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I have a 5x4 matrix and I want to select all elements (to be set to 0) except those in rows 2 to 4 AND columns 2 to 3. Basically, all the elements along the "edges" of the matrix should be set to 0. Currently, my code is
mat[ -(2:4), -(2:3) ] <- 0
However, this (de)selects the elements in an OR fashion so instead, only the corners of the matrix are set to 0. How can I choose them in AND fashion?
533
To get a specific row of a matrix, specify the row number followed by a comma, in square brackets, after the matrix variable name. This expression returns the required row as a vector.
Typically, it would be: xdatatemp =xdata(:,77:86) - to select columns 77 to 86.
When we create a matrix in R, its column names are not defined but we can name them or might import a matrix that might have column names. If the column names are not defined then we simply use column numbers to extract the columns but if we have column names then we can select the column by name as well as its name.
Use functions row and col together with logical operations. The following works because R's matrices are in column-first order.
mat <- matrix(seq.int(5*4), nrow = 5)
mat[ !(row(mat) %in% 2:4) | !(col(mat) %in% 2:3) ] <- 0
mat
# [,1] [,2] [,3] [,4]
#[1,] 0 0 0 0
#[2,] 0 7 12 0
#[3,] 0 8 13 0
#[4,] 0 9 14 0
#[5,] 0 0 0 0
Another option with replace
replace(mat * 0, as.matrix(expand.grid(2:4, 2:3)), mat[2:4, 2:3])
-output
# [,1] [,2] [,3] [,4]
#[1,] 0 0 0 0
#[2,] 0 7 12 0
#[3,] 0 8 13 0
#[4,] 0 9 14 0
#[5,] 0 0 0 0
mat <- matrix(seq.int(5*4), nrow = 5)
Here is a two-step base R option
rs <- 2:4
cs <- 2:3
`<-`(mat[setdiff(seq(nrow(mat)), rs), ], 0)
`<-`(mat[, setdiff(seq(ncol(mat)), cs)], 0)
which gives
> mat
[,1] [,2] [,3] [,4]
[1,] 0 0 0 0
[2,] 0 7 12 0
[3,] 0 8 13 0
[4,] 0 9 14 0
[5,] 0 0 0 0
Data
mat <- matrix(seq.int(5*4), nrow = 5)
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