diagU2N {Matrix} | R Documentation

## Transform Triangular Matrices from Unit Triangular to General Triangular and Back

### Description

Transform a triangular matrix `x`, i.e., of `class` `"triangularMatrix"`, from (internally!) unit triangular (“unitriangular”) to “general” triangular (`diagU2N(x)`) or back (`diagN2U(x)`). Note that the latter, `diagN2U(x)`, also sets the diagonal to one in cases where `diag(x)` was not all one.

`.diagU2N(x)` assumes but does not check that `x` is a `triangularMatrix` with `diag` slot `"U"`, and should hence be used with care.

### Usage

     diagN2U(x, cl = getClassDef(class(x)), checkDense = FALSE)

     diagU2N(x, cl = getClassDef(class(x)), checkDense = FALSE)
    .diagU2N(x, cl, checkDense = FALSE)

### Arguments

`x` | a `triangularMatrix`, often sparse.
`cl` | (optional, for speedup only:) class (definition) of `x`.
`checkDense` | logical indicating if dense (see `denseMatrix`) matrices should be considered at all; i.e., when false, as per default, the result will be sparse even when `x` is dense.

### Details

The concept of unit triangular matrices with a `diag` slot of `"U"` stems from LAPACK.

### Value

a triangular matrix of the same `class` but with a different `diag` slot. For `diagU2N` (semantically) with identical entries as `x`, whereas in `diagN2U(x)`, the off-diagonal entries are unchanged and the diagonal is set to all `1` even if it was not previously.

### Note

Such internal storage details should rarely be of relevance to the user. Hence, these functions really are rather internal utilities.

### See Also

`"triangularMatrix"`, `"dtCMatrix"`.

### Examples

    (T <- Diagonal(7) + triu(Matrix(rpois(49, 1/4), 7,7), k = 1))
    (uT <- diagN2U(T)) # "unitriangular"
    (t.u <- diagN2U(10*T))# changes the diagonal!
    stopifnot(all(T == uT), diag(t.u) == 1,
              identical(T, diagU2N(uT)))
    T[upper.tri(T)] <- 5
    T <- diagN2U(as(T,"triangularMatrix"))
    stopifnot(T@diag == "U")
    dT <- as(T, "denseMatrix")
    dt. <- diagN2U(dT)
    dtU <- diagN2U(dT, checkDense=TRUE)
    stopifnot(is(dtU, "denseMatrix"), is(dt., "sparseMatrix"),
              all(dT == dt.), all(dT == dtU),
              dt.@diag == "U", dtU@diag == "U")

[Package Matrix version 1.2-6 Index]