[R] apply to row and column of matrix

[Eric Berger]

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 using 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 using 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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> > and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.