I want to write something like this when expressed in python.
a = int(input())
for i in range(a):
b = input()
print(b)
And this is what I actually wrote.
(let [a][(read-line)]
(for[i (range [a])]
(defn b[string]
(= (read-line) b)
(println [b]))))
But now there are many errors.
Similar to the Python flow.
(doseq [_ (range (Integer. (read-line)))
:let [b (read-line)]]
(println b))
Even closer to Python code:
(let [a (Integer. (read-line))]
(doseq [i (range a)
:let [b (read-line)]]
(println b)))
More functional Code
(mapv println (repeatedly (Integer. (read-line)) read-line))
But more idiomatic (per Sean Corfield)
(run! println (repeatedly (Integer. (read-line)) read-line))
or this...
(repeatedly
(read-string (read-line))
(comp println read-line))
Off the top of my head, you could do something like:
(map (fn [_] (println (read-line))) (range (Integer/parseInt (read-line))))
There may be something more appropriate than a map here, read the clojure documentation. The clojure standard library has a lot of cool stuff :)
Edit: #SeanCorfield brought up a good point in the comments, using run! would be a better choice here since we don't care about the return value of map.
Related
I'm trying to dump the results of a core.async channel to stdout.
Here is what I have (simplified example):
(use 'clojure.core.async)
(def mychan (to-chan (range 100)))
(loop []
(let [a (<!! mychan)]
(if (not (nil? a))
(do
(println a)
(recur)))))
Now I think I should be able to replace this with map:
(map (fn [a] (println a) a) [mychan])
But this seems to be lazy and doesn't return any results.
I can't help but feel that my loop function is somewhat of a workaround. My question is - What is the optimal way to iterate over a core.async channel for printing?
Better use go-loop to run logging process in go block:
(def c (async/chan 10))
(go-loop []
(when-let [msg (<! c)]
(println msg)
(recur)))
(async/onto-chan c (range 100))
or you can use transducers on 1.7
(def prn-chan (async/chan 10 (map println)))
(async/onto-chan prn-chan (range 100))
I'm working through a book on clojure and ran into a stumbling block with "->>". The author provides an example of a comp that converts camelCased keywords into a clojure map with a more idiomatic camel-cased approach. Here's the code using comp:
(require '[clojure.string :as str])
(def camel->keyword (comp keyword
str/join
(partial interpose \-)
(partial map str/lower-case)
#(str/split % #"(?<=[a-z])(?=[A-Z])")))
This makes a lot of sense, but I don't really like using partial all over the place to handle a variable number of arguments. Instead, an alternative is provided here:
(defn camel->keyword
[s]
(->> (str/split s #"(?<=[a-z])(?=[A-Z])")
(map str/lower-case)
(interpose \-)
str/join
keyword))
This syntax is much more readable, and mimics the way I would think about solving a problem (front to back, instead of back to front). Extending the comp to complete the aforementioned goal...
(def camel-pairs->map (comp (partial apply hash-map)
(partial map-indexed (fn [i x]
(if (odd? i)
x
(camel->keyword x))))))
What would be the equivalent using ->>? I'm not exactly sure how to thread map-indexed (or any iterative function) using ->>. This is wrong:
(defn camel-pairs->map
[s]
(->> (map-indexed (fn [i x]
(if (odd? i)
x
(camel-keyword x)))
(apply hash-map)))
Three problems: missing a parenthesis, missing the > in the name of camel->keyword, and not "seeding" your ->> macro with the initial expression s.
(defn camel-pairs->map [s]
(->> s
(map-indexed
(fn [i x]
(if (odd? i)
x
(camel->keyword x))))
(apply hash-map)))
Is this really more clear than say?
(defn camel-pairs->map [s]
(into {}
(for [[k v] (partition 2 s)]
[(camel->keyword k) v])))
I have a situation where I am creating and destroying objects in one clojure namespace, and want another namespace to co-ordinate. However I do not want the first namespace to have to call the second explicitly on object destruction.
In Java, I could use a listener. Unfortunately the underlying java libraries do not signal events on object destruction. If I were in Emacs-Lisp, then I'd use hooks which do the trick.
Now, in clojure I am not so sure. I have found the Robert Hooke library https://github.com/technomancy/robert-hooke. But this is more like defadvice in elisp terms -- I am composing functions. More over the documentation says:
"Hooks are meant to extend functions you don't control; if you own the target function there are obviously better ways to change its behaviour."
Sadly, I am not finding it so obvious.
