. The most likely cause of failure is specifying an array_ptr that points to a full (nonsparse) mxArray.

Description

Use mxGetJc to obtain the starting address of the jc array. The jc array is an integer array having n+1 elements where n is the number of columns in the sparse mxArray. The values in the jc array indirectly indicate columns containing nonzero elements. For a detailed explanation of the jc array, see mxSetJc.

Examples

Consider a MEX-file named SecndCol that displays the number of nonzero elements in the second column of an input mxArray:

void
mexFunction(int nlhs,mxArray *plhs[],int nrhs,const mxArray 
*prhs[])
{
 int     *jc;
 int      starting_row_index, stopping_row_index;
 int      elements_in_second_col;

 /* Get the starting positions of the data in the sparse array. */  
   jc = mxGetJc(prhs[0]);

 /* How many elements are in the second column of the sparse array. 
*/
   starting_row_index = *(jc + 1);
   stopping_row_index = *(jc + 2);
   elements_in_second_col = stopping_row_index - 
starting_row_index;
   mexPrintf("There are %d elements in the second column.\n", 
              elements_in_second_col);
}

In MATLAB, create a sparse mxArray and put four nontrivial elements in the second column:

>> sp = sparse(eye(100));
>> sp(23,2)=1;
>> sp(57,2)=1;
>> sp(84,2)=1;

Now call SecndCol, passing the sparse mxArray as an input argument:

>> SecndCol(sp);
There are 4 elements in the second column.

For an additional example, see mxGetJc.c in the mx subdirectory of the examples directory.