Hi,
A quick thank you to both Gary and Sean and everyone! I will come back to
this once I have achieved a deeper understanding.
Apologies for the delayed response.
--
Kind regards
Stephen.
On 14 November 2017 at 09:59, Gary Verhaegen <gary.verhae...@gmail.com>
wrote:
> 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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