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<mailto: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/clojure.core/sorted-map-by#example-542692d5c026201cdc327094
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