Stefano Stefano - 1 year ago 99
R Question

R - Concatenate string elements based on another array

I have a simple issue and I found a very complicated (C stylish) solution and I am wondering if there is something much simpler than this.
I have two arrays:

y <- c("A","B","C","D","E")
d <- c(1,3,1)

and I need to get an array as


so based on the distribution
, aggregate the elements of
separated by a
(or any other separator).

I did this

old_y <- y
y <- NULL

k <- 1
for (i in 1:length(d)) {
y[i] <- as.character(old_y[k])
j <- 1
while(d[i]>j) {
y[i] <- paste0(y[i], "-", as.character(old_y[k+j]))
j <- j + 1
k <- k + d[i]

Answer Source

We can use tapply on 'y' grouped by the sequence of 'd' replicated by 'd' and paste the elements of 'y' together.

unname(tapply(y, rep(seq_along(d), d), FUN = paste, collapse="-"))
#[1] "A"     "B-C-D" "E" 
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