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

rbindlist from data.table package can also be used for the task and seems to be much more efficient.

# EXAMPLE DATA 
df1 <- read.table(text = '
Col1 Col2 Col3 Col4
1    2    3    4
5    6    7    8
1    2    3    4
5    6    7    8', header = TRUE)

library(data.table)
library(microbenchmark)
library(purrr)
microbenchmark(
    Map = as.data.frame(Map(c, df1[,1:2], df1[, 3:4])),
    Reshape = reshape(df1, direction="long", varying=split(names(df1), rep(seq_len(ncol(df1)/2), 2))),
    Purr = df1 %>% unclass() %>%    # convert to non-data.frame list
        split(names(.)[seq(length(.) / 2)]) %>%     # split columns by indexed names
        map_df(simplify), 
    DataTable = rbindlist(list(df1[,1:2], df1[, 3:4])),
    Mapply = data.frame(mapply(c, df1[,1:2], df1[, 3:4], SIMPLIFY=FALSE)),
    Rbind = rbind(df1[, 1:2],setnames(df1[, 3:4],names(df1[,1:2])))
)

The results are:

Unit: microseconds
expr     min         lq       mean         median     uq         max neval      cld
Map        214.724   232.9380  246.2936    244.1240   255.9240   343.611    100  bc  
Reshape    716.962   739.8940  778.7912    749.7550   767.6725   2834.192   100     e
Purr       309.559   324.6545  339.2973    334.0440   343.4290   551.746    100    d 
DataTable  98.228    111.6080  122.7753    119.2320   129.2640   189.614    100 a    
Mapply     233.577   258.2605  271.1881    270.7895   281.6305   339.291    100   c  
Rbind      206.001   221.1515  228.5956    226.6850   235.2670   283.957    100  b