On Nov 19, 2011, at 1:15 PM, Bert Gunter wrote:
Folks:
David: I believe your approach needs to be modified to accommodate
hundreds of matrices per the OP's specification.
I suggested that my approach was most applicable if the questioner had
his matrices already in a list. (He didn't say, so neither of us knows
yet what his/her use case is.)
However, I would say that this is exactly the situation in which
arrays should be used. Not only does it appear simpler, but it also
could be faster since once the array is created, (vectorized) indexing
is used. The key is to take advantage of column major ordering of
arrays in R as follows:
1. The OP's example specification is wrong -- he wants positions 1 and
5 of the first COLUMN of each matrix (which both solutions gave, but
just didn't mention).
2. The code below creates the array assuming that the global workspace
has only the matrices in it. Subscript ls() appropriately if this is
not the case.
for(i in 1:3) assign(LETTERS[i], 15*(i-1)+seq_len(15)) ## toy data
rm(i) ## want only the matrices in the global workspace
ar <- array( sapply(ls(), get), dim=c(5,3,3)) ## the dim argument
could also be specified programatically
I wonder if you are doing ".jpg" a favor by putting ls() at the center
of your strategy unless you offer the tools to isolate these objects
from the rest of the global environment. Who works with no other
objects in the workspace?
Perhaps this would add the extra level of encapsulation needed for the
sapply(ls() method of access the created matrices in a probably less
disruptive fashion:
e <- new.env()
with(e, { for(i in 1:3) assign( LETTERS[i], 15*(i-1)+seq_len(15) )
rm(i)
ar <- array( sapply(ls(), get), dim=c(5,3,3)) })
# ----------------
e$ar[c(1,5),1, ]
[,1] [,2] [,3]
[1,] 1 16 31
[2,] 5 20 35
--
David.
ar[c(1,5),1,]
[,1] [,2] [,3]
[1,] 1 16 31
[2,] 5 20 35
Cheers,
Bert
On Sat, Nov 19, 2011 at 9:19 AM, David Winsemius <dwinsem...@comcast.net
> wrote:
On Nov 19, 2011, at 9:32 AM, R. Michael Weylandt wrote:
Here's one approach:
A=matrix(1:15,5)
B=matrix(15:29,5)
C=matrix(30:44,5)
do.call(cbind, lapply(c("A","B","C"),function(x) get(x)[c(1,5),1]))
Also:
sapply( list(A,B,C), function(x) do.call("[", list(x, c(1,5))) )
Notice that this actually was extracting using what might be called
the
"vector positions". If you wanted to use the i,j version of "["
then you
would need an extra column (this example pulling the second columns
in the
rows selected:
sapply(list(A,B,C),function(x) do.call("[", list(x, c(1,5), 2)) )
[,1] [,2] [,3]
[1,] 6 20 35
[2,] 10 24 39
Comments on the differences: Michaels version used cbind to get the
ruslt in
a matrix, whereas mine used sapply. His used the get function to
extract the
objects from a character vector, whereas mine never constructed a
character
vector. Which one you deploy will depend on your data setup. If you
already
have these in a list, mine might be easier, but if they constitute
an easily
constructed set of names you might use LETTERS[] numbers and
paste() to
build your list of object names.
--
david.
Michael
On Thu, Nov 17, 2011 at 9:44 PM, .Jpg <jporob...@gmail.com> wrote:
Hi everyone, I tried to solve this problem but I could not find the
solution. I have about 105 matrices of equal size in the memory
of**R, I
need to do is extract from these matrices some known positions and
create a new matrix with these columns. Show you an example with
only
three matrices (but in my case I have hundreds of them).
A=matrix(1:15,5)
[,1] [,2] [,3]
[1,] 1 6 11
[2,] 2 7 12
[3,] 3 8 13
[4,] 4 9 14
[5,] 5 10 15
B=matrix(15:29,5)
[,1] [,2] [,3]
[1,] 15 20 25
[2,] 16 21 26
[3,] 17 22 27
[4,] 18 23 28
[5,] 19 24 29
C=matrix(30:44,5)
[,1] [,2] [,3]
[1,] 30 35 40
[2,] 31 36 41
[3,] 32 37 42
[4,] 33 38 43
[5,] 34 39 44
The positions I wish to extract are 1 and 5 of the first row of
each
matrix (in my case are 25positions) and with this generate a new
matrix
with the form
d=
[,1] [,2] [,3]
[1,] 1 15 30
[2,] 5 19 34
The ideais to builda loop toextract thisinformation from
hundredsmatrices, butI failed todo so.
Any helpwould be greatthank you very muchin advance
regards
.jpg
--
Bert Gunter
Genentech Nonclinical Biostatistics
Internal Contact Info:
Phone: 467-7374
Website:
http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm
David Winsemius, MD
West Hartford, CT
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