how do I get the difference between two R named lists?

Question

OK, I've got two named lists, one is "expected" and one is "observed". They may be complex in structure, with arbitrary data types. I want to get a new list containing just those elements of the observed list that are different from what's in the expected list. Here's an example:

Lexp <- list(a=1, b="two", c=list(3, "four"))
Lobs <- list(a=1, c=list(3, "four"), b="ni")
Lwant <- list(b="ni")

Lwant is what I want the result to be. I tried this:

> setdiff(Lobs, Lexp)
[[1]]
[1] "ni"

Nope, that loses the name, and I don't think setdiff pays attention to the names. Order clearly doesn't matter here, and I don't want a=1 to match with b=1.

Not sure what a good approach is... Something that loops over a list of names(Lobs)? Sounds clumsy and non-R-like, although workable... Got any elegant ideas?

Solution

At least in this case

Lobs[!(Lobs %in% Lexp)]

gives you what you want.

Comments

[Marek]

You don't need plyr: Lobs[sapply(names(Lobs), function(x) !identical(Lobs[[x]], Lexp[[x]]))]

[JD Long]

I was just using the %in% function yesterday for debugging and was about the recommend that.

[Chetan Arvind Patil]

.@Harlan - Is it possible to find the percentage difference between two list having same structure? I am trying to compare how much the expected differ from the observed?