Another possibility would be to use add-watch, but this is marked as alpha.
Am I missing another obvious solution?
Example Added:
So First namespace....
(ns scratch-clj.first
(:require [scratch-clj.another]))
(def listf (ref ()))
(defn add-object []
(dosync
(ref-set listf (conj
#listf (Object.))))
(println listf))
(defn remove-object []
(scratch-clj.another/do-something-useful (first #listf))
(dosync
(ref-set listf (rest #listf)))
(println listf))
(add-object)
(remove-object)
Second namespace
(ns scratch-clj.another)
(defn do-something-useful [object]
(println "object removed is:" object))
The problem here is that scratch-clj.first has to require another and explicitly push removal events across. This is a bit clunky, but also doesn't work if I had "yet-another" namespace, which also wanted to listen.
Hence I thought of hooking the first function.
Is this solution suitable to your requirements?
scratch-clj.first:
(ns scratch-clj.first)
(def listf (atom []))
(def destroy-listeners (atom []))
(def add-listeners (atom []))
(defn add-destroy-listener [f]
(swap! destroy-listeners conj f))
(defn add-add-listener [f]
(swap! add-listeners conj f))
(defn add-object []
(let [o (Object.)]
(doseq [f #add-listeners] (f o))
(swap! listf conj o)
(println #listf)))
(defn remove-object []
(doseq [f #destroy-listeners] (f (first #listf)))
(swap! listf rest)
(println #listf))
Some listeners:
(ns scratch-clj.another
(:require [scratch-clj.first :as fst]))
(defn do-something-useful-on-remove [object]
(println "object removed is:" object))
(defn do-something-useful-on-add [object]
(println "object added is:" object))
Init binds:
(ns scratch-clj.testit
(require [scratch-clj.another :as another]
[scratch-clj.first :as fst]))
(defn add-listeners []
(fst/add-destroy-listener another/do-something-useful-on-remove)
(fst/add-add-listener another/do-something-useful-on-add))
(defn test-it []
(add-listeners)
(fst/add-object)
(fst/remove-object))
test:
(test-it)
=> object added is: #<Object java.lang.Object#c7aaef>
[#<Object java.lang.Object#c7aaef>]
object removed is: #<Object java.lang.Object#c7aaef>
()
It sounds a lot like what you're describing is callbacks.
Something like:
(defn make-object
[destructor-fn]
{:destructor destructor-fn :other-data "data"})
(defn destroy-object
[obj]
((:destructor obj) obj))
; somewhere at the calling code...
user> (defn my-callback [o] (pr [:destroying o]))
#'user/my-callback
user> (destroy-object (make-object my-callback))
[:destroying {:destructor #<user$my_callback user$my_callback#73b8cdd5>, :other-data "data"}]
nil
user>
So, here is my final solution following mobytes suggestion. A bit more work, but
I suspect that I will want this in future.
Thanks for all the help
;; hook system
(defn make-hook []
(atom []))
(defn add-hook [hook func]
(do
(when-not
(some #{func} #hook)
(swap! hook conj func))
#hook))
(defn remove-hook [hook func]
(swap! hook
(partial
remove #{func})))
(defn clear-hook [hook]
(reset! hook []))
(defn run-hook
([hook]
(doseq [func #hook] (func)))
([hook & rest]
(doseq [func #hook] (apply func rest))))
(defn phils-hook []
(println "Phils hook"))
(defn phils-hook2 []
(println "Phils hook2"))
(def test-hook (make-hook))
(add-hook test-hook phils-hook)
(add-hook test-hook phils-hook2)
(run-hook test-hook)
(remove-hook test-hook phils-hook)
(run-hook test-hook)
I'm trying to compute folder size in parallel.
Maybe it's naive approach.
What I do, is that I give computation of every branch node (directory) to an agent.
All leaf nodes have their file sizes added to my-size.
Well it doesn't work. :)
'scan' works ok, serially.
'pscan' prints only files from first level.
(def agents (atom []))
(def my-size (atom 0))
(def root-dir (clojure.java.io/file "/"))
(defn scan [listing]
(doseq [f listing]
(if (.isDirectory f)
(scan (.listFiles f))
(swap! my-size #(+ % (.length f))))))
(defn pscan [listing]
(doseq [f listing]
(if (.isDirectory f)
(let [a (agent (.listFiles f))]
(do (swap! agents #(conj % a))
(send-off a pscan)
(println (.getName f))))
(swap! my-size #(+ % (.length f))))))
Do you have any idea, what have i done wrong?
