I know this is a recurring question (here, here, and more), and I know that the problem is related to creating lazy sequencies, but I can't see why it fails.
The problem: I had written a (not very nice) quicksort algorithm to sort strings that uses loop/recur. But applied to 10000 elements, I get a StackOverflowError:
(defn qsort [list]
(loop [[current & todo :as all] [list] sorted []]
(cond
(nil? current) sorted
(or (nil? (seq current)) (= (count current) 1)) (recur todo (concat sorted current))
:else (let [[pivot & rest] current
pred #(> (compare pivot %) 0)
lt (filter pred rest)
gte (remove pred rest)
work (list* lt [pivot] gte todo)]
(recur work sorted)))))
I used in this way:
(defn tlfnum [] (str/join (repeatedly 10 #(rand-int 10))))
(defn tlfbook [n] (repeatedly n #(tlfnum)))
(time (count (qsort (tlfbook 10000))))
And this is part of the stack trace:
[clojure.lang.LazySeq seq "LazySeq.java" 49]
[clojure.lang.RT seq "RT.java" 521]
[clojure.core$seq__4357 invokeStatic "core.clj" 137]
[clojure.core$concat$fn__4446 invoke "core.clj" 706]
[clojure.lang.LazySeq sval "LazySeq.java" 40]
[clojure.lang.LazySeq seq "LazySeq.java" 49]
[clojure.lang.RT seq "RT.java" 521]
[clojure.core$seq__4357 invokeStatic "core.clj" 137]]}
As far as I know, loop/recur performs tail call optimization, so no stack is used (is, in fact, an iterative process written using recursive syntax).
Reading other answers, and because of the stack trace, I see there's a problem with concat and adding a doall before concat solves the stack overflow problem. But... why?
Here's part of the code for the two-arity version of concat.
(defn concat [x y]
(lazy-seq
(let [s (seq x)]
,,,))
)
Notice that it uses two other functions, lazy-seq, and seq. lazy-seq is a bit like a lambda, it wraps some code without executing it yet. The code inside the lazy-seq block has to result in some kind of sequence value. When you call any sequence operation on the lazy-seq, then it will first evaluate the code ("realize" the lazy seq), and then perform the operation on the result.
(def lz (lazy-seq
(println "Realizing!")
'(1 2 3)))
(first lz)
;; prints "realizing"
;; => 1
Now try this:
(defn lazy-conj [xs x]
(lazy-seq
(println "Realizing" x)
(conj (seq xs) x)))
Notice that it's similar to concat, it calls seq on its first argument, and returns a lazy-seq
(def up-to-hundred
(reduce lazy-conj () (range 100)))
(first up-to-hundred)
;; prints "Realizing 99"
;; prints "Realizing 98"
;; prints "Realizing 97"
;; ...
;; => 99
Even though you asked for only the first element, it still ended up realizing the whole sequence. That's because realizing the outer "layer" results in calling seq on the next "layer", which realizes another lazy-seq, which again calls seq, etc. So it's a chain reaction that realizes everything, and each step consumes a stack frame.
(def up-to-ten-thousand
(reduce lazy-conj () (range 10000)))
(first up-to-ten-thousand)
;;=> java.lang.StackOverflowError
You get the same problem when stacking concat calls. That's why for instance (reduce concat ,,,) is always a smell, instead you can use (apply concat ,,,) or (into () cat ,,,).
Other lazy operators like filter and map can exhibit the exact same problem. If you really have a lot of transformation steps over a sequence consider using transducers instead.
;; without transducers: many intermediate lazy seqs and deep call stacks
(->> my-seq
(map foo)
(filter bar)
(map baz)
,,,)
;; with transducers: seq processed in a single pass
(sequence (comp
(map foo)
(filter bar)
(map baz))
my-seq)
Arne had a good answer (and, in fact, I'd never noticed cat before!). If you want a simpler solution, you can use the glue function from the Tupelo library:
Gluing Together Like Collections
The concat function can sometimes have rather surprising results:
(concat {:a 1} {:b 2} {:c 3} )
;=> ( [:a 1] [:b 2] [:c 3] )
In this example, the user probably meant to merge the 3 maps into one. Instead, the three maps were mysteriously converted into length-2 vectors, which were then nested inside another sequence.
The conj function can also surprise the user:
(conj [1 2] [3 4] )
;=> [1 2 [3 4] ]
Here the user probably wanted to get [1 2 3 4] back, but instead got a nested vector by mistake.
