[R] sum of some matrix columns

[David Winsemius]

On Aug 29, 2010, at 8:47 PM, Lorenzo Cattarino wrote:

> Hi,
>
>
>
> I have the following matrix
>
>
>
> cc <- matrix (1:21, 3)
>
> cc[,3:4]<- 0
>
> cc
>
>     [,1] [,2] [,3] [,4] [,5] [,6] [,7]
>
> [1,]    1    4    0    0   13   16   19
>
> [2,]    2    5    0    0   14   17   20
>
> [3,]    3    6    0    0   15   18   21
>
>
>
> and I would like to sum just the values in columns 2, 3 and 4, so to
> have something like
>
>
>
> 1  4  13  16  19
>
> 2  5  14  17  20
>
> 3  6  15  18  21

To take only selected columns:

 > CC <-cc[,c(1,2,5,6,7)]
 > CC
      [,1] [,2] [,3] [,4] [,5]
[1,]    1    4   13   16   19
[2,]    2    5   14   17   20
[3,]    3    6   15   18   21

I'm having difficulty understanding the point of summing columns which  
you have just set to zero so am taking the liberty of changing the  
task to summing columns 2-4 and leaving in column 2 of the shortened  
matrix:

 > CC[,2] <-apply(cc[ , 2:4], 1, sum)
 > CC
      [,1] [,2] [,3] [,4] [,5]
[1,]    1   21   13   16   19
[2,]    2   24   14   17   20
[3,]    3   27   15   18   21
>
>
>
> Thanks
>
>
>
> Lorenzo
>
>
> 	[[alternative HTML version deleted]]
>
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