[R] Initializing vector and matrices

[Bert Gunter]

"It would be really really helpful to have a clearer idea of what you
are trying to do."

Amen!

But in R, "constructing" objects by extending them piece by piece is
generally very inefficient (e.g.
https://r-craft.org/growing-objects-and-loop-memory-pre-allocation/),
although sometimes?/often? unavoidable (hence the relevance of your
comment above). R generally prefers to take a "whole object" point of
view ( see R books by Chambers, et. al.) and provides code for basic
operations like vector/matrix, etc. construction to do so. When this
is not possible, I suspect "optimal" efficient strategies for
allocating space to build objects gets you into the weeds of how R
works.

Cheers,
Bert




On Sat, Mar 2, 2024 at 1:02 AM Richard O'Keefe <raoknz using gmail.com> wrote:
>
> The matrix equivalent of
>   x <- ...
>   v <- ...
>   x[length(x)+1] <- v
> is
>   m <- ...
>   r <- ...
>   m <- rbind(m, r)
> or
>   m <- ...
>   k <- ...
>   m <- cbind(m, c)
>
> A vector or matrix so constructed never has "holes" in it.
> It's better to think of CONSTRUCTING vectors and matrices rather than
> INITIALISING them,
> because always being fully defined is important.
>
> It would be really really helpful to have a clearer idea of what you
> are trying to do.
>
> On Fri, 1 Mar 2024 at 03:31, Ebert,Timothy Aaron <tebert using ufl.edu> wrote:
> >
> > 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 using r-project.org> On Behalf Of Richard O'Keefe
> > Sent: Thursday, February 29, 2024 5:29 AM
> > To: Steven Yen <styen using ntu.edu.tw>
> > Cc: R-help Mailing List <r-help using 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 using 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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