How to replace certain data frame value with it's unknown column name?

Question

I have a large data frame with unknown column names and numeric values 1, 2, 3, or 4. Now I want to replace all 4 values with it's column name and all 1, 2 and 3's with an empty value.

Ofcourse I can make a loop of some kind, like this:

df <- data.frame(id=1:8,unknownvarname1=c(1:4,1:4),unknownvarname2=c(4:1,4:1))
for (i in 2:length(df)){
  df[,i] <- as.character(df[,i])
  df[,i] <- mgsub::mgsub(df[,i],c(1,2,3,4),c("","","",names(df)[i]))  
}

This would be the result:

  id unknownvarname1 unknownvarname2
1  1                 unknownvarname2
2  2                                
3  3                                
4  4 unknownvarname1                
5  5                 unknownvarname2
6  6                                
7  7                                
8  8 unknownvarname1 unknownvarname2

For a data frame this size that's no problem at all. But when I try this loop on large data frames with up to 30k and up to 40 uknown variables, the loop takes ages to complete.

Does anyone know of a faster way to do this? I tried functions like mutate() of dplyr package but I could not manage to make it work.

Many thanks in advance!

Answer 1

One way using base R

#Replace all the values with 1:3 with blank
df[-1][sapply(df[-1], `%in%`, 1:3)] <- ""
#Get the row/column indices where value is 4
mat <- which(df == 4, arr.ind = TRUE)
#Exclude values from first column
mat <- mat[mat[, 2] != 1, ]
#Replace remaining entries with it's corresponding column names
df[mat] <- names(df)[mat[, 2]]
df

#  id unknownvarname1 unknownvarname2
#1  1                 unknownvarname2
#2  2                                
#3  3                                
#4  4 unknownvarname1                
#5  5                 unknownvarname2
#6  6                                
#7  7                                
#8  8 unknownvarname1