Hi Gerald Jean,
Glad to hear it!
The number of elements in each group is contained in a special variable `.N`.
IIUC, all you’ve to do is replace ’n’ by ‘.N’ in your DT code.
BTW, we are also adding detailed (HTML) vignettes, which you can find here:
https://github.com/Rdatatable/data.table/wiki/Getting-started
You can see here: https://github.com/Rdatatable/data.table/issues/944 to get an
idea on the vignettes that are yet to be done.
Hope this is of some help.
--
Arun
On 16 Feb 2015 at 20:55:04, Gerald Jean ([email protected]) wrote:
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
Gerald Jean, M. Sc. en statistiques
Conseiller senior en statistiques
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