Keep columns with matching rows

In R, I have a dataframe with e.g. 4 variables:

df <- data.frame(
  v1=c(1,2,3,4),
  v2=c("x","y","z","q"),
  v3=c("x","b","c","d"),
  v4=c("a","y","c","d"),
  v5=c("a","b","z","d"),
  v6=c("a","b","c","q")
)

Suppose I use v2 as reference and I want to know what other columns match the values of v2.

How do I match the values from v3, v4, etc… to v2, in such a way that I know from which column the match came?
The result would look something like:

I tried match, %in%, and creating matrices of combinations, however I could not find the solution.

>Solution :

You could try:

library(tidyverse)

ref_col <- 'v2'

df %>%
  pivot_longer(-c('v1', ref_col)) %>%
  group_by(across(c('v1', ref_col))) %>%
  summarise(match = name[as.character(.data[[ref_col]]) == as.character(value)])

Output:

# A tibble: 4 x 3
# Groups:   v1 [4]
     v1 v2    match
  <dbl> <fct> <chr>
1     1 x     v3   
2     2 y     v4   
3     3 z     v5   
4     4 q     v6