[R] What's the BEST way in R to adapt this vector?

Jagat.K.Sheth at wellsfargo.com Jagat.K.Sheth at wellsfargo.com
Mon Nov 24 03:00:00 CET 2008

bEST is up to you to define. Here is one simple way

y.new <- c(t(model.matrix(~factor(y)-1))) 

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of zerfetzen
Sent: Saturday, November 22, 2008 12:00 PM
To: r-help at r-project.org
Subject: [R] What's the BEST way in R to adapt this vector?

Suppose you have a vector that is a discrete variable with values
ranging from 1 to 3, and length of 10.  We'll use this as the example:

y <- c(1,2,3,1,2,3,1,2,3,1)

...and suppose you want your new vector (y.new) to be equal in length to
the possible discrete values (3) times the length (10), and formatted in
such a way that if y[1] == 1, then y.new[1:3] == c(1,0,0), and if y[2]
== 2, then y.new[4:6] == c(0,1,0).  For example, the final goal should

y.new <- c(1,0,0,0,1,0,0,0,1,1,0,0,0,1,0,0,0,1,1,0,0,0,1,0,0,0,1,1,0,0)

Note: I know how to do this with loops, but that's not taking advantage
of R's capabilities with vectors and, I suspect, matrices.

So far, my best guess would be to start as follows:

y1 <- ifelse(y == 1, 1, 0)
y2 <- ifelse(y == 2, 1, 0)
y3 <- ifelse(y == 3, 1, 0)

>From here, maybe put these into a 10x3 matrix, and read them out by row

y.new?  Is that even the most efficient way?  If it is, I'm sure I can
get them into a matrix, but how do I read them out correctly?  Thanks
for any input.
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