Thanks.
No need to keep state using atoms. Pure functional:
(defn psize [f]
(if (.isDirectory f)
(apply + (pmap psize (.listFiles f)))
(.length f)))
So counting filesizes in parallel should be so easy?
It's not :)
I tried to solve this issue better. I realized that i'm doing blocking I/O operations so pmap doesn't do the job.
I was thinking maybe giving chunks of directories (branches) to agents to process it independently would make sense. Looks it does :)
Well I haven't benchmarked it yet.
It works, but, there might be some problems with symbolic links on UNIX-like systems.
(def user-dir (clojure.java.io/file "/home/janko/projects/"))
(def root-dir (clojure.java.io/file "/"))
(def run? (atom true))
(def *max-queue-length* 1024)
(def *max-wait-time* 1000) ;; wait max 1 second then process anything left
(def *chunk-size* 64)
(def queue (java.util.concurrent.LinkedBlockingQueue. *max-queue-length* ))
(def agents (atom []))
(def size-total (atom 0))
(def a (agent []))
(defn branch-producer [node]
(if #run?
(doseq [f node]
(when (.isDirectory f)
(do (.put queue f)
(branch-producer (.listFiles f)))))))
(defn producer [node]
(future
(branch-producer node)))
(defn node-consumer [node]
(if (.isFile node)
(.length node)
0))
(defn chunk-length []
(min (.size queue) *chunk-size*))
(defn compute-sizes [a]
(doseq [i (map (fn [f] (.listFiles f)) a)]
(swap! size-total #(+ % (apply + (map node-consumer i))))))
(defn consumer []
(future
(while #run?
(when-let [size (if (zero? (chunk-length))
false
(chunk-length))] ;appropriate size of work
(binding [a (agent [])]
(dotimes [_ size] ;give us all directories to process
(when-let [item (.poll queue)]
(set! a (agent (conj #a item)))))
(swap! agents #(conj % a))
(send-off a compute-sizes))
(Thread/sleep *max-wait-time*)))))
You can start it by typing
(producer (list user-dir))
(consumer)
For result type
#size-total
You can stop it by (there are running futures - correct me if I'm wrong)
(swap! run? not)
If you find any errors/mistakes, you're welcome to share your ideas!
I have just start reading Let over lambda and I thought I would try and write a clojure version of the block-scanner in the closures chapter.
I have the following so far:
(defn block-scanner [trigger-string]
(let [curr (ref trigger-string) trig trigger-string]
(fn [data]
(doseq [c data]
(if (not (empty? #curr))
(dosync(ref-set curr
(if (= (first #curr) c)
(rest #curr)
trig)))))
(empty? #curr))))
(def sc (block-scanner "jihad"))
This works I think, but I would like know what I did right and what I could do better.
I would not use ref-set but alter because you don't reset the state to a completely new value, but update it to a new value which is obtained from the old one.
(defn block-scanner
[trigger-string]
(let [curr (ref trigger-string)
trig trigger-string]
(fn [data]
(doseq [c data]
(when (seq #curr)
(dosync
(alter curr
#(if (-> % first (= c))
(rest %)
trig)))))
(empty? #curr))))
Then it is not necessary to use refs since you don't have to coordinate changes. Here an atom is a better fit, since it can be changed without all the STM ceremony.
(defn block-scanner
[trigger-string]
(let [curr (atom trigger-string)
trig trigger-string]
(fn [data]
(doseq [c data]
(when (seq #curr)
(swap! curr
#(if (-> % first (= c))
(rest %)
trig))))
(empty? #curr))))
Next I would get rid of the imperative style.
it does more than it should: it traverses all data - even if we found a match already. We should stop early.
it is not thread-safe, since we access the atom multiple times - it might change in between. So we must touch the atom only once. (Although this is probably not interesting in this case, but it's good to make it a habit.)
it's ugly. We can do all the work functionally and just save the state, when we come to a result.
(defn block-scanner
[trigger-string]
(let [state (atom trigger-string)
advance (fn [trigger d]
(when trigger
(condp = d
(first trigger) (next trigger)
; This is maybe a bug in the book. The book code
; matches "foojihad", but not "jijihad".
(first trigger-string) (next trigger-string)
trigger-string)))
update (fn [trigger data]
(if-let [data (seq data)]
(when-let [trigger (advance trigger (first data))]
(recur trigger (rest data)))
trigger))]
(fn [data]
(nil? (swap! state update data)))))