Hi Frank,
The data.frame way of referring to column names or numbers requires with=FALSE.
This is because the default data.table-like operations are more common where
with=TRUE. Also, data.tables are designed with really huge data sets in mind
and avoiding as many copies as possible. So, unless there’s a strong reason,
the data.table philosophy would be to avoid copies (ex: DT[, 1, with=FALSE]
will create a copy).
That being said, another alternative is to subset the data.table way (where
with=TRUE by default):
DT2 = DT[, list(A,D,B)] # list(1,4,2) won't work. Read FAQ 1.1-1.5
setnames(DT2, c("a", "d", "B"))
It’s also generally considered a bad practice to subset columns by using column
numbers - prone to errors.
Hope this helps.
Arun
From: Frank S. [email protected]
Reply: Frank S. [email protected]
Date: July 21, 2014 at 4:48:54 PM
To: Chris Neff [email protected], [email protected]
[email protected]
Subject: Re: [datatable-help] Construct a new data table from another
Thanks Chris,
But, is there any option in order to avoid writing "with=FALSE" every time
in the next situation?:
# USING DATA FRAME
set.seed(100)
DF = data.frame(A=letters[1:5],B=rnorm(5),C=rexp(5),D=runif(5))
DF2 <- data.frame(a=DF[,1],d=DF[,4],B=DF[,2])
If I do the equivalent operations under data table format, I'm onñy able to
obtain
the same result with the following code:
# USING DATA TABLE
set.seed(100)
DT = data.table(A=letters[1:5],B=rnorm(5),C=rexp(5),D=runif(5))
DT2 <- data.table(DT[,1,with=FALSE], DT[,4,with=FALSE], DT[,2,with=FALSE])
setnames(DT2, c('a','d','B'))
Thank you!
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