R: Drop "Explicit Zeroes" from a Sparse Matrix

drop0 {Matrix} | R Documentation

## Drop "Explicit Zeroes" from a Sparse Matrix

### Description

Returns a sparse matrix with no “explicit zeroes”, i.e., all zero or `FALSE` entries are dropped from the explicitly indexed matrix entries.

### Usage

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

### Arguments

`x` | a Matrix, typically sparse, i.e., inheriting from `sparseMatrix`.

`tol` | non-negative number to be used as tolerance for checking if an entry x[i,j] should be considered to be zero.
`is.Csparse` | logical indicating prior knowledge about the “Csparseness” of `x`. This exists for possible speedup reasons only.

### Value

a Matrix like `x` but with no explicit zeros, i.e., `!any(x@x == 0)`, always inheriting from `CsparseMatrix`.

### Note

When a sparse matrix is the result of matrix multiplications, you may want to consider combining `drop0()` with `zapsmall()`, see the example.

### See Also

`spMatrix`, class `sparseMatrix`

### Examples

    m <- spMatrix(10,20, i= 1:8, j=2:9, x = c(0:2,3:-1))
    m
    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 1.1-4 Index]