How can I stack columns per x columns in R

Question

I\'m looking to transform a data frame of 660 columns into 3 columns just by stacking them on each other per 3 columns without manually re-arranging (since I have 660 column

Answer 1

A classical split-apply-combine approach will scale flexibly:

as.data.frame(lapply(split(unclass(df), 
                           names(df)[seq(ncol(df) / 2)]), 
                     unlist, use.names = FALSE))
##   A  B
## 1 1  4
## 2 2  5
## 3 3  6
## 4 7 10
## 5 8 11
## 6 9 12

or with a hint of purrr,

library(purrr)

df %>% unclass() %>%    # convert to non-data.frame list
    split(names(.)[seq(length(.) / 2)]) %>%     # split columns by indexed names
    map_df(simplify)    # simplify each split to vector, coerce back to data.frame
## # A tibble: 6 × 2
##       A     B
##    
## 1     1     4
## 2     2     5
## 3     3     6
## 4     7    10
## 5     8    11
## 6     9    12