Add a column from one dataframe in a list to another dataframe in another list with map2

Question

I have two lists with different dataframes (original data has 70 dataframes for each list, totalising 2 million rows).

df1 <- data.frame(colA = LETTERS[1:5], colB = seq(1,5))
df2 <- data.frame(colA = LETTERS[6:10], colB = seq(6,10))
list1 <- list(df1,df2)
df3 <- data.frame(colA = LETTERS[1:5], colB = seq(11,15))
df4 <- data.frame(colA = LETTERS[6:10], colB = seq(16,20))
list2 <- list(df3,df4)

I want either to create a new list (eg, desired_list ) or to update the existing list1 , adding colB from each dataframe of list2 ( df3 and df4 ) naming the new column accordingly and re-naming the old ones. list1 and list2 have the same number of dataframes, each of them has the same number of rows ( df1 has the same number of rows of df3 , df2 of df4 , and so on). The desired output should looks like this.

desired_df1 <- data.frame(colA = LETTERS[1:5], colB_1 = seq(1,5), colB_2 = seq(11,15))
desired_df2 <- data.frame(colA = LETTERS[6:10], colB_1 = seq(6,10), colB_2 = seq(16,20))
desired_list <- list(desired_df1,desired_df2)

I think I can do this with purrr::map2 but I´am not very familiar with lists and I am having difficulties with map2 formatting and indexing. So far I tried:

desired_list <- lapply(list1, 
                       function(df) {purrr::map2(.x = list1, .y = list2, .f = list1$colB_2 <- list2$colB)})

But I get:

Error in `as_mapper()`:
! Can't convert `.f`, NULL, to a function.

Answer 1

Something like the following:

map2(list1, list2,
     ~data.frame(colA = .x$colA,
                 colB_1 = .x$colB,
                 colB_2 = .y$colB))

##> [[1]]
##>   colA colB_1 colB_2
##> 1    A      1     11
##> 2    B      2     12
##> 3    C      3     13
##> 4    D      4     14
##> 5    E      5     15
##> 
##> [[2]]
##>   colA colB_1 colB_2
##> 1    F      6     16
##> 2    G      7     17
##> 3    H      8     18
##> 4    I      9     19
##> 5    J     10     20

Answer 2

You can use plain old Map for this:

Map(function(a, b) { a$colB_2 <- b$colB; a }, list1, list2)
#> [[1]]
#>   colA colB colB_2
#> 1    A    1     11
#> 2    B    2     12
#> 3    C    3     13
#> 4    D    4     14
#> 5    E    5     15
#> 
#> [[2]]
#>   colA colB colB_2
#> 1    F    6     16
#> 2    G    7     17
#> 3    H    8     18
#> 4    I    9     19
#> 5    J   10     20

Answer 3

Or may also extract colB from list2 and transform the list1

 Map(transform, list1, colB_2 = lapply(list2, `[[`, "colB"))

-output

[[1]]
  colA colB colB_2
1    A    1     11
2    B    2     12
3    C    3     13
4    D    4     14
5    E    5     15

[[2]]
  colA colB colB_2
1    F    6     16
2    G    7     17
3    H    8     18
4    I    9     19
5    J   10     20