[datatable-help] Subsetting within groups.

[Arunkumar Srinivasan]

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 (gerald.jean at dgag.ca) 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
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