R - how to expand timestamps to each id in a dataframe?

Question

I want to expand a df with time column.

Here I have 2 df:

book1 <- data.frame(
  id = c('A1', 'A2', 'A3')
)
book2 <- data.frame(
  year = c('2000', '2010', '2020')  
)

> book1
  id
1 A1
2 A2
3 A3

> book2
  year
1 2000
2 2010
3 2020

What I expect is

  id year
1 A1 2000
2 A1 2010
3 A1 2020
4 A2 2000
5 A2 2010
6 A2 2020
7 A3 2000
8 A3 2010
9 A3 2020

Could anyone help me to do this? Thank you!

Answer 1

Use expand.grid:

df <- expand.grid(book1$id, book2$year)
df

Answer 2

You can use merge(). For desired ordering of the result, we use book2 as the first argument and then reverse the column order of the result.

rev(merge(book2, book1))
#   id year
# 1 A1 2000
# 2 A1 2010
# 3 A1 2020
# 4 A2 2000
# 5 A2 2010
# 6 A2 2020
# 7 A3 2000
# 8 A3 2010
# 9 A3 2020