[R] Sum efficiently from large matrix according to re-occuring levels of factor?

[Ralph S.]

The first and second column are actually indices of another matrix (my example may make this not sufficiently clear). I want to compare the sum with that corresponding entry, and then record the result of that.

Any idea?

Best,

Ralph



----------------------------------------
> Date: Sun, 20 Jul 2008 16:50:41 -0700
> From: h.wickham at gmail.com
> To: ruffel1 at hotmail.com
> Subject: Re: [R] Sum efficiently from large matrix according to re-occuring levels of factor?
> CC: r-help at r-project.org
> 
> On Sun, Jul 20, 2008 at 4:47 PM, hadley wickham  wrote:
>> On Sun, Jul 20, 2008 at 4:16 PM, Ralph S.  wrote:
>>>
>>> Hi,
>>>
>>> I am trying to calculate the sum for each occurrence of the level of a factor in a very large matrix. In addition, I want to save that sum together with the information of the level of the factor and the level of a second factor.
>>>
>>> My matrix looks like this:
>>>
>>> x<-matrix(c(1,1,1,2,2,3,3,1,1,7,7,7,4,4,2,2,7,7,1,1,1,1,1,1,2,5,5),9,3)
>>>
>>> I want to sum according to the levels in the first column and save the sum with the information of the level in the first and the second column in a new matrix.
>>>
>>> That is, I want output in the matrix of form:
>>>
>>> 1 7 3
>>> 2 4 2
>>> 3 2 3
>>> 1 7 10
>>>
>>
>> Why that and not:
>>
>> 1 7 13
>> 2 4 2
>> 3 2 3
>>
>> ?
> 
> Here's a solution for that case:
> 
> index <- x[, 2] + x[, 1] * max(x[, 2])
> cbind(x[!duplicated(index), 1:2], tapply(x[, 3], index, sum))
> 
> It takes about half a second for a million row matrix.
> 
> Hadley
> 
> 
> 
> -- 
> http://had.co.nz/

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