如何在 tidyverse package 中使用左连接仅匹配两个 dfs 中具有多个不同变量的第一个？

Question

Say I have to 2 dfs

df_1 <- data_frame(dates = c(as.Date("2018-07-01"), as.Date("2018-06-01"), as.Date("2018-06-01"), as.Date("2018-06-01"), as.Date("2018-05-01")), x1 = c(10L, 11L, 21L, 21L, 13L), text1 = c("text a", "text b", "text c", "text d", "text e"))
df_2 <- data_frame(dates = c(as.Date("2018-07-01"), as.Date("2018-06-01"), as.Date("2018-05-01"), as.Date("2018-04-01")),x2 = c(10L, 21L, 22L, 23L),text2 = c("text aa", "text bb", "text cc", "text dd"))

I know I could use join function in plyr package to match the first with only one variable

plyr::join(df_2, df_1, type = 'left', match = 'first', by = 'dates')

But with 2 variables, "dates" and "x", it will throw error:

plyr::join(df_2, df_1, type = 'left', match = 'first', by = c('dates' = 'dates', 'x2' = 'x1'))

Also I could use left_join in dplyr with multiple variables

df_2 %>% 
  left_join(df_1, by = c('dates' = 'dates', 'x2' = 'x1'))

But no the first match argument. Any helps, thanks

Answer 1

You can't, not directly, multiple matching combinations are always returned using join operations with dplyr . If you want only the first match you could group by the joining variables and use slice() on the second table before the join.

df_2 %>% 
  left_join(df_1 %>%
              group_by(dates, x1) %>%
              slice(1), by = c('dates' = 'dates', 'x2' = 'x1'))

# A tibble: 4 x 4
  dates         x2 text2   text1 
  <date>     <int> <chr>   <chr> 
1 2018-07-01    10 text aa text a
2 2018-06-01    21 text bb text c
3 2018-05-01    22 text cc NA    
4 2018-04-01    23 text dd NA    

Answer 2

One (rather cheating) solution would be to rename the column that throws an error like so,

colnames(df_1)[2] = "x2" 

and then do

plyr::join(df_2, df_1, type = 'left', match = 'first', by = c("dates", "x2"))

which yields

       dates x2   text2  text1
1 2018-07-01 10 text aa text a
2 2018-06-01 21 text bb text c
3 2018-05-01 22 text cc   <NA>
4 2018-04-01 23 text dd   <NA>