External Interfaces/API Reference

mxSetJc

Set the jc array of a sparse mxArray

C Syntax

• #include "matrix.h"
  void mxSetJc(mxArray *array_ptr, int *jc);
  

Arguments

array_ptr
   Pointer to a sparse mxArray.

jc
   Pointer to the jc array.

Description

Use mxSetJc to specify a new jc array for a sparse mxArray. 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 have the meanings:

• jc[j] is the index in ir, pr (and pi if it exists) of the first nonzero entry in the jth column.
• jc[j+1]-1 is the index of the last nonzero entry in the jth column.
• jc[number of columns + 1] is equal to nnz, which is the number of nonzero entries in the entire spare mxArray.

The number of nonzero elements in any column (denoted as column C) is

• jc[C] - jc[C-1];
  

For example, consider a 7-by-3 sparse mxArray named Sparrow containing six nonzero elements, created by typing

• Sparrow=zeros(7,3);
  Sparrow(2,1)=1;
  Sparrow(5,1)=1;
  Sparrow(3,2)=1;
  Sparrow(2,3)=2;
  Sparrow(5,3)=1;
  Sparrow(6,3)=1;
  Sparrow=sparse(Sparrow);
  

The contents of the ir, jc, and pr arrays are:

Subscript	ir	pr	jc	Comment
(2,1)	1	1	0	Column 1 contains two entries, at ir[0],ir[1]
(5,1)	4	1	2	Column 2 contains one entry, at ir[2]
(3,2)	2	1	3	Column 3 contains three entries, at ir[3],ir[4], ir[5]
(2,3)	1	2	6	There are six nonzero elements.
(5,3)	4	1
(6,3)	5	1

As an example of a much sparser mxArray, consider an 8,000 element sparse mxArray named Spacious containing only three nonzero elements. The ir, pr, and jc arrays contain:

Subscript	ir	pr	jc	Comment
(73,2)	72	1	0	Column 1 contains zero entries
(50,3)	49	1	0	Column 2 contains one entry, at ir[0]
(64,5)	63	1	1	Column 3 contains one entry, at ir[1]
			2	Column 4 contains zero entries.
			2	Column 5 contains one entry, at ir[3]
			3	Column 6 contains zero entries.
			3	Column 7 contains zero entries.
			3	Column 8 contains zero entries.
			3	There are three nonzero elements.

Examples

See mxsetdimensions.c in the mx subdirectory of the examples directory. For an additional example, see explore.c in the mex subdirectory of the examples directory.

See Also

mxGetIr, mxGetJc, mxSetIr

mxSetIr

mxSetLogical (Obsolete)