# [R] Specification of factors in tapply

Hedderik van Rijn rijn at swi.psy.uva.nl
Wed Feb 21 18:14:56 CET 2001

```First of all, thanks for the helpful reply (also to Peter Dalgaard).

Ripley:

> I think you are just discovering that if you collapse an array to a
> vector, you get the results in Fortran order.

Dalgaard:

> tapply() gives a *table*, in this case each element is a vector of
> logicals. Tables are stored with first index varying the fastest. So
> where's the surprise?

Obviously, I overlooked the exact representation of the result of tapply.
A nice example of conformation bias as the unlist command seemed the right
function to use but clouded my the actual return structure. :-)

If I understand correctly, my code depends on the "accidental" order of
table representations. Browsing through of R scripts I wrote, it seems that
I've been very lucky not to stumble upon this earlier as I use this
"feature" quite often.

So, two questions pop up.

First, can I use this "feature" savely, or is should it be considered
"undocumented" and instable? (I do not like the idea of possible different
results if I ever happen to run the scripts using later versions of R.)

Second, what are the functions/methods I could use to get these results in a
more R-ish way? I quite often want to perform some function per subset of a
dataframe and include the results back into the frame. As in the example I
sent, finding the first value equal to or larger than 0 per
subject/condition combination.

Thanks a lot for all the help sofar,
Hedderik.

(Just to be sure, I include the example again:)

x <- as.data.frame(list(data=c(-9,0,3,1,-9,1,0,-9,0,3,1,-9,1,0),
subj=c(rep(1,7),rep(2,7)),
cond=rep(c(rep(1,4),rep(2,3)),2)))

x\$first <- unlist(tapply(x\$data,list(x\$subj,x\$cond),
function(x) {
retval<-rep(F,length(x));
if (length(x[x>=0])>0) {
retval[min(which(x>=0))]<-T;
}
print(cbind(x,retval)); # Print some debug info
retval}))

--
http://swipc30.swi.psy.uva.nl/~rijn

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```