Re: How is this code to create a stateful lazy sequence?
My version of distinct-by just took clojure.core/distinct and added a keyfn
(and optional max-n). It's hard to claim this code is readable. I took this
approach mostly because I knew it would be unlikely to be buggy, since it
was a simple change or two from clojure.core/distinct
(defn distinct-by
"Like distinct, but calls keyfn to determine distinctness, much like
sort-by.
Takes an optional max-n, indicating how many duplicates are acceptible."
([keyfn coll]
(distinct-by keyfn 1 coll))
([keyfn max-n coll]
(let [step (fn step [xs seen]
(lazy-seq
((fn [[f :as xs] seen]
(when-let [s (seq xs)]
(if (>= (get seen (keyfn f) 0) max-n)
(recur (rest s) seen)
(cons f (step (rest s) (update-in seen [(keyfn
f)] (fnil inc 0)))
xs seen)))]
(step coll {}
On Sun, Jul 23, 2017 at 8:25 PM, Rob Nikander
wrote:
> Yes, I find that much clearer too. Thanks!
>
> On Saturday, July 22, 2017 at 4:50:23 PM UTC-4, tbc++ wrote:
>>
>> If we think about what we're doing here is a stateful filter, then maybe
>> we could leverage a few more core Clojure functions:
>>
>>
>> (defn distinct-by [f coll]
>> (let [seen (atom #{})]
>> (filter (fn [itm]
>>(let [m (f itm)]
>> (when-not (@seen m)
>> (swap! seen conj m
>> coll)))
>>
>> --
> You received this message because you are subscribed to the Google
> Groups "Clojure" group.
> To post to this group, send email to clojure@googlegroups.com
> Note that posts from new members are moderated - please be patient with
> your first post.
> To unsubscribe from this group, send email to
> clojure+unsubscr...@googlegroups.com
> For more options, visit this group at
> http://groups.google.com/group/clojure?hl=en
> ---
> You received this message because you are subscribed to the Google Groups
> "Clojure" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to clojure+unsubscr...@googlegroups.com.
> For more options, visit https://groups.google.com/d/optout.
>
--
You received this message because you are subscribed to the Google
Groups "Clojure" group.
To post to this group, send email to clojure@googlegroups.com
Note that posts from new members are moderated - please be patient with your
first post.
To unsubscribe from this group, send email to
clojure+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/clojure?hl=en
---
You received this message because you are subscribed to the Google Groups
"Clojure" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to clojure+unsubscr...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.
Re: How is this code to create a stateful lazy sequence?
Yes, I find that much clearer too. Thanks!
On Saturday, July 22, 2017 at 4:50:23 PM UTC-4, tbc++ wrote:
>
> If we think about what we're doing here is a stateful filter, then maybe
> we could leverage a few more core Clojure functions:
>
>
> (defn distinct-by [f coll]
> (let [seen (atom #{})]
> (filter (fn [itm]
>(let [m (f itm)]
> (when-not (@seen m)
> (swap! seen conj m
> coll)))
>
>
--
You received this message because you are subscribed to the Google
Groups "Clojure" group.
To post to this group, send email to clojure@googlegroups.com
Note that posts from new members are moderated - please be patient with your
first post.
To unsubscribe from this group, send email to
clojure+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/clojure?hl=en
---
You received this message because you are subscribed to the Google Groups
"Clojure" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to clojure+unsubscr...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.
Re: How is this code to create a stateful lazy sequence?
Perhaps you want to study the implementation in Medley, those are always very good quality: https://github.com/weavejester/medley/blob/254989ed3de83c30ce0101d66c7ce1b6ee257b4d/src/medley/core.cljc#L173 David -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
Re: How is this code to create a stateful lazy sequence?
If we think about what we're doing here is a stateful filter, then maybe we
could leverage a few more core Clojure functions:
(defn distinct-by [f coll]
(let [seen (atom #{})]
(filter (fn [itm]
(let [m (f itm)]
(when-not (@seen m)
(swap! seen conj m
coll)))
Now it's true, we're using a atom and a bit of mutability where none
existed before, but personally I think the intent is a bit clearer in this
code. What we're saying is "filter out all results we've seen before". If
this was production code I would probably replace the atom with a
`volatile!` but that's just for (small) performance reasons.
On Sat, Jul 22, 2017 at 7:36 AM, Rob Nikander
wrote:
> Hi,
>
> Here's a function and a simple use of it. It works, but the code may not
> be good Clojure. I'm wondering how it might be better.
>
> (defn distinct-by
> "Returns a sequence with no repeats of (f item).
> A set is built up internally as the sequence is consumed, so don't use
> it on
> an infinite sequence or you will run out of memory."
> ([f coll]
> (letfn [(get-more [seen input-seq]
>(if (not (empty? input-seq))
> (let [x (first input-seq)
>fx (f x)]
>(if (contains? seen fx)
> (lazy-seq (get-more seen (rest input-seq)))
> (lazy-seq (cons x (get-more (conj seen fx) (rest
> input-seq]
>(get-more #{} (seq coll)
>
> (def xs (list {:n 1, :a 'a} {:n 2, :a 'b} {:n 3, :a 'a} {:n 4, :a 'c} {:n
> 5, :a 'b}))
>
> (distinct-by :a xs)
> => ({:n 1, :a a} {:n 2, :a b} {:n 4, :a c})
>
> Rob
>
> --
> You received this message because you are subscribed to the Google
> Groups "Clojure" group.
> To post to this group, send email to clojure@googlegroups.com
> Note that posts from new members are moderated - please be patient with
> your first post.
> To unsubscribe from this group, send email to
> clojure+unsubscr...@googlegroups.com
> For more options, visit this group at
> http://groups.google.com/group/clojure?hl=en
> ---
> You received this message because you are subscribed to the Google Groups
> "Clojure" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to clojure+unsubscr...@googlegroups.com.
> For more options, visit https://groups.google.com/d/optout.
>
--
“One of the main causes of the fall of the Roman Empire was that–lacking
zero–they had no way to indicate successful termination of their C
programs.”
(Robert Firth)
--
You received this message because you are subscribed to the Google
Groups "Clojure" group.
To post to this group, send email to clojure@googlegroups.com
Note that posts from new members are moderated - please be patient with your
first post.
To unsubscribe from this group, send email to
clojure+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/clojure?hl=en
---
You received this message because you are subscribed to the Google Groups
"Clojure" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to clojure+unsubscr...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.
