Rowwise cumulative sum

Question

I have a data.table dt as follows.

df <- data.frame(t1 = rep(0,5), t3 = c(12, 5, 8,9, 5), t7= c(25, 48, 7, 9, 14))
dt <- setD         


        

Answer 1

If we need to do this by row, then one option is group by row, unlist, get the cumsum, convert to list and assign it to the columns

dt[, (1:3) := as.list(cumsum(unlist(.SD))), 1:nrow(dt)]
dt
#    t1 t3 t7
#1:  0 12 37
#2:  0  5 53
#3:  0  8 15
#4:  0  9 18
#5:  0  5 19

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Or another option is rowCumsums from matrixStats, which can be applied to a matrix

library(matrixStats)
dt[, (1:3) := as.data.table(rowCumsums(as.matrix(.SD)))]

Answer 2

Another option use Reduce with accumulate=TRUE:

dt[, names(dt) := Reduce(`+`, dt, accumulate = TRUE)]

dt
#   t1 t3 t7
#1:  0 12 37
#2:  0  5 53
#3:  0  8 15
#4:  0  9 18
#5:  0  5 19