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

[Frank Erickson]

Yeah, I would use mapply pretty much the same way:

dt[, mapply(sub, pattern = shift(col1, type="lead", fill=""), replacement =
"", x = col1, USE.NAMES = FALSE)]

I expect it to be faster than a loop, since `dt[]` has some overhead that
you would incur for each row of a loop. Once you get used to mapply/Map, it
won't seem so hard to read, I think.

On Mon, Oct 19, 2015 at 11:28 AM, Nicolas Paris <niparisco at gmail.com> wrote:

> 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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>>
>>
>
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