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20.1.3 Sparse Matrix Properties

There are a number of functions that return basic information concerning sparse matrix objects. The most basic of these is issparse which identifies whether a particular Octave object is a sparse matrix.

The function nnz returns the number of non-zero entries in a sparse matrix, and nzmax returns the amount of storage allocated to it.

The function spstats returns some basic statistics on the columns of a sparse matrix, including the number of elements, mean value and variance of each column.

Loadable Function: issparse (expr)

Return 1 if the value of the expression expr is a sparse matrix.

Function File: nonzeros (s)

Returns a vector of the non-zero values of the sparse matrix s.

Built-in Function: scalar = nnz (a)

Returns the number of non zero elements in a.

See also sparse

Built-in Function: scalar = nzmax (SM)

Return the amount of storage allocated to the sparse matrix SM. Note that Octave tries to free unused memory at the first opportunity for sparse objects. The values returned by nzmaz and nnz will typically be the same, although they may differ for user-created sparse objects.

See also sparse, spalloc

Mapping Function: spmax (x, y, dim)

Mapping Function: [w, iw] = spmax (x)

For a vector argument, return the maximum value. For a matrix argument, return the maximum value from each column, as a row vector, or over the dimension dim if defined. For two matrices (or a matrix and scalar), return the pair-wise maximum. Thus,

max (max (x))

returns the largest element of x, and

max (2:5, pi)
    =>  3.1416  3.1416  4.0000  5.0000

compares each element of the range 2:5 with pi, and returns a row vector of the maximum values.

For complex arguments, the magnitude of the elements are used for comparison.

If called with one input and two output arguments, max also returns the first index of the maximum value(s). Thus,

[x, ix] = max ([1, 3, 5, 2, 5])
    =>  x = 5
        ix = 3

Mapping Function: spmin (x, y, dim)

Mapping Function: [w, iw] = spmin (x)

For a vector argument, return the minimum value. For a matrix argument, return the minimum value from each column, as a row vector, or over the dimension dim if defined. For two matrices (or a matrix and scalar), return the pair-wise minimum. Thus,

min (min (x))

returns the smallest element of x, and

min (2:5, pi)
    =>  2.0000  3.0000  3.1416  3.1416

compares each element of the range 2:5 with pi, and returns a row vector of the minimum values.

For complex arguments, the magnitude of the elements are used for comparison.

If called with one input and two output arguments, min also returns the first index of the minimum value(s). Thus,

[x, ix] = min ([1, 3, 0, 2, 5])
    =>  x = 0
        ix = 3

Function File: [count, mean, var] = spstats (s)

Function File: [count, mean, var] = spstats (s, j)

Return statistics for the non-zero elements of the sparse matrix s. count is the number of non-zeros in each column, mean is the mean of the non-zeros in each column, and var is the variance of the non-zeros in each column.

Called with two input arguments, if s is the data and j is the bin number for the data, compute the statistics for each bin. In this case, bins can contain data values of zero, whereas with spstats (s) the zeros are not included.