[datatable-help] DT 1.9.5 - refers previous/next/whatever row

[Nicolas Paris]

Thanks all,

Actualy my use case is a little different

> dt <- data.table(col1=c("FOOBARBAZ","BARBAZ","BAZ"))
        col1
1: FOOBARBAZ
2:    BARBAZ
3:       BAZ


What I want to get is :
> data.table(col1=c("FOO","BAR","BAZ"))
   col1
1:  FOO
2:  BAR
3:  BAZ

(I remove next row from actual row)

This works :
dt[,col3:=mapply(function(x,y){gsub(x,"",y,fixed=T)},
shift(col1,fill=" ",type="lead"),
col1)]


Have you in mind a better solution ? Will it be faster than loop (yet this
is less readable)?

Thanks again




2015-10-19 16:12 GMT+02:00 Frank Erickson <fperickson at wisc.edu>:

> I think `shift` is the best option:
>
> dt <-data.table(col1=c(1,2,3))
> dt[, col3 := shift(col1, type="lag") > 2]
>
> On Mon, Oct 19, 2015 at 10:10 AM, jim holtman <jholtman at gmail.com> wrote:
>
>> does this do what you want:
>>
>> > dt <-data.table(col1=c(1,2,3))
>> > dt
>>    col1
>> 1:    1
>> 2:    2
>> 3:    3
>> > dt[2:nrow(dt), col3:=dt[1:(nrow(dt) - 1), list(col1)]>2]
>> > dt
>>    col1  col3
>> 1:    1    NA
>> 2:    2 FALSE
>> 3:    3 FALSE
>>
>>
>>
>> Jim Holtman
>> Data Munger Guru
>>
>> What is the problem that you are trying to solve?
>> Tell me what you want to do, not how you want to do it.
>>
>> On Mon, Oct 19, 2015 at 9:59 AM, Nicolas Paris <niparisco at gmail.com>
>> wrote:
>>
>>> Hello,
>>>
>>> I wonder if there is a way in data.table (or more generaly in R) to work
>>> on previous row without loops
>>> E.G. something equivalent to :
>>>
>>> dt <-data.table(col1=c(1,2,3))
>>> for (i in 2:nrow(dt))
>>> {
>>> dt[i,col3:=dt[i-1,list(col1)]>2]
>>> }
>>>
>>> Thanks a lot !
>>>
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>>
>>
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>
>
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