On Aug 10, 2011, at 10:03 PM, Rolf Turner wrote:
On 11/08/11 13:27, David Winsemius wrote:
On Aug 10, 2011, at 9:22 PM, Rolf Turner wrote:
On 11/08/11 13:11, David Winsemius wrote:
On Aug 10, 2011, at 8:23 PM, Andra Isan wrote:
Hi All,
I would like to create a matrix in R but I dont know the size of
my matrix. I only know the size of the columns but not the size
of the rows. So, is there any way to create a dynamic matrix of
size NULL by n_cols? and then add to that matrix?
I know for a vector, I can do this: x= NULL but is there any way
to do the same for a matrix as well?
No. You cannot make an R matrix without knowing the number of
rows. By definition an R matix has two integer dimensions.
Alternatives: You can rbind to an existing matrix, or you can
make a larger than necessary matrix filled with NA's and then
fill and later extract a subset of the rows.
Wrong-oh, David. :-) Check this out:
0 is an integer.
Yes it is. But you are being pedantic. My suggestion does exactly
what I am sure the OP wanted.
He used the phrase ``NULL by n_cols'' simply because of unclear
thinking. What he wanted was an
``empty matrix'', with no rows and a given number of columns. Which
is what I gave him.
He asked for a "dynamic matrix", by which I imagined (in the absence
of an example) that he wanted to be able to define a matrix and then
assign values to rows without allocating any rows. Your zero row
matrix does not accomplish what I imagined his request to represent.
Your offering is rather useless unless you want to adopt what I
suggested as one strategy:
> m <- matrix(0,nrow=0,ncol=5)
> m[1,1] <- 1
Error in m[1, 1] <- 1 : subscript out of bounds
Alternative 1:
> m <- rbind(m, c(1,NA,NA,NA,NA) )
> m
[,1] [,2] [,3] [,4] [,5]
[1,] 1 NA NA NA NA
It is interesting that the syntax works with matrices but not with
data frames. If you want an
empty data frame, with no rows and a given number of columns, you
need to create a matrix,
in the manner indicated, and then use as.data.frame, which yields
the desired result.
cheers,
Rolf
David Winsemius, MD
West Hartford, CT
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