If you're going to call that for multiple elements of the same vector, it's
worth thinking about doing some work upfront so that each look-up is faster:
(defn indices [vect]
(->> vect
(map-indexed vector)
(reduce (fn [acc [idx el]]
(update acc el (fnil conj []) idx))
{})))
(indices [1 "is" 1 :same]){1 [0 2], "is" [1], :same [3]}
would walk through the vector only once and create a map of value -> list
of indices.
On 14 November 2017 at 05:49, Sean Corfield <s...@corfield.org> wrote:
> I don’t think anyone addressed your question about finding all the indices
> in a vector where the element matches a given search value?
>
>
>
> There are a number of ways to tackle that (given it’s going to be a linear
> search). Since you want the indices, you are either going to need to track
> them directly or use map-indexed to produce them automatically.
>
>
>
> (defn indices [x l] ; produces a vector
>
> (loop [i 0 l l r []]
>
> (if (seq l)
>
> (recur (inc i) (rest l) (if (= x (first l)) (conj r i) r))
>
> r)))
>
>
>
> (defn indices [x l] ; produces a lazy sequence
>
> (keep identity (map-indexed (fn [i v] (when (= x v) i)) l)))
>
>
>
> (defn indices [x l] ; produces a vector
>
> (transduce (comp (map-indexed vector)
>
> (filter (comp (partial = x) second))
>
> (map first))
>
> conj
>
> []
>
> l))
>
>
>
> Hopefully that’ll give you some options to think about…
>
>
>
> Sean Corfield -- (970) FOR-SEAN -- (904) 302-SEAN
> An Architect's View -- http://corfield.org/
>
> "If you're not annoying somebody, you're not really alive."
> -- Margaret Atwood
>
>
> ------------------------------
> *From:* clojure@googlegroups.com <clojure@googlegroups.com> on behalf of
> Stephen Feyrer <stephen.fey...@gmail.com>
> *Sent:* Monday, November 13, 2017 5:19:32 PM
> *To:* clojure@googlegroups.com
> *Subject:* Re: Map Keywords are functions, why not vector elements?
>
> Hi Alex, Didier,
>
> Thanks for your patience.
>
> That covers everything which I can think of and a fair bit more :) I have
> a bit of reading and thinking to do now.
>
> Again, thank you.
>
>
> --
> Rule of thumb simple question, complicated answer
>
> Stephen.
>
> On 13 November 2017 at 22:09, Didier <didi...@gmail.com> wrote:
>
>> Yo are looking for indexOf (.indexOf vector value).
>>
>> (.indexOf ["a" "b"] "b")
>> => 1
>>
>> (.indexOf ["a" "b" "b"] "b")
>> => 1
>>
>> Note how indexOf searches for the index of the first value which matches
>> value.
>>
>> To do what you ask, is a query over a vector, which requires a search on
>> each element. This will take O(N) time. For a small list like in your
>> example its probably good enough and not an issue.
>>
>> If you want the name of the horse in a given position, that's a key
>> lookup, which is ~O(1) time. You can use get:
>>
>> (get ["a" "b"] 1)
>> => "b"
>>
>> If you really needed performance, you would need to combine a LinkedList
>> and a map. Some datastructures do it for you under the hood, like Apache
>> LinkedMap, amalloy ordered, java LinkedHashMap, etc.
>>
>> Its possible to also just use sorted-map-by in a closure. But this only
>> works if you're not going to add/update things to the datastructure after
>> first creation.
>>
>> See the example on clojuredocs: https://clojuredocs.org/clojur
>> e.core/sorted-map-by#example-542692d5c026201cdc327094
>>
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