[datatable-help] Filtering Based on Previous Observation

[Arunkumar Srinivasan]

Perhaps do:

DT[, GRP := .GRP, by=a]

And then continue as Chinmay's earlier solution on the column GRP?

Arun

From: Michael Smith my.r.help at gmail.com
Reply: Michael Smith my.r.help at gmail.com
Date: April 30, 2014 at 11:00:21 AM
To: datatable-help at lists.r-forge.r-project.org datatable-help at lists.r-forge.r-project.org
Subject:  Re: [datatable-help] Filtering Based on Previous Observation  

... and here's a new challenge. What if `a` is character?  

DT <- data.table(a = as.character(c(1, 2, 2, 3)), b = 8:5)  
DT[, L.a := c(NA, head(a, -1))][a != L.a | is.na(L.a)][, L.a := NULL][]  

I still would like to compare equality with the previous  
observation/row, and remove it if it's the same. (`diff` doesn't work  
with characters.)  

M  



On 04/30/2014 04:55 PM, Michael Smith wrote:  
> Chinmay,  
>  
> Kudos, that's a nice one! It also can be generalized to longer lags.  
> Thanks!  
>  
> M  
>  
> On 04/30/2014 03:59 PM, Chinmay Patil wrote:  
>> You can try  
>>  
>> DT  
>> ## a b  
>> ## 1: 1 8  
>> ## 2: 2 7  
>> ## 3: 2 6  
>> ## 4: 3 5  
>>  
>> DT[c(T,diff(a)!=0),]  
>> ## a b  
>> ## 1: 1 8  
>> ## 2: 2 7  
>> ## 3: 3 5  
>>  
>>  
>>  
>>  
>> On Tue, Apr 29, 2014 at 10:04 PM, Michael Smith <my.r.help at gmail.com  
>> <mailto:my.r.help at gmail.com>> wrote:  
>>  
>> All,  
>>  
>> Is there some data.table-idiomatic way to filter based on a previous  
>> observation/row? For example, I want to remove a row if  
>> DT$a[row]==DT$a[row-1].  
>>  
>> It could be done by first calculating the lag and then filtering based  
>> on that, but I wonder if there's a more direct way.  
>>  
>> The following example works, but my feeling is there should be a more  
>> elegant solution:  
>>  
>> ( DT <- data.table(a = c(1, 2, 2, 3), b = 8:5) )  
>> DT[, L.a := c(NA, head(a, -1))][a != L.a | is.na  
>> <http://is.na>(L.a)][, L.a := NULL][]  
>>  
>> Thanks,  
>> M  
>> _______________________________________________  
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>> datatable-help at lists.r-forge.r-project.org  
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>>  
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