R将最后一行附加到数据帧

Question

I have a data frame (df) that shares a key column ($Name) with a list of data frames:

head(df)
# A tibble: 6 x 3 ##truncating to show first 2 rows only
  Name      var1  var2
 <chr>      <chr> <chr>
1 Tom Marks LAX   ORD
2 Bob Sells MIA   CHI

I have a list of data frames that contains historical data for each person contained in df$Name.

head(employees$'Tom Marks')
Name      date       var3
Tom Marks 2017-01-01 250
Tom Marks 2017-01-02 457

head(employees$'Bob Sells')
Name      date       var3
Bob Sells 2017-01-01 385
Bob Sells 2017-01-02 273

I would like to append the value in var3 from employees list to the df by the most recent date (which is always the last row in an employees list). For example, the output, after matching Tom Marks from df$Name to employees$'Tom Marks' would look like this:

head(df)
  Name      var1  var2  var3
 <chr>      <chr> <chr> <num>
1 Tom Marks LAX   ORD   457
2 Bob Sells MIA   CHI   273

I have spent a decent amount of time researching filtering joins, mutating joins, bind_rows, reduce() functions but have been unsuccessful in accomplishing what is probably an easy task for a decent programmer. I'm hoping someone out there can put me out of my misery and provide some better direction or better yet, an answer!

Thank you!

Answer 1

If you're always after the last row, you can use tail to get it:

library(tidyverse)
left_join(
  df, 
  map_df(employees, ~ tail(.x, 1))  
)

Answer 2

This solution relies on the fact that your data arranged as you said they were, but you can easily arrange the list by date if they were not so.

library(tidyverse)
df %>% left_join(
  df_list$employees %>% 
    bind_rows() %>% 
    group_by(Name) %>% 
    summarise_at(vars(var3), last))

#        Name var1 var2 var3
# 1 Tom Marks  LAX  ORD  457
# 2 Bob Sells  MIA  CHI  273

---

Data

df <- data.frame(Name = c("Tom Marks", "Bob Sells"),
                 var1 = c("LAX", "MIA"),
                 var2 = c("ORD", "CHI"))

df_list <- list(employees = list(
  `Tom Marks` = data.frame(Name = "Tom Marks",
                           date = c("2017-01-01", "2017-01-02"),
                           var3 = c(250, 457)),
  `Bob Sells` = data.frame(Name = "Bob Sells",
                           date = c("2017-01-01", "2017-01-02"),
                           var3 = c(385, 273))
))