[datatable-help] Sum first 3 non zero elements of row

[Matthew Dowle]

Yes, think it was meant for r-help.
It can be fairly easy to mix up in Nabble since datatable-help is a 
sub-forum of R there.
But the notices are quite clear upon subscribing (which is required to 
post).

Matthew

On 21.05.2013 13:28, Gabor Grothendieck wrote:
> On Tue, May 21, 2013 at 5:10 AM, JNV <jose at memo2.nl> wrote:
>> Hi there,
>> I've got this matrix D with, say 10 rows and 20 columns. For each 
>> row I want
>> to sum the first 3 non zero elements and put them in a vector z.
>>
>> So if the first row D[1,] is
>> 0 3 5 0 8 9 3 2 4 0
>>
>> then I want z
>> z<-D[1,2]+D[1,3]+D[1,5]
>>
>> But if there are less than 3 non zero elements, those should be 
>> summed. If
>> there are no non zero elements, the result must be zero.
>>
>> So if the first row D[1,] is
>> 0 0 3 0 1 0 0 0 0 0
>>
>> then I want z
>> z<-D[1,3]+D[1,5]
>>
>
> Here is a matrix, D, with those two rows  The t(apply(...)) replaces
> the first non-zero element in each row with 1, the 2nd with 2, etc.
> (It puts garbage into the elements that are 0.)    We then convert
> this to T/F according to whether each element less than or equal to 3
> or not and multiply by the original data which both zaps the garbage
> in the zero positions and zaps those positions which are a 4th or 
> more
> non-zero in each row.  This multiplication also inserts the correct
> values into the good positions.  Finally we sum the rows using what 
> is
> left:
>
>> D <- matrix( c(0, 0, 3, 0, 5, 3, 0, 0, 8, 1, 9, 0, 3,
> + 0, 2, 0, 4, 0, 0, 0), 2)
>>
>> D
>      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
> [1,]    0    3    5    0    8    9    3    2    4     0
> [2,]    0    0    3    0    1    0    0    0    0     0
>>
>> as.data.table(D)[, rowSums((t(apply(.SD > 0, 1, cumsum)) <= 3) * 
>> .SD)]
> [1] 16  4
>
> Not sure if this really benefits from data.table as we could have
> written this without data.table:
>
>> rowSums((t(apply(D > 0, 1, cumsum)) <= 3) * D)
> [1] 16  4
>
>
> --
> Statistics & Software Consulting
> GKX Group, GKX Associates Inc.
> tel: 1-877-GKX-GROUP
> email: ggrothendieck at gmail.com
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