print column name when specific row of a column contains string in R

Question

I have the following dataset

A    B    C    D
1!   0!   1!   0!
0!   .    1!   1!

I need to print the column name to each match as follows

matches_1     matches_0
A,C           B,D
C,D           A

I'm using:

df$matches_1 <- colSums(apply(df, 1, stringr::str_count, "1!"))

to count occurences of 1! in each row, but i'm not sure how to print the column name instead of the sum

Answer 1

Using == instead of stringr, then extracting the matching colnames, finally paste the result to get the desired output.

srch <- c("1!", "0!")

data.frame(
  sapply(srch, \(n) 
    apply(df == n, 1, \(x) paste(colnames(df)[x], collapse=","))), check.names=F)
   1!  0!
1 A,C B,D
2 C,D   A

Answer 2

We can write a customized function:

my_matches <- function(df, match){
  tmp <- data.frame(which(df == match, arr.ind = TRUE))
  tmp <- split(tmp, tmp$row)
  tmp <- sapply(1:length(tmp), \(x) paste0(names(df)[tmp[[x]][,2]], collapse = ","))
  return(tmp)
}

library(dplyr)
df %>% 
  mutate(matches_1 = my_matches(., "1!"),
         matches_0 = my_matches(., "0!"))

which produces:

   A  B  C  D matches_1 matches_0
1 1! 0! 1! 0!       A,C       B,D
2 0!  . 1! 1!       C,D         A