[datatable-help] Extracting specific values from a matrix

[Mike.Gahan]

Here is one way to approach this problem (there are lots of ways)

#Convert to model matrix to account for factor variables
iris <- model.matrix(~.-1,data=iris)

#Convert to data.table
iris <- data.table(iris)
candidates <- names(iris)

#Setup interaction variables for every combination
all.interactions <- as.data.table(t(combn(candidates,2)))
all.interactions[ , interaction := paste(V1,V2,sep="__")]
all.interactions[ , multiplier := paste(V1,V2,sep="*")]

#Add interactions to data
for (x in 1:nrow(all.interactions))
  iris[ , paste0(all.interactions$interaction[x]) :=
eval(parse(text=all.interactions$multiplier[x]))]



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