Instead of having to wonder if the items to be combined will be merged, nested, or converted into another data type, we provide the glue function to always combine like collections together into a result collection of the same type:
; Glue together like collections:
(is (= (glue [ 1 2] '(3 4) [ 5 6] ) [ 1 2 3 4 5 6 ] )) ; all sequential (vectors & lists)
(is (= (glue {:a 1} {:b 2} {:c 3} ) {:a 1 :c 3 :b 2} )) ; all maps
(is (= (glue #{1 2} #{3 4} #{6 5} ) #{ 1 2 6 5 3 4 } )) ; all sets
(is (= (glue "I" " like " \a " nap!" ) "I like a nap!" )) ; all text (strings & chars)
; If you want to convert to a sorted set or map, just put an empty one first:
(is (= (glue (sorted-map) {:a 1} {:b 2} {:c 3}) {:a 1 :b 2 :c 3} ))
(is (= (glue (sorted-set) #{1 2} #{3 4} #{6 5}) #{ 1 2 3 4 5 6 } ))
An Exception will be thrown if the collections to be 'glued' are not all of the same type. The allowable input types are:
all sequential: any mix of lists & vectors (vector result)
all maps (sorted or not)
all sets (sorted or not)
all text: any mix of strings & characters (string result)
I put glue into your code instead of concat and still got a StackOverflowError. So, I also replaced the lazy filter and remove with eager versions keep-if and drop-if to get this result:
(defn qsort [list]
(loop [[current & todo :as all] [list] sorted []]
(cond
(nil? current) sorted
(or (nil? (seq current)) (= (count current) 1))
(recur todo (glue sorted current))
:else (let [[pivot & rest] current
pred #(> (compare pivot %) 0)
lt (keep-if pred rest)
gte (drop-if pred rest)
work (list* lt [pivot] gte todo)]
(recur work sorted)))))
(defn tlfnum [] (str/join (repeatedly 10 #(rand-int 10))))
(defn tlfbook [n] (repeatedly n #(tlfnum)))
(def result
(time (count (qsort (tlfbook 10000)))))
-------------------------------------
Clojure 1.8.0 Java 1.8.0_111
-------------------------------------
"Elapsed time: 1377.321118 msecs"
result => 10000
Related
I want to convert {a 1, b 2} clojure.lang.PersistentArrayMap into
[a 1 b 2] clojure.lang.PersistentVector in clojure.
I have tried to write a function in clojure which converts {a 1, b 2} into [a 1 b 2]. I have also written a macro which gives me expected end result. In clojure we cannot pass the values generated inside functions to macros. For that I wanted to know a way in which I can implement a macro directly which can convert {a 1, b 2} into (let [a 1 b 2] (println a)), which will return 1.
Dummy Macro:
(defmacro mymacro [binding & body]
--some implemetation---)
Execution :
(mymacro '{a 1, b 2} (println a))
output:
1
nil
My Implementaion:
Function which converts into desired output.
(defn myfn [x]
(let [a (into (vector) x) b (vec (mapcat vec a))] b))
Execution:
(myfn '{a 1, b 2})
Output:
[a 1 b 2]
MACRO:
(defmacro list-let [bindings & body] `(let ~(vec bindings) ~#body))
Execution:
(list-let [a 1 b 2] (println a))
Output:
1
nil
I wanted to know how can I implement the same inside the macro itself and avoid the function implementation to get the require output. Something same as dummy macro given above. I am also interested in knowing if there is any which through which I can pass the value from my funtion to the macro without using
(def)
in general, macro code is plain clojure code (with the difference that it returns clojure code to be evaluated later). So, almost anything you can think of coding in clojure, you can do inside macro to the arguments passed.
for example, here is the thing you're trying to do (if i understood you correctly):
(defmacro map-let [binding-map & body]
(let [b-vec (reduce into [] binding-map)]
`(let ~b-vec ~#body)))
(map-let {a 10 b 20}
(println a b)
(+ a b))
;;=> 10 20
30
or like this:
(defmacro map-let [binding-map & body]
`(let ~(reduce into [] binding-map) ~#body))
or even like this:
(defmacro map-let [binding-map & body]
`(let [~#(apply concat binding-map)] ~#body))
You don't need a macro for this, and you should always prefer functions over macros when possible.
For your particular case, I have already written a function keyvals which you may find handy:
(keyvals m)
"For any map m, returns the keys & values of m as a vector,
suitable for reconstructing via (apply hash-map (keyvals m))."
(keyvals {:a 1 :b 2})
;=> [:b 2 :a 1]
(apply hash-map (keyvals {:a 1 :b 2}))
;=> {:b 2, :a 1}
And, here are the full API docs.
