[datatable-help] Subsetting columns in data.table

[Matthew Dowle]

Hi,

I'm not following this one I'm afraid. The example data is good, and 
it's nice
to see your attempts, but the desired result isn't clear, at least to 
me.

I can't think of any reason to do .SD<=7. .SD is a data.table, so it 
doesn't
make much sense to compare a list of vectors to the single value 7.

Matthew


On 16.11.2012 14:15, Berto wrote:
> Dear all,
>
> I am naive in data.table usage, so I write to you to get help on a 
> perhaps
> trivial question, that it's not clear to me after reading the 
> "Introduction
> to data.table" or the "FAQ" vignettes.
>
> I'd like to subset the rows with contain a value above a certain 
> threshold
> for a certain column, but below another threshold for all the rest 
> and then
> do some operations (e.g. sum):
>
> #Example data
> DT = data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9, 
> w=3:11,
> z=LETTERS[1:9])
> setkey(DT,z)
> DT
>
>    x y v  w z
> 1: a 1 1  3 A
> 2: a 3 2  4 B
> 3: a 6 3  5 C
> 4: b 1 4  6 D
> 5: b 3 5  7 E
> 6: b 6 6  8 F
> 7: c 1 7  9 G
> 8: c 3 8 10 H
> 9: c 6 9 11 I
>
> #Easy case, 1 variable
> DT[y>=3&v<=7,sum(y), by=x]
>
> #Particular solution (low ncols)
> DT[y>=3&v<=7&w<=7,sum(y), by=x]
>
> x V1
> 1: a  9
> 2: b  3
>
> #Attempt subgroup for general solution
> DT[y>=3, .SD<=7,]
>
>          x    y     v     w     z
> [1,] FALSE TRUE  TRUE  TRUE FALSE
> [2,] FALSE TRUE  TRUE  TRUE FALSE
> [3,] FALSE TRUE  TRUE  TRUE FALSE
> [4,] FALSE TRUE  TRUE FALSE FALSE
> [5,] FALSE TRUE FALSE FALSE FALSE
> [6,] FALSE TRUE FALSE FALSE FALSE
>
> ##gives a matrix with logical for <=7.
>
> #Second attempt based on FAQ 2.1
> DT[y>=3,lapply(.SD<=7,sum),by=x]
>
>    x V1 V2 V3 V4 V5 V6 V7 V8
> 1: a  1  1  1  1  1  1  0  0
> 2: b  1  1  1  1  1  0  0  0
> 3: c  1  1  0  0  0  0  0  0
>
> I would appreciate any suggestions, maybe this has been covered 
> already in
> some post of SO that I've missed.
>
> Thanks in advance for your support!!
>
>
>
> --
> View this message in context:
> 
> http://r.789695.n4.nabble.com/Subsetting-columns-in-data-table-tp4649736.html
> Sent from the datatable-help mailing list archive at Nabble.com.
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