r - Create unique identifier from the interchangeable combination of two variables

Question

I need to create a unique identifier from the combination of two variables in a data frame. Consider the following data frame:

 df <- data.frame(col1 = c("a", "a", "b", "c"), col2 = c("c", "b", "c", "a"), id = c(1,2,3,1))

The variable "id" is not in the data set; that's the one I would like to create. Essentially, I want every combination of the variables col1 and col2 to be treated interchangeably, e.g. the combination of c("a", "c") is the same as c("c", "a").

Answer 1

You could do:

labels <- apply(df[, c("col1", "col2")], 1, sort)
df$id <- as.numeric(factor(apply(labels, 2, function(x) paste(x, collapse=""))))