Lontananza
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Memory images, once they are fixed in words, are erased.
Assigning values to a matrix
byrow: default FALSE.
M <- matrix(data=1:9,nrow=3,ncol=3,byrow=TRUE)
Adding and Removing Rows/Columns
We can add row or column using rbind() and cbind() function respectively. Similarly, it can be removed through reassignment.
> cbind(x, c(1, 2, 3)) # add column
[,1] [,2] [,3] [,4]
[1,] 0 0 7 1
[2,] 0 10 8 2
[3,] 0 6 9 3
> rbind(x,c(1,2,3)) # add row
[,1] [,2] [,3]
[1,] 0 0 7
[2,] 0 10 8
[3,] 0 6 9
[4,] 1 2 3
> x <- x[1:2,] # remove last row
Basic Operations
> t(x)
# transpose a matrix
> det(x)
# return error if not a square matrix
Turning a list into matrix
> x <- c(1,2,3,4,5,6)
> x
[1] 1 2 3 4 5 6
> class(x)
[1] "numeric"
> dim(x) <- c(2,3)
> x
[,1] [,2] [,3]
[1,] 1 3 5
[2,] 2 4 6
> class(x)
[1] "matrix"
Property: drop=FALSE
if the matrix returned after indexing is a row matrix or column matrix, the result is given as a vector.
> x[1,]
[1] 1 4 7
> class(x[1,])
[1] "integer"
This behavior can be avoided by using the argument drop = FALSE while indexing.
> x[1,,drop=FALSE]
# now the result is a 1X3 matrix rather than a vector
[,1] [,2] [,3]
[1,] 1 4 7
> class(x[1,,drop=FALSE])
[1] "matrix"