Removing items from a map based on the contents of another map

Question

Still working through Programming Collective Intelligence and using Clojure to write the code. I've got it working, but some parts are really ugly, so I thought I'd ask some of the experts around here to help clean it up.

Let's suppose I have a map that looks like this (bound to "recs"):

{"Superman Returns" 3.902419556891574, "Lady in the Water" 2.8325499182641614, 
"Snakes on a Plane" 3.7059737842895792, "The Night Listener" 3.3477895267131017,
"You, Me and Dupree" 2.651006036204627, "Just My Luck" 2.5309807037655645}

and I want to remove those items with keys that are also in the map (bound to "mymovies"):

{"Snakes on a Plane" 4.5, "You, Me and Dupree" 1.0, "Superman Returns" 4.0}

so that I get the map:

{"Lady in the Water" 2.8325499182641614, "The Night Listener" 3.3477895267131017,
"Just My Luck" 2.5309807037655645}

the code that I managed to get to do this looks like:

(apply merge (map #(hash-map (first %) (second %))
              (remove #(contains? mymovies (first %))
                      recs)))

That seems pretty ugly to me. It doesn't seem like it should be necessary to create a map from the value I get back from "remove". Is there a cleaner way to do this?

UPDATE: Joost's answer below sparked another idea. If I turn the keys of the two maps into sets I can use select-keys like this:

(select-keys recs (difference (set (keys recs))
                              (set (keys mymovies))))

Joost, thanks for turning me on to select-keys. I didn't know about that function before. Now to go rewrite several other sections with this new found knowledge!

Answer 1

(apply dissoc recs (keys mymovies))

Answer 2

The following first builds a seq of keys to keep, then extracts the "submap" for those keys from recs using select-keys. It also takes advantage of the fact that sets are predicates.

(select-keys recs (remove (apply hash-set (keys mymovies)) (keys recs)))

Answer 3

I think ponzao's answer is best for this case, but I wouldn't have thought to apply dissoc. Here are the two solutions I might have come up with: hopefully looking over them will help with similar future problems.

Note that the second solution will fail if your mymovies map contains nil or false values.

(into {}
      (for [[k v] recs
            :when (not (contains? mymovies k))]
        [k v]))

(into {}
      (remove (comp mymovies key) recs))