MATLAB Application Program Interface Reference

Set the jc array of a sparse mxArray

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

Arguments

array_ptr

Pointer to a sparse mxArray.

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 2 entries, at `ir[0`],`ir[1]`
`(5,1)`	`4`	`1`	`2`	Column 2 contains 1 entry, at `ir[2]`
`(3,2)`	`2`	`1`	`3`	Column 3 contains 3 entries, at `ir[3]`,`ir[4]`, `ir[5]`
`(2,3)`	`1`	`2`	`6`	There are 6 nonzero elements.
`(5,3)`	`4`	`1`
`(6,3)`	`5`	`1`

Now onto a much sparser mxArray. Consider an 8,000 element sparse mxArray named Spacious containing only 3 nonzero elements. The ir, pr, and jc arrays contain


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

Example

See the example on the mxSetNzmax reference page.