try this:
x <- matrix(c(1,0,0, 0,0,1, 0,1,0, 0,0,1, 0,1,0, 1,0,0),
ncol = 3, byrow = TRUE)
which(x == 1, arr.ind = TRUE)[, "col", drop = FALSE]
I hope it helps.
Best,
Dimitris
----
Dimitris Rizopoulos
Biostatistical Centre
School of Public Health
Catholic University of Leuven
Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/(0)16/336899
Fax: +32/(0)16/337015
Web: http://med.kuleuven.be/biostat/
http://www.student.kuleuven.be/~m0390867/dimitris.htm
----- Original Message -----
From: <[EMAIL PROTECTED]>
To: <r-help@r-project.org>
Sent: Wednesday, June 11, 2008 10:10 AM
Subject: [R] Matrix transformation problem
ng,
I have a matrix (x) with binary content. Each row of the matrix
holds exactly one 1, and the rest of the row is zeros. The thing is
that I need to 'collapse' the matrix to one column where each row
holds the original column index of the 1's (y). Sometimes, the
matrix is quite large, so I have a perfomance problem.
x <- matrix(c(1,0,0, 0,0,1, 0,1,0, 0,0,1, 0,1,0,
1,0,0),ncol=3,byrow=T)
x
[,1] [,2] [,3]
[1,] 1 0 0
[2,] 0 0 1
[3,] 0 1 0
[4,] 0 0 1
[5,] 0 1 0
[6,] 1 0 0
In the matrix above, on the first row, the 1 is in column 1, hence
'1' on the first row in the matrix below. On the second row in the
matrix above, the 1 is in column 3, hence the '3' on the second row
in the matrix below. And so on...
y
[,1]
[1,] 1
[2,] 3
[3,] 2
[4,] 3
[5,] 2
[6,] 1
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