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Confusing warning message #2763

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MichaelChirico opened this issue Apr 18, 2018 · 2 comments · Fixed by #3685
Closed

Confusing warning message #2763

MichaelChirico opened this issue Apr 18, 2018 · 2 comments · Fixed by #3685
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@MichaelChirico
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@MichaelChirico MichaelChirico commented Apr 18, 2018

I'm trying to split data according to 15-minute timestamps; my approach is:

library(data.table)
set.seed(210349)
DT = data.table(t = sample(10000, 100), V = rnorm(100))
# t is in seconds, so 900 seconds is 15 minutes
DT[ , ID := .GRP, keyby = .(t %/% 900L)]

Warning message:
In [.data.table(DT, , :=(ID, .GRP), keyby = .(t%/%900L)) :
:= keyby not straightforward character column names or list() of column names, treating as a by:t%/%900

It's weird that this gives me a warning since it's purely related to the use of keyby:

# (no warning)
DT[order(t), ID := .GRP, by = .(t %/% 900L)]

(i in this case doesn't need to match that in by exactly)

For sanity:

DT[ , ID := .GRP, keyby = .(t %/% 900L)]
DT[order(t), ID2 := .GRP, by = .(t %/% 900L)]
DT[ , identical(ID, ID2)]
# [1] TRUE

What's the reason for this warning?

@franknarf1
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@franknarf1 franknarf1 commented Apr 18, 2018

I'm guessing it's that when you use := with keyby, it ...

  1. does the column edit with :=
  2. sets the key to the keyby= columns

If your keyby= columns do not exist in the table, it cannot set them as the key. Dunno if there's some clearer way to write the warning.

@MichaelChirico
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@MichaelChirico MichaelChirico commented Apr 18, 2018

MichaelChirico referenced this issue in tidyverse/dtplyr Jul 3, 2019
By using extended j expression as suggested by @MichaelChirico
@mattdowle mattdowle added this to the 1.12.4 milestone Jul 16, 2019
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3 participants