R help - Oct 2014 - Using a function for rows over a subset of columns in data.table

Hi everyone,

This may be a trivial solution but I have not been able to figure what is wrong
with this code.

The basic premise of the code is to use data.table to append a new column to a
data.table that indicates if the values  per row at two other columns are within
a 2d hull generated with the package alphahull.

I get the right results when I apply the function to a data.table with only the
two columns to consider, But the results are off when I try to apply the same
function selecting the two columns from a data.table with numerous columns. I
think my problem is how the columns are selected for use in the function. Any
help will be much appreciated.

library (alphahull)
library(data.table)

#creates a hull
DT=data.frame(x=c(0.25,0.25,0.75,0.75),y=c(0.25,0.75,0.75,0.25))
Hull <- ahull(DT, alpha = 0.5)

#applying the "inahull" function to a datatable with just the two
colunms
TwoColumn<- data.table(x=c(0.25,0.5,0.5,0.5,0),y=c(0.5,0.5,0.5,.5,0))
TwoColumn[, INHULL := apply(TwoColumn, 1, function(x) inahull(Hull, x))]
#the results are correct


#now if I apply the same function to a selection of two  columns  in a
data.table with several columns, the codes dies not work
ThreColumn<- data.table(w=c(1,2,3,4,5),
z=c(1,2,3,4,5),x=c(0.25,0.5,0.5,0.5,0),y=c(0.5,0.5,0.5,.5,0))
ThreColumn[, INHULL := lapply(.SD, function(x) inahull(Hull, x)),
.SDcols=c(x,y)]



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