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

reshape to the rescue:

reshape(df, direction="long", varying=split(names(df), rep(seq_len(ncol(df)/2), 2)))

#    time A  B id
#1.1    1 1  4  1
#2.1    1 2  5  2
#3.1    1 3  6  3
#1.2    2 7 10  1
#2.2    2 8 11  2
#3.2    2 9 12  3