[datatable-help] Row wise operation in data.table

[Serdar Akin]

Thanks Arunkumar,

Very good and fast :)

/Serdar


2013/8/30 Arunkumar Srinivasan <aragorn168b at gmail.com>

>  It'd be easier if you just `melt` your `data.table` by `id = Respid`.
> Then you get a long format with which you can use fast grouping to count
> the number of 3s.
>
> require(reshape2)
> require(data.table)
>
> setkey(as.data.table(melt(DT, id="Respid")), Respid)[value == 3, .N,
> by=Respid][DT]
>
>
> Arun
>
> On Friday, August 30, 2013 at 9:04 AM, Serdar Akin wrote:
>
> Hi,
>
> Currently I'm trying to find a way to make row wise operation within
> data.table to find value that have
> a certain pattern, and to count those.
>
> For instance find the number of 3 for each Respid and count those for each
> row.
> set.seed(1)
> DT    <- data.table( Respid = seq(1,100, 1),
>                     Q1 = rep(1:5, each = 20),
>                     Q2 = as.integer(runif(100, min = 1, max = 5) ),
>                     Q3   = sample(1:5, 100, replace = T)
> )
>
> DT1    <- DT[, lapply(.SD, function(x) length(grep(3, x))), by = 'Respid']
>
> A do get 1 for each column that has a 3 in it but no column that counts
> it.
>
> Regards Serdar
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