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

From: <Jagat.K.Sheth_at_wellsfargo.com>
Date: Sun, 23 Nov 2008 20:00:00 -0600

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?

Goal:
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 be:

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

>into

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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