[R] newbie: remove column with low mean from a matrix

[Prof Brian D Ripley]

On Tue, 5 Mar 2002, Tord Snall wrote:

> Sorry to bother you with one more newbie question.

A `newbie' of long standing, I believe.

> I have a dataobject with several hundreds of columns. I want to remove

What sort of object?  matrix or data frame or private class "dataobject" or
....

> columns with a mean of the column values below a certain value:
>
> > a<- c(1,2,3,4,5,6)
> > b<-c(2,4,6,8,10,12)
> > c<- c(3,6,9,12,15,18)
> > test<- as.matrix(cbind(a, b, c))
> > mean(a)
> [1] 3.5
> > mean(b)
> [1] 7
> > mean(c)
> [1] 10.5
>
> Say the "certain value" is 5, I would like a new matrix as follows:
>
> > new.test
>       b  c
> [1,]  2  3
> [2,]  4  6
> [3,]  6  9
> [4,]  8 12
> [5,] 10 15
> [6,] 12 18
>
>
> I have searched in the help archive, in help files, and in the
> documentation without finding it.

Let's first assume a matrix, as you used one

mns <- colMeans(test)  # apply(test, 2, mean) in 1.4.x
test[, mns >= 5, drop = F]

For a data frame:

mns <- sapply(dfr, mean)
dfr[mns >= 5]

I know a good book or two about this kind of thing.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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