dataframe - Stacking columns by pair in R

Question

id <- rep(c(1, 2, 3, 4), 3)
name <- rep(c("a", "b", "c", "d"), 3)
variable_a <- c(1:4, 9:12, 17:20)
variable_b <- c(5:8, 13:16, 21:24)

test1 <- data.frame(id, name, variable_a, variable_b)

question from:https://stackoverflow.com/questions/65865932/stacking-columns-by-pair-in-r

Answer 1

A base R option with reshape

reshape(
  setNames(test,gsub("(\d)(.)","\2.\1",names(test))),
  direction = "long",
  idvar = c("id","name"),
  varying = -(1:2)
)

gives

      id name time variable_a variable_b
1.a.1  1    a    1          1          5
2.b.1  2    b    1          2          6
3.c.1  3    c    1          3          7
4.d.1  4    d    1          4          8
1.a.2  1    a    2          9         13
2.b.2  2    b    2         10         14
3.c.2  3    c    2         11         15
4.d.2  4    d    2         12         16
1.a.3  1    a    3         17         21
2.b.3  2    b    3         18         22
3.c.3  3    c    3         19         23
4.d.3  4    d    3         20         24