Filter map collection by list of keys

[Jeff Sigmon]

Given the following collection as input

(def input ({:a "a", :b "b"} {:b "b", :c "c"} {:d "d", :e "e"}))

I want to be able to return a subset of the contained maps for each map that contains at least one key from a list of provided keys.

for example:

(filter-keys input [:a :b])

;; out -> ({:a "a", :b "b"} {:b "b", :c "c"}))

(filter-keys input [:e f])

;; out -> ({:d "d", :e "e"})

Here is how I implemented this:

VERSION 1

(defn filter-keys [coll kset]

  (letfn [(keys-found [m]

           (-> m keys set (clojure.set/intersection kset))) 

          (any-keys-found? [m]

            (< 0 (count (keys-found m))))]

    (filter any-keys-found? coll)))

VERSION 2

(defn filter-keys [coll keyseq]

  (letfn [(any-keys-found? [m]

            (< 0 (count (select-keys m keyseq))))]

    (filter any-keys-found? coll)))

VERSION 3 ??

Is there something in clojure core or contrib that would make this easier?

[Sean Corfield]

On Sun, Jun 12, 2011 at 5:01 PM, Jeff Sigmon <je...@jeffsigmon.com> wrote:
> VERSION 3 ??
> Is there something in clojure core or contrib that would make this easier?

How about:

(defn filter-keys [coll keys] (filter #(some % keys) coll))
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Sean A Corfield -- (904) 302-SEAN
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[Jeff Sigmon]

Wonderful! Pretty cool how the map itself is the predicate to the some function. 

Thanks Sean!

[Meikel Brandmeyer]

Hi,

Am 13.06.2011 um 04:36 schrieb Jeff Sigmon:

> Wonderful! Pretty cool how the map itself is the predicate to the some function.

The usual disclaimer about keys with false or nil as value applies.

Sincerely
Meikel

[Sean Corfield]

On Sun, Jun 12, 2011 at 7:36 PM, Jeff Sigmon <je...@jeffsigmon.com> wrote:
> Wonderful! Pretty cool how the map itself is the predicate to the some
> function.

Yes, a map is a function from its keys to its to values, just as a
keyword is also a function (from a map to a value identified by that
keyword).

And, as Meikel notes, if you have a value of false or nil in your map,
my "shortcut" won't work.

If you're on Clojure 1.3.0, a solution that addresses Meikel's concern would be:

(defn filter-keys [coll keys] (filter (apply some-fn (map #(fn [c]
(find c %)) keys)) coll))

Maybe someone can clean that up?

[Meikel Brandmeyer]

Hi,

Am 13.06.2011 um 08:00 schrieb Sean Corfield:

> (defn filter-keys [coll keys] (filter (apply some-fn (map #(fn [c]
> (find c %)) keys)) coll))
>
> Maybe someone can clean that up?

Just use contains? in your original solution.

(defn filter-keys
[ks coll]
(filter #(some (partial contains? %) ks) coll))

Not as nice as before, but it should work.

Sincerely
Meikel

[Sean Corfield]

On Mon, Jun 13, 2011 at 12:02 AM, Meikel Brandmeyer <m...@kotka.de> wrote:
> Just use contains? in your original solution.
>
> (defn filter-keys
>  [ks coll]
>  (filter #(some (partial contains? %) ks) coll))

Needs to be (defn filter-keys [coll ks] ...) but, yes, that is cleaner
than mine. Nice.

So many functions...

[Meikel Brandmeyer]

Hi,

Am 13.06.2011 um 09:19 schrieb Sean Corfield:

>> (defn filter-keys
>> [ks coll]
>> (filter #(some (partial contains? %) ks) coll))
>
> Needs to be (defn filter-keys [coll ks] ...)

I disagree. Clojure puts sequence arguments usually last. See filter, remove, take-while, ... I would exchange the arguments and follow the usual Clojure style.

> So many functions...

Hehe. :) That's why it's called “functional.” ;)

Sincerely
Meikel

[Sean Corfield]

On Mon, Jun 13, 2011 at 2:36 AM, Meikel Brandmeyer <m...@kotka.de> wrote:
>> Needs to be (defn filter-keys [coll ks] ...)
> I disagree. Clojure puts sequence arguments usually last. See filter, remove, take-while, ... I would exchange the arguments and follow the usual Clojure style.

The OP specifically asked for (filter-keys input [:a :b]) - I was
merely pointing out your solution had the arguments the wrong way
round _for his question_.

I do agree that the usual practice is to have the collection last and
actually kept doing that while I was trying to solve his puzzle - and
then I'd run his example and it wouldn't work, until I swapped the
arguments :)

[Jonathan Fischer Friberg]

I find that functions that extract or update data from some collection often has the collection first.
nth, assoc, contains?, get-in, assoc-in, update-in ...

Jonathan

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