dplyr join by exclusion? - r

Question

When using the various join functions from dplyr you can either join all variables with the same name (by default) or specify those ones using by = c("a" = "b"). Is there a way to join by exclusion? For example, I have 1000 variables in two data frames and I want to join them by 999 of them, leaving one out. I don't want to do by = c("a1" = "b1", ...,"a999" = "b999"). Is there a way to join by excluding the one variable that is not used?
Ok, using this example from one answer:
set.seed(24)
df1 <- data_frame(alala= LETTERS[1:3], skks= letters[1:3], sskjs=
letters[1:3], val = rnorm(3))
df2 <- data_frame(alala= LETTERS[1:3], skks= letters[1:3], sskjs=
letters[1:3], val = rnorm(3))
I want to join them using all variables excluding val. I'm looking for a more general solution. Assuming there are 1000 variables and I only remember the name of the one that I want to exclude in the join, while not knowing the index of that variable. How can I perform the join while only knowing the variable names to exclude. I understand I can find the column index first but is there a simply way to add exclusions in by =?

Answer 1

We create a named vector to do this
library(dplyr)
grps <- setNames(paste0("b", 1:999), paste0("a", 1:999))
Note the 'grps' vector is created with paste as the OP's post suggested a pattern. If there is no pattern, but we know the column that is not to be grouped
nogroupColumn <- "someColumn"
grps <- setNames(setdiff(names(df1), nogroupColumn),
setdiff(names(df2), nogroupColumn))
inner_join(df1, df2, by = grps)
Using a reproducible example
set.seed(24)
df1 <- data_frame(a1 = LETTERS[1:3], a2 = letters[1:3], val = rnorm(3))
df2 <- data_frame(b1 = LETTERS[3:4], b2 = letters[3:4], valn = rnorm(2))
grps <- setNames(paste0("b", 1:2), paste0("a", 1:2))
inner_join(df1, df2, by = grps)
# A tibble: 1 x 4
# a1 a2 val valn
# <chr> <chr> <dbl> <dbl>
#1 C c 0.420 -0.584

Answer 2

To exclude a certain field(s), you need to identify the index of the columns you want. Here's one way:
which(!names(df1) %in% "sskjs" ) #<this excludes the column "sskjs"
[1] 1 2 4 #<and shows only the desired index columns
Use unite to create a join_id in each dataframe, and join by it.
df1 <- df1 %>%
unite(join_id, which(!names(.) %in% "sskjs"), remove = F)
df2 <- df2 %>%
unite(join_id, which(!names(.) %in% "sskjs"), remove = F)
left_join(df1, df2, by = "join_id" )