apply to row and column of matrix
Bert provided an excellent answer to your question. FYI here is a different approach to do the calculation. It uses data.frame rather than matrix. A data frame is a list of its columns. Here the function supplied to sapply operates on each column of the data.frame.
m <- as.data.frame(t(matrix(1:10,ncol=2))) m
V1 V2 V3 V4 V5 1 1 2 3 4 5 2 6 7 8 9 10
sapply(m, cumsum)
V1 V2 V3 V4 V5 [1,] 1 2 3 4 5 [2,] 7 9 11 13 15
On Mon, Sep 26, 2022 at 5:57 PM Bert Gunter <bgunter.4567 at gmail.com> wrote:
from ?apply: "If each call to FUN returns a vector of length n, and simplify is TRUE, then apply returns an array of dimension c(n, dim(X)[MARGIN]) ." For margin = 1 (cumsum over rows), each call to cumsum return a vector of length 2. Hence the array returned will be of dimension c(2, c(5,2)[1]) = c(2,5). Cheers, Bert On Mon, Sep 26, 2022 at 7:43 AM Jinsong Zhao <jszhao at yeah.net> wrote:
Hi there, I try to calculate the cumsum of row and column of a matrix as follows.
> m <- matrix(1:10, ncol = 2) > m
[,1] [,2] [1,] 1 6 [2,] 2 7 [3,] 3 8 [4,] 4 9 [5,] 5 10
> apply(m, 1, cumsum)
[,1] [,2] [,3] [,4] [,5] [1,] 1 2 3 4 5 [2,] 7 9 11 13 15
> apply(m, 2, cumsum)
[,1] [,2] [1,] 1 6 [2,] 3 13 [3,] 6 21 [4,] 10 30 [5,] 15 40 My question is why the dim of the return value of apply(m, 1, cumsum) is not 5x2, but 2x5. Best, Jinsong
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