Try this:

rowSums(rowsum(t(m), rep(1:3, c(2, 2, 1)), na.rm = TRUE))


On Fri, Jan 7, 2011 at 2:29 PM, emj83 <stp08emj@shef.ac.uk> wrote:

>
> Hi,
>
> I would like to sum some specific columns in my matrix- for example, my
> matrix looks like this:
>     [,1]  [,2]  [,3] [,4]  [,5]
> [1,]    1   NA   NA   NA   NA
> [2,]    2    1   NA    1   NA
> [3,]    3    2    1     2    1
> [4,]    4    3    2     3    2
> [5,]   NA   NA   NA    4    3
> [6,]   NA   NA   NA    5   NA
>
> I would like to find the sum of the first two columns, the second two
> columns and the last column:
> i.e I am left with a vector of c(16, 18, 6).
>
> I know about colSums and sum overall- I just wondered if this type of
> grouping can be included somehow in a vector such as c(2,2,1)? I don't
> really want to have to use a loop for this.
>
> Many thanks Emma
> --
> View this message in context:
> http://r.789695.n4.nabble.com/Summing-over-specific-columns-in-a-matrix-tp3179400p3179400.html
> Sent from the R help mailing list archive at Nabble.com.
>
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> and provide commented, minimal, self-contained, reproducible code.
>



-- 
Henrique Dallazuanna
Curitiba-Paraná-Brasil
25° 25' 40" S 49° 16' 22" O

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