applying a function over a matrix 2N x M to produce a matrix N x M
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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