vec_1 <- 1:5
vec_2 <- c('foo', 'bar', 'baz')
vec_3 <- '?'
s <- stack(list(col_1 = vec_1, col_2 = vec_2, col_3 = vec_3))
s
# values ind
# 1 1 col_1
# 2 2 col_1
# 3 3 col_1
# 4 4 col_1
# 5 5 col_1
# 6 foo col_2
# 7 bar col_2
# 8 baz col_2
# 9 ? col_3
u <- unstack(s)
u
# $col_1
# [1] "1" "2" "3" "4" "5"
#
# $col_2
# [1] "foo" "bar" "baz"
#
# $col_3
# [1] "?"