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

Here is another base R option

i1 <- c(TRUE, FALSE)
`row.names<-`(data.frame(A= unlist(df1[i1]), B = unlist(df1[!i1])), NULL)
#  A  B
#1 1  4
#2 2  5
#3 3  6
#4 7 10
#5 8 11
#6 9 12

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Or another option is melt from data.table

library(data.table)
i1 <- seq(1, ncol(df1), by = 2)
i2 <- seq(2, ncol(df1), by = 2)
melt(setDT(df1), measure = list(i1, i2), value.name = c("A", "B"))