[datatable-help] Subsetting within groups.

[Gerald Jean]

Hello,

I am fairly new to data.table, it's fast and I love it!!!  Here is what I am trying to do.  Suppose I have a data.table DT, with columns a, b, c, v, t and g.  I want to add a new column, x, say, where for each group defined by g, in vector notation:

x = c(0, (v[-1] - v[-n]) / (t[-1] - t[-n]))

where n is the number of rows for the groups, I don't know n yet.  Obviously

DT[, x :=  c(0, (v[-1] - v[-n]) / (t[-1] - t[-n])), by = g]

won't work.  I have read the doc I found so far but couldn't find examples of subsetting the groups, maybe it could be done using .SD but I am not familiar enough with data.table yet to figure out how to do it.

By the way my data.tables are large, 50000 to over 1000000 rows and I have over 60000 of them to process and many more operations to perform, I just hope data.table will do the trick!!!

Thanks for your help,

Gérald

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Gerald Jean, M. Sc. en statistiques
Conseiller senior en statistiques

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