You could declare a matrix much larger than you intend to use. This works with a
few megabytes of data. It is not very efficient, so scaling up may become a
problem.
m22 <- matrix(NA, 1:600000, ncol=6)
It does not work to add a new column to the matrix, as in you get an error if
you try m22[ , 7] but convert to data frame and add a column
m23 <- data.frame(m22)
m23$x7 <- 12
The only penalty that I know of to having unused space in a matrix is the amount
of memory it takes. One side effect is that your program may have a mistake that
you would normally catch with a subscript out of bounds error but with the extra
space it now runs without errors.
Tim
-----Original Message-----
From: R-help <r-help-bounces at r-project.org> On Behalf Of Richard
O'Keefe
Sent: Thursday, February 29, 2024 5:29 AM
To: Steven Yen <styen at ntu.edu.tw>
Cc: R-help Mailing List <r-help at r-project.org>
Subject: Re: [R] Initializing vector and matrices
[External Email]
x <- numeric(0)
for (...) {
x[length(x)+1] <- ...
}
works.
You can build a matrix by building a vector one element at a time this way, and
then reshaping it at the end. That only works if you don't need it to be a
matrix at all times.
Another approach is to build a list of rows. It's not a matrix, but a list
of rows can be a *ragged* matrix with rows of varying length.
On Wed, 28 Feb 2024 at 21:57, Steven Yen <styen at ntu.edu.tw>
wrote:>
> Is there as way to initialize a vector (matrix) with an unknown length
> (dimension)? NULL does not seem to work. The lines below work with a
> vector of length 4 and a matrix of 4 x 4. What if I do not know
> initially the length/dimension of the vector/matrix?
>
> All I want is to add up (accumulate) the vector and matrix as I go
> through the loop.
>
> Or, are there other ways to accumulate such vectors and matrices?
>
> > x<-rep(0,4) # this works but I like to leave the length open
>
> for (i in 1:3){
> + x1<-1:4
> + x<-x+x1
> + }
> > x
> [1] 3 6 9 12
>
> > y = 0*matrix(1:16, nrow = 4, ncol = 4); # this works but I like to
> leave the dimension open
> [,1] [,2] [,3] [,4]
> [1,] 0 0 0 0
> [2,] 0 0 0 0
> [3,] 0 0 0 0
> [4,] 0 0 0 0
> > for (i in 1:3){
> + y1<-matrix(17:32, nrow = 4, ncol = 4)
> + y<-y+y1
> + }
> > y
> [,1] [,2] [,3] [,4]
> [1,] 51 63 75 87
> [2,] 54 66 78 90
> [3,] 57 69 81 93
> [4,] 60 72 84 96
> >
>
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