Join data but ignore missing values - r

Question

I am having some trouble with joining data frames with dplyr, where I would like to ignore the NAs.
The data that I have is quite big, but a simplified version looks like:
id <- c("id1", "id2", "id3", "id4")
A <- c("E", "F", "G", NA)
B <- c("T", NA, "N", "T")
C <- c(NA, "T", "U", NA)
df <- data.frame(A, B, C)
id A B C
1 id1 E T NA
2 id2 F NA T
3 id3 G N U
4 id4 NA T NA
I have an entry that I would like to match with df, which is e.g.:
df2 <- data.frame(A = "E", B = "T", C = "M")
A B C
1 E T M
As a result I would like to obtain all rows from df that match with df2, but the NAs should be ignored. So the result should look like this:
id A B C
1 id1 E T NA
2 id4 NA T NA
I was trying to do this with semi_join, but it did not work so far:
result <- df %>%
group_by(n = seq(n())) %>%
do(modify_if(., is.na, ~NULL) %>%
semi_join(df2, by = c("A", "B", "C"))) %>%
ungroup %>%
select(-n)
Which results in:
Error: `by` can't contain join column `C` which is missing from LHS
Call `rlang::last_error()` to see a backtrace
Who knows the answer?

Answer 1

Here's a solution with a mix of tidyverse and base R. I think this is pretty clear, but I'd be interested in a pure tidyverse implementation that isn't completely contrived.
The idea is to first expand all entries in df and df2 and then filter through all the columns using a loop.
The data:
id <- c("id1", "id2", "id3", "id4")
A <- c("E", "F", "G", NA)
B <- c("T", NA, "N", "T")
C <- c(NA, "T", "U", NA)
df <- data.frame(id, A, B, C, stringsAsFactors = F) # Make sure to use strings not factors
df2 <- data.frame(A = "E", B = "T", C = "M", stringsAsFactors = F)
Code:
library(tidyr)
results <- crossing(df, df2)
select_columns <- c("A", "B", "C")
for(col in select_columns) {
keep <- is.na(results[[col]]) | results[[col]] == results[[paste0(col, 1)]]
results <- results[keep,, drop=F]
}
results <- results %>% dplyr::select(id, A:C) %>% distinct
results
id A B C
1 id1 E T <NA>
2 id4 <NA> T <NA>

Answer 2

If you only need to do this for a single set of values this is probably the most straightforward approach:
d[A %in% c("E",NA) & B %in%c("T",NA) & C %in% c("M",NA),]

Answer 3

Another example using tidyverse and base (dplyr, tidyr, base):
In this I convert your df2 into a dataframe that includes all combinations of values you want to accept ( (E or NA) & (T or NA) & (M or NA) ) and then I do an inner join with this full set. There are other ways to create a dataframe of all possible combinations but this one uses tidyr fairly easily.
library(dplyr)
library(tidyr)
id <- c("id1", "id2", "id3", "id4")
A <- c("E", "F", "G", NA)
B <- c("T", NA, "N", "T")
C <- c(NA, "T", "U", NA)
df <- data.frame(A, B, C, stringsAsFactors = FALSE)
df2 <- data.frame(A = "E", B = "T", C = "M",stringsAsFactors = FALSE)
df2_expanded <- df2 %>%
rowwise() %>%
mutate(combinations = list(expand.grid(A = c(A,NA),B = c(B,NA),C = c(C,NA),stringsAsFactors = FALSE))) %>%
select(-A,-B,-C) %>%
unnest(combinations)
# A tibble: 8 x 3
# A B C
# <chr> <chr> <chr>
# 1 E T M
# 2 NA T M
# 3 E NA M
# 4 NA NA M
# 5 E T NA
# 6 NA T NA
# 7 E NA NA
# 8 NA NA NA
df %>%
inner_join(df2_expanded)
# A B C
# 1 E T <NA>
# 2 <NA> T <NA>