script.r

#!/usr/bin/env Rscript

df <- data.frame(
  "V" = c("A", "B"),
  "X" = c("A", "A"),
  "Y" = c("B","B"),
  "Z" = c("B", "C"),
  "X1" = c(2, 4),
  "Y2" = c(3,4),
  "Z3" = c(5, 5)
)

print(df, row.names = FALSE)

fun <- function (row, colnames) {
  # Loop over columns (except first column)
  for (colid in 2:length(colnames)) {
    # If value in current column == value in first column
    if (row[colid] == row[1]) {
      # Then get the name of the current column
      return(colnames[colid])
    }
  }
}

df2 <- apply(df, 1, fun, colnames = colnames(df))

print(df2, row.names = FALSE)