R: Drop Non-Structural Zeros from a Sparse Matrix

drop0 {Matrix}

R Documentation

Drop Non-Structural Zeros from a Sparse Matrix

Description

Deletes “non-structural” zeros (i.e., zeros stored explicitly, in memory) from a sparse matrix and returns the result.

Usage

drop0(x, tol = 0, is.Csparse = NA, give.Csparse = TRUE)

Arguments

`x`	a `Matrix`, typically inheriting from virtual class `sparseMatrix`. `denseMatrix` and traditional vectors and matrices are coerced to `CsparseMatrix`, with zeros dropped automatically, hence users passing such `x` should consider `as(x, "CsparseMatrix")` instead, notably in the `tol = 0` case.
`tol`	a non-negative number. If `x` is numeric, then entries less than or equal to `tol` in absolute value are deleted.
`is.Csparse`	a logical used only if `give.Csparse` is `TRUE`, indicating if `x` already inherits from virtual class `CsparseMatrix`, in which case coercion is not attempted, permitting some (typically small) speed-up.
`give.Csparse`	a logical indicating if the result must inherit from virtual class `CsparseMatrix`. If `FALSE` and `x` inherits from `RsparseMatrix`, `TsparseMatrix`, or `indMatrix`, then the result preserves the class of `x`. The default value is `TRUE` only for backwards compatibility.

Value

A sparseMatrix, the result of deleting non-structural zeros from x, possibly after coercion.

Note

drop0 is sometimes called in conjunction with zapsmall, e.g., when dealing with sparse matrix products; see the example.

Examples

(m <- sparseMatrix(i = 1:8, j = 2:9, x = c(0:2, 3:-1),
                   dims = c(10L, 20L)))
drop0(m)

## A larger example:
t5 <- new("dtCMatrix", Dim = c(5L, 5L), uplo = "L",
          x = c(10, 1, 3, 10, 1, 10, 1, 10, 10),
          i = c(0L,2L,4L, 1L, 3L,2L,4L, 3L, 4L),
          p = c(0L, 3L, 5L, 7:9))
TT <- kronecker(t5, kronecker(kronecker(t5, t5), t5))
IT <- solve(TT)
I. <- TT %*% IT ;  nnzero(I.) # 697 ( == 625 + 72 )
I.0 <- drop0(zapsmall(I.))
## which actually can be more efficiently achieved by
I.. <- drop0(I., tol = 1e-15)
stopifnot(all(I.0 == Diagonal(625)), nnzero(I..) == 625)

[Package Matrix version 1.7-1 Index]