修改列表列中的数据框

Question

I have a dataframe with a list-column which itself contains dataframes (see below). Essentially, I am trying to add values from another column in the parent dataframe into the smaller dataframe by creating another column.

This is a simplified example- my real application is more complex.

library(tidyverse)
# What I am trying to do: add column "a" to dataframe within the list column
add_column(mtcars, a = 1)
#>                      mpg cyl  disp  hp drat    wt  qsec vs am gear carb a
#> Mazda RX4           21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4 1
#> Mazda RX4 Wag       21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4 1
#> Datsun 710          22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1 1
#> Hornet 4 Drive      21.4   6 258.0 110 3.08 3.215 19.44  1  0    3    1 1
#> Hornet Sportabout   18.7   8 360.0 175 3.15 3.440 17.02  0  0    3    2 1
#> Valiant             18.1   6 225.0 105 2.76 3.460 20.22  1  0    3    1 1
#> Duster 360          14.3   8 360.0 245 3.21 3.570 15.84  0  0    3    4 1
#> Merc 240D           24.4   4 146.7  62 3.69 3.190 20.00  1  0    4    2 1
#> Merc 230            22.8   4 140.8  95 3.92 3.150 22.90  1  0    4    2 1
#> Merc 280            19.2   6 167.6 123 3.92 3.440 18.30  1  0    4    4 1
#> Merc 280C           17.8   6 167.6 123 3.92 3.440 18.90  1  0    4    4 1
#> Merc 450SE          16.4   8 275.8 180 3.07 4.070 17.40  0  0    3    3 1
#> Merc 450SL          17.3   8 275.8 180 3.07 3.730 17.60  0  0    3    3 1
#> Merc 450SLC         15.2   8 275.8 180 3.07 3.780 18.00  0  0    3    3 1
#> Cadillac Fleetwood  10.4   8 472.0 205 2.93 5.250 17.98  0  0    3    4 1
#> Lincoln Continental 10.4   8 460.0 215 3.00 5.424 17.82  0  0    3    4 1
#> Chrysler Imperial   14.7   8 440.0 230 3.23 5.345 17.42  0  0    3    4 1
#> Fiat 128            32.4   4  78.7  66 4.08 2.200 19.47  1  1    4    1 1
#> Honda Civic         30.4   4  75.7  52 4.93 1.615 18.52  1  1    4    2 1
#> Toyota Corolla      33.9   4  71.1  65 4.22 1.835 19.90  1  1    4    1 1
#> Toyota Corona       21.5   4 120.1  97 3.70 2.465 20.01  1  0    3    1 1
#> Dodge Challenger    15.5   8 318.0 150 2.76 3.520 16.87  0  0    3    2 1
#> AMC Javelin         15.2   8 304.0 150 3.15 3.435 17.30  0  0    3    2 1
#> Camaro Z28          13.3   8 350.0 245 3.73 3.840 15.41  0  0    3    4 1
#> Pontiac Firebird    19.2   8 400.0 175 3.08 3.845 17.05  0  0    3    2 1
#> Fiat X1-9           27.3   4  79.0  66 4.08 1.935 18.90  1  1    4    1 1
#> Porsche 914-2       26.0   4 120.3  91 4.43 2.140 16.70  0  1    5    2 1
#> Lotus Europa        30.4   4  95.1 113 3.77 1.513 16.90  1  1    5    2 1
#> Ford Pantera L      15.8   8 351.0 264 4.22 3.170 14.50  0  1    5    4 1
#> Ferrari Dino        19.7   6 145.0 175 3.62 2.770 15.50  0  1    5    6 1
#> Maserati Bora       15.0   8 301.0 335 3.54 3.570 14.60  0  1    5    8 1
#> Volvo 142E          21.4   4 121.0 109 4.11 2.780 18.60  1  1    4    2 1

Then create list-column:

(df <- tibble(data = rep(list(mtcars), times = 3), a = 1:3))
#> # A tibble: 3 x 2
#>   data                       a
#>   <list>                 <int>
#> 1 <data.frame [32 x 11]>     1
#> 2 <data.frame [32 x 11]>     2
#> 3 <data.frame [32 x 11]>     3

But this doesn't work:

df %>% 
  rowwise() %>% 
  modify_at("data", ~ add_column(., a = a))
# Error in eval_tidy(xs[[i]], unique_output): object 'a' not found

Answer 1

We may use

df %>% mutate(data = data %>% map2(a, ~add_column(.x, a = .y)))

In this way we start by mutating a column as usual, but then recognising that it's a list we use map2 along with the a column.