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 [cid:[email protected]] Gerald Jean, M. Sc. en statistiques Conseiller senior en statistiques Actuariat corporatif, Modélisation et Recherche Assurance de dommages Mouvement Desjardins Lévis (siège social) 418 835-4900, poste 5527639 1 877 835-4900, poste 5527639 Télécopieur : 418 835-6657 Faites bonne impression et imprimez seulement au besoin! Ce courriel est confidentiel, peut être protégé par le secret professionnel et est adressé exclusivement au destinataire. Il est strictement interdit à toute autre personne de diffuser, distribuer ou reproduire ce message. Si vous l'avez reçu par erreur, veuillez immédiatement le détruire et aviser l'expéditeur. Merci.
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