extract positions from matrix

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

ar[c(1,5),1,]

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 <jporobicg at 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


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David Winsemius, MD
West Hartford, CT