[datatable-help] Data Table Subset Question

[Bernstein, Elliot J]

Thanks!

- Elliot

From: by.hook.or at gmail.com [mailto:by.hook.or at gmail.com] On Behalf Of Frank Erickson
Sent: Wednesday, August 16, 2017 6:24 PM
To: Bernstein, Elliot J
Cc: datatable-help at lists.r-forge.r-project.org
Subject: Re: [datatable-help] Data Table Subset Question

One idiom for testing group-level conditions is:

data[, if (mean(x) < 10) .SD, by=g]

This might be slower in the special case of taking a mean. See ?GForce.

There's a request for an idiom like SQL HAVING over here: https://github.com/Rdatatable/data.table/issues/788

--Frank

On Wed, Aug 16, 2017 at 4:44 PM, Bernstein, Elliot J <EJBernstein at wellington.com<mailto:EJBernstein at wellington.com>> wrote:
Is there a way to subset a data table by the result of a grouped aggregation without adding an interim column to the table? For example, if I want to select all rows for which the group mean value of x is less than 10, I can do the following:

data <- data.table(x = 1:20, g = rep(c("a", "b"), each = 10))
data[, mean.x := mean(x), by = .(g)]
data[mean.x < 10,]

But I’m not really interested in “mean.x”. Can I do the same thing without adding it to the table?

Thanks.

- Elliot

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