how to add up the column (cumulative sum) of the matrix in R?

Question

I have a question about adding up the column of the matrix for example:

I have a matrix
      [,1] [,2] [,3]
[1,]    1    3    1
[2,]    2    4    2

I want it to         


        

Answer 1

We can apply cumsum on each row by looping over the rows with apply and MARGIN specified as 1 and transpose the output

t(apply(m1, 1, cumsum))
#     [,1] [,2] [,3]
#[1,]    1    4    5
#[2,]    2    6    8

Or with a for loop

for(i in seq_len(ncol(m1))[-1]) m1[,i] <- m1[, i] + m1[, i-1]

Or another option is to split it a list of vectors with asplit and then Reduce with + and accumulate = TRUE

do.call(cbind, Reduce(`+`, asplit(m1, 2), accumulate = TRUE))
#     [,1] [,2] [,3]
#[1,]    1    4    5
#[2,]    2    6    8

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or with a convenient function rowCumsums from matrixStats

library(matrixStats)
rowCumsums(m1)
#      [,1] [,2] [,3]
#[1,]    1    4    5
#[2,]    2    6    8

data

m1 <- cbind(1:2, 3:4, 1:2)