[R] array indexing and which

[Bill.Venables@csiro.au]

You need to think about it just a bit harder.

[Hint: what happens if you leave out the first 'which' and just make

ids <- (d[, 1] > 0)

does it work then...?]

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Werner Wernersen
Sent: Monday, 18 April 2005 3:13 AM
To: r-help at stat.math.ethz.ch
Subject: [R] array indexing and which


Hi R friends!

I am stuck with a stupid question: I can circumvent it
but I would like to 
understand why it is wrong. It would be nice if you
could give me a hint...

I have an 2D array d and do the following:
ids <- which(d[,1]>0)

then I have a vector gk with same column size as d and
do:
ids2 <- which(gk[ids]==1)

but I can't interprete the result I get in ids2.

I get the expected result when I use:
which(gk==1 & d[,1]>0)

Why is the first version wrong?

The reason why I try to use the ids vectors is that I
want to avoid recomputation.

Thanks for your help!
   Werner

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