R help - Sep 2006 - how to delete some columns from a matrix based on some other indicator variable

Hello,

I am not very familiar with R and need help in deleting a few columns in a 
matrix.

Suppose I have a indicator variable called r and it's defined as r = (0, 0, 
1, 1). A matrix D is a 3X4 matrix. If I want a new matrix which contains 
only the columns of D corresponding to the elements of r that equal to 1. 
how can i write a loop which creat a new matrix that contains only the last 
2 columns of D in this case? thanks

You don't need a loop.  You could try
> r <- c(0,0,1,1)
> matD <- matrix(1:12,nrow=3)
> matD
     [,1] [,2] [,3] [,4]
[1,]    1    4    7   10
[2,]    2    5    8   11
[3,]    3    6    9   12
> matD[,r==1]
> matD[,r==1]
     [,1] [,2]
[1,]    7   10
[2,]    8   11
[3,]    9   12


On 20/09/06, Ya-Hsiu Chuang <amichuang at hotmail.com>
wrote:
> Hello,
>
> I am not very familiar with R and need help in deleting a few columns in a
> matrix.
>
> Suppose I have a indicator variable called r and it's defined as r =
(0, 0,
> 1, 1). A matrix D is a 3X4 matrix. If I want a new matrix which contains
> only the columns of D corresponding to the elements of r that equal to 1.
> how can i write a loop which creat a new matrix that contains only the last
> 2 columns of D in this case? thanks
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

-- 
================================David Barron
Said Business School
University of Oxford
Park End Street
Oxford OX1 1HP