[R] applying a function over a matrix 2N x M to produce a matrix N x M
David Winsemius
dwinsemius at comcast.net
Fri Apr 22 18:28:23 CEST 2011
On Apr 22, 2011, at 12:13 PM, Christine SINOQUET wrote:
> Hello,
>
> mat1 only consists of 0s and 1s:
> 0 0 1 0 0 0
> 1 1 0 1 1 0
> 1 1 1 0 1 0
> 0 1 1 0 0 1
> 1 0 0 1 0 0
> 0 1 0 1 0 1
>
> N = 3
> M = 6
>
> I would like to "compress" mat1 every two rows, applying summation
> over the two rows (per column), at each step, to yield:
>
> mat2
> 1 1 1 1 1 0
> 1 2 2 0 1 1
> 1 1 0 2 0 1
> mat[seq(1, nrow(mat), by=2), ]+mat[seq(2, nrow(mat), by=2), ]
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] 1 1 1 1 1 0
[2,] 1 2 2 0 1 1
[3,] 1 1 0 2 0 1
>
> Then, in mat2, I shall have to count the number of 0s, 1s and 2s,
> per column, which is my final aim.
>
> I am aware of possibilities such as
>
> counts <- sapply(mat2,2,fun1)
>
> but I do not know how to write fun1.
>
>
> Besides, it was perhaps not necessary to waste memory producing the
> temporary matrix mat2.
>
> Can somebody help ?
>
> I thank you in advance for your answer.
>
> Best regards,
>
> Christine Sinoquet
>
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David Winsemius, MD
West Hartford, CT
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