How did I not know about stack(), unstack(), and applying formula to a data.frame?

stack.R

tmp <- data.frame(a = c(1, 2, 3), b = c(4, 3, 5), c = c(4, 4, 5))
tmp
#   a b c
# 1 1 4 4
# 2 2 3 4
# 3 3 5 5

formula(tmp)
# a ~ b + c

library(dplyr)
formula(mtcars %>% add_rownames("model"))
# model ~ mpg + cyl + disp + hp + drat + wt + qsec + vs + am + gear + carb

tmp.stack <- stack(tmp)
tmp.stack
#   values ind
# 1      1   a
# 2      2   a
# 3      3   a
# 4      4   b
# 5      3   b
# 6      5   b
# 7      4   c
# 8      4   c
# 9      5   c

stack(tmp, select = -b)
#   values ind
# 1      1   a
# 2      2   a
# 3      3   a
# 4      4   c
# 5      4   c
# 6      5   c

unstack(tmp.stack)
#   a b c
# 1 1 4 4
# 2 2 3 4
# 3 3 5 5