The problem with David's proposal is revealed by:
mat[7:1,]
# [,1] [,2] [,3]
# [1,] 7 14 21
# [2,] 6 13 20
# [3,] 5 12 19
# [4,] 4 11 18
# [5,] 3 10 17
# [6,] 2 9 16
# [7,] 1 8 15
which simply reverses the rows. Then:
c(mat[7:1,])
# [1] 7 6 5 4 3 2 1 14 13 12 11 10 9 8 21 20 19 18 17 16 15
since a matrix is stored "by columns" -- i.e. as a vector with its
elements ordered down each column, per col1 then col2 then ...
And the following won't work either:
mat[,1:3]
# [,1] [,2] [,3]
# [1,] 1 8 15
# [2,] 2 9 16
# [3,] 3 10 17
# [4,] 4 11 18
# [5,] 5 12 19
# [6,] 6 13 20
# [7,] 7 14 21
which is simply the original matrix, and hence:
c(mat[,1:3])
# [1] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
for the same reson as before. What you need to do is based on the
following:
t(mat[7:1,])
# [,1] [,2] [,3] [,4] [,5] [,6] [,7]
# [1,] 7 6 5 4 3 2 1
# [2,] 14 13 12 11 10 9 8
# [3,] 21 20 19 18 17 16 15
which now has the elements (down the columns) in the order you want.
So:
c(t(mat[7:1,]))
# [1] 7 14 21 6 13 20 5 12 19 4 11 18 3 10 17 2 9 16 1 8 15
As desired.
Ted.
On 23-Aug-09 18:53:38, Bogaso wrote:
No no, I actually want following result :
7, 14, 21, 6, 13, 20, 5, 12, 19,............
David Winsemius wrote:
On Aug 23, 2009, at 2:37 PM, Bogaso wrote:
I have suppose a matrix like that
mat <- matrix(1:21, 7)
mat
[,1] [,2] [,3]
[1,] 1 8 15
[2,] 2 9 16
[3,] 3 10 17
[4,] 4 11 18
[5,] 5 12 19
[6,] 6 13 20
[7,] 7 14 21
From this matrix, I want to create a vector like tha :
c(mat[7,], mat[6,], mat[5,], ....., mat[1,])
Can anyone please guide me, how to do that?
c( mat[7:1,] )
# [1] 7 6 5 4 3 2 1 14 13 12 11 10 9 8 21 20 19 18 17 16
15
--
David Winsemius, MD
Heritage Laboratories
West Hartford, CT
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Date: 23-Aug-09 Time: 20:17:58
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