If you are curious about the implementation, it is very simple:
(s/defn keyvals :- [s/Any]
"For any map m, returns the (alternating) keys & values of m as a vector, suitable for reconstructing m via
(apply hash-map (keyvals m)). (keyvals {:a 1 :b 2} => [:a 1 :b 2] "
[m :- tsk/Map ]
(reduce into [] (seq m)))
Say that I have a function:
(defn get-token [char]
(defn char->number? []
(re-matches #"\d" (str char)))
(defn whitespace? []
(or
(= \space char)
(= \newline char)))
(defn valid-ident-char? []
(re-matches #"[a-zA-Z_$]" (str char)))
(cond
(whitespace?)
nil
(= \' char)
:quote
(= \) char)
:rparen
(= \( char)
:lparen
(char->number?)
:integer
(valid-ident-char?)
:identifier
:else
(throw (Exception. "invalid character"))))
When I run this function on, for instance, the string "(test 1 2)", I get a list of symbols for each character:
'(:lparen :identifier :identifier :identifier nil :integer nil :integer :rparen)
Seeing that this is not entirely what I want, I am trying to write a function that takes a collection and "condenses" the collection to combine adjacent elements that are equal.
A final example might do this:
(defn combine-adjacent [coll]
implementation...)
(->>
"(test 1 2)"
(map get-token)
(combine-adjacent)
(remove nil?))
; => (:lparen :identifier :integer :integer :rparen)
What is the idiomatic Clojure way to achieve this?
Clojure 1.7 will introduce a new function called dedupe to accomplish exactly this:
(dedupe [0 1 1 2 2 3 1 2 3])
;= (0 1 2 3 1 2 3)
If you're prepared to use 1.7.0-alpha2, you could use it today.
The implementation relies on transducers (dedupe produces a transducer when called with no arguments; the unary overload is defined simply as (sequence (dedupe) coll)), so it wouldn't be straightforward to backport.
Not sure how idiomatic it is but one way to do it would be to use partition-by to group elements of the incoming sequence into lists containing subsequences of the same element and then use map to get the first element from each of those lists.
So, in code
(defn combine-adjacent [input]
(->> input (partition-by identity) (map first)))
or
(defn combine-adjacent [input]
(->> (partition-by identity input) (map first))
should work.
There are a couple of tricks for comparing items to the one next door:
first, we can compare it to its tail:
(defn combine-adjacent
[s]
(mapcat #(when (not= % %2) [%]) (rest s) s))
alternatively, we can take the sequence by twos, and drop repeats
(defn combine-adjacent
[s]
(mapcat (fn [[a b]] (if (not= a b) [a]) ())
(partition 2 1[0 1 2 2 2 3 2 3])))
both of these take advantage of the helpful property of concat when combined with map that you can return zero or more elements for the result sequence for each input. The empty list for the false case in the second version is not needed, but may help with clarity.
The ClojureDocs page for lazy-seq gives an example of generating a lazy-seq of all positive numbers:
(defn positive-numbers
([] (positive-numbers 1))
([n] (cons n (lazy-seq (positive-numbers (inc n))))))
This lazy-seq can be evaluated for pretty large indexes without throwing a StackOverflowError (unlike the sieve example on the same page):
user=> (nth (positive-numbers) 99999999)
100000000
If only recur can be used to avoid consuming stack frames in a recursive function, how is it possible this lazy-seq example can seemingly call itself without overflowing the stack?
A lazy sequence has the rest of the sequence generating calculation in a thunk. It is not immediately called. As each element (or chunk of elements as the case may be) is requested, a call to the next thunk is made to retrieve the value(s). That thunk may create another thunk to represent the tail of the sequence if it continues. The magic is that (1) these special thunks implement the sequence interface and can transparently be used as such and (2) each thunk is only called once -- its value is cached -- so the realized portion is a sequence of values.
Here it is the general idea without the magic, just good ol' functions:
(defn my-thunk-seq
([] (my-thunk-seq 1))
([n] (list n #(my-thunk-seq (inc n)))))
(defn my-next [s] ((second s)))
(defn my-realize [s n]
(loop [a [], s s, n n]
(if (pos? n)
(recur (conj a (first s)) (my-next s) (dec n))
a)))
user=> (-> (my-thunk-seq) first)
1
user=> (-> (my-thunk-seq) my-next first)
2
user=> (my-realize (my-thunk-seq) 10)
[1 2 3 4 5 6 7 8 9 10]
user=> (count (my-realize (my-thunk-seq) 100000))
100000 ; Level stack consumption
The magic bits happen inside of clojure.lang.LazySeq defined in Java, but we can actually do the magic directly in Clojure (implementation that follows for example purposes), by implementing the interfaces on a type and using an atom to cache.
