# Matrix: Construct a Classed Matrix In Matrix: Sparse and Dense Matrix Classes and Methods

## Description

Construct a Matrix of a class that inherits from `Matrix`.

## Usage

 ```1 2``` ```Matrix(data=NA, nrow=1, ncol=1, byrow=FALSE, dimnames=NULL, sparse = NULL, doDiag = TRUE, forceCheck = FALSE) ```

## Arguments

 `data` an optional numeric data vector or matrix. `nrow` when `data` is not a matrix, the desired number of rows `ncol` when `data` is not a matrix, the desired number of columns `byrow` logical. If `FALSE` (the default) the matrix is filled by columns, otherwise the matrix is filled by rows. `dimnames` a `dimnames` attribute for the matrix: a `list` of two character components. They are set if not `NULL` (as per default). `sparse` logical or `NULL`, specifying if the result should be sparse or not. By default, it is made sparse when more than half of the entries are 0. `doDiag` logical indicating if a `diagonalMatrix` object should be returned when the resulting matrix is diagonal (mathematically). As class `diagonalMatrix` `extends` `sparseMatrix`, this is a natural default for all values of `sparse`. Otherwise, if `doDiag` is false, a dense or sparse (depending on `sparse`) symmetric matrix will be returned. `forceCheck` logical indicating if the checks for structure should even happen when `data` is already a `"Matrix"` object.

## Details

If either of `nrow` or `ncol` is not given, an attempt is made to infer it from the length of `data` and the other parameter. Further, `Matrix()` makes efforts to keep `logical` matrices logical, i.e., inheriting from class `lMatrix`, and to determine specially structured matrices such as symmetric, triangular or diagonal ones. Note that a symmetric matrix also needs symmetric `dimnames`, e.g., by specifying `dimnames = list(NULL,NULL)`, see the examples.

Most of the time, the function works via a traditional (full) `matrix`. However, `Matrix(0, nrow,ncol)` directly constructs an “empty” sparseMatrix, as does `Matrix(FALSE, *)`.

Although it is sometime possible to mix unclassed matrices (created with `matrix`) with ones of class `"Matrix"`, it is much safer to always use carefully constructed ones of class `"Matrix"`.

## Value

Returns matrix of a class that inherits from `"Matrix"`. Only if `data` is not a `matrix` and does not already inherit from class `Matrix` are the arguments `nrow`, `ncol` and `byrow` made use of.

The classes `Matrix`, `symmetricMatrix`, `triangularMatrix`, and `diagonalMatrix`; further, `matrix`.
Special matrices can be constructed, e.g., via `sparseMatrix` (sparse), `bdiag` (block-diagonal), `bandSparse` (banded sparse), or `Diagonal`.
 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34``` ```Matrix(0, 3, 2) # 3 by 2 matrix of zeros -> sparse Matrix(0, 3, 2, sparse=FALSE)# -> 'dense' ## 4 cases - 3 different results : Matrix(0, 2, 2) # diagonal ! Matrix(0, 2, 2, sparse=FALSE)# (ditto) Matrix(0, 2, 2, doDiag=FALSE)# -> sparse symm. "dsCMatrix" Matrix(0, 2, 2, sparse=FALSE, doDiag=FALSE)# -> dense symm. "dsyMatrix" Matrix(1:6, 3, 2) # a 3 by 2 matrix (+ integer warning) Matrix(1:6 + 1, nrow=3) ## logical ones: Matrix(diag(4) > 0) # -> "ldiMatrix" with diag = "U" Matrix(diag(4) > 0, sparse=TRUE) # (ditto) Matrix(diag(4) >= 0) # -> "lsyMatrix" (of all 'TRUE') ## triangular l3 <- upper.tri(matrix(,3,3)) (M <- Matrix(l3)) # -> "ltCMatrix" Matrix(! l3) # -> "ltrMatrix" as(l3, "CsparseMatrix")# "lgCMatrix" Matrix(1:9, nrow=3, dimnames = list(c("a", "b", "c"), c("A", "B", "C"))) (I3 <- Matrix(diag(3)))# identity, i.e., unit "diagonalMatrix" str(I3) # note 'diag = "U"' and the empty 'x' slot (A <- cbind(a=c(2,1), b=1:2))# symmetric *apart* from dimnames Matrix(A) # hence 'dgeMatrix' (As <- Matrix(A, dimnames = list(NULL,NULL)))# -> symmetric forceSymmetric(A) # also symmetric, w/ symm. dimnames stopifnot(is(As, "symmetricMatrix"), is(Matrix(0, 3,3), "sparseMatrix"), is(Matrix(FALSE, 1,1), "sparseMatrix")) ```