If j for ColumnLinkedMatrix or i for
RowLinkedMatrix is passed, it will only generate entries for the
given indices. sort, which is set by default, determines whether
j or i should be sorted before building the index.
index(x, ...)Either a ColumnLinkedMatrix or a RowLinkedMatrix object.
Additional arguments (see Details).
A matrix.
ColumnLinkedMatrix-class,
RowLinkedMatrix-class, and LinkedMatrix-class
for more information on the ColumnLinkedMatrix,
RowLinkedMatrix, and LinkedMatrix classes.