(deftype MyLazySeq [thunk-mem]
clojure.lang.Seqable
(seq [_]
(if (fn? #thunk-mem)
(swap! thunk-mem (fn [f] (seq (f)))))
#thunk-mem)
;Implementing ISeq is necessary because cons calls seq
;on anyone who does not, which would force realization.
clojure.lang.ISeq
(first [this] (first (seq this)))
(next [this] (next (seq this)))
(more [this] (rest (seq this)))
(cons [this x] (cons x (seq this))))
(defmacro my-lazy-seq [& body]
`(MyLazySeq. (atom (fn [] ~#body))))
Now this already works with take, etc., but as take calls lazy-seq we'll make a my-take that uses my-lazy-seq instead to eliminate any confusion.
(defn my-take
[n coll]
(my-lazy-seq
(when (pos? n)
(when-let [s (seq coll)]
(cons (first s) (my-take (dec n) (rest s)))))))
Now let's make a slow infinite sequence to test the caching behavior.
(defn slow-inc [n] (Thread/sleep 1000) (inc n))
(defn slow-pos-nums
([] (slow-pos-nums 1))
([n] (cons n (my-lazy-seq (slow-pos-nums (slow-inc n))))))
And the REPL test
user=> (def nums (slow-pos-nums))
#'user/nums
user=> (time (doall (my-take 10 nums)))
"Elapsed time: 9000.384616 msecs"
(1 2 3 4 5 6 7 8 9 10)
user=> (time (doall (my-take 10 nums)))
"Elapsed time: 0.043146 msecs"
(1 2 3 4 5 6 7 8 9 10)
Keep in mind that lazy-seq is a macro, and therefore does not evaluate its body when your positive-numbers function is called. In that sense, positive-numbers isn't truly recursive. It returns immediately, and the inner "recursive" call to positive-numbers doesn't happen until the seq is consumed.
user=> (source lazy-seq)
(defmacro lazy-seq
"Takes a body of expressions that returns an ISeq or nil, and yields
a Seqable object that will invoke the body only the first time seq
is called, and will cache the result and return it on all subsequent
seq calls. See also - realized?"
{:added "1.0"}
[& body]
(list 'new 'clojure.lang.LazySeq (list* '^{:once true} fn* [] body)))
I think the trick is that the producer function (positive-numbers) isn't getting called recursively, it doesn't accumulate stack frames as if it was called with basic recursion Little-Schemer style, because LazySeq is invoking it as needed for the individual entries in the sequence. Once a closure gets evaluated for an entry then it can be discarded. So stack frames from previous invocations of the function can get garbage-collected as the code churns through the sequence.
I want to reverse a sequence in Clojure without using the reverse function, and do so recursively.
Here is what I came up with:
(defn reverse-recursively [coll]
(loop [r (rest coll)
acc (conj () (first coll))]
(if (= (count r) 0)
acc
(recur (rest r) (conj acc (first r))))))
Sample output:
user> (reverse-recursively '(1 2 3 4 5 6))
(6 5 4 3 2 1)
user> (reverse-recursively [1 2 3 4 5 6])
(6 5 4 3 2 1)
user> (reverse-recursively {:a 1 :b 2 :c 3})
([:c 3] [:b 2] [:a 1])
Questions:
Is there a more concise way of doing this, i.e. without loop/recur?
Is there a way to do this without using an "accumulator" parameter in the loop?
References:
Whats the best way to recursively reverse a string in Java?
http://groups.google.com/group/clojure/browse_thread/thread/4e7a4bfb0d71a508?pli=1
You don't need to count. Just stop when the remaining sequence is empty.
You shouldn't pre-populate the acc, since the original input may be empty (and it's more code).
Destructuring is cool.
(defn reverse-recursively [coll]
(loop [[r & more :as all] (seq coll)
acc '()]
(if all
(recur more (cons r acc))
acc)))
As for loop/recur and the acc, you need some way of passing around the working reversed list. It's either loop, or add another param to the function (which is really what loop is doing anyway).
Or use a higher-order function:
user=> (reduce conj '() [1 2 3 4])
(4 3 2 1)
For the sake of exhaustivenes, there is one more method using into. Since into internally uses conj it can be used as follows :
(defn reverse-list
"Reverse the element of alist."
[lst]
(into '() lst))
Yes to question 1, this is what I came up with for my answer to the recursion koan (I couldn't tell you whether it was good clojure practice or not).
(defn recursive-reverse [coll]
(if (empty? coll)
[]
(conj (recursive-reverse (rest coll)) (first coll) )))
In current version of Clojure there's a built-in function called rseq. For anyone who passes by.
(defn my-rev [col]
(loop [ col col
result []]
(if (empty? col)
result
(recur (rest col) (cons (first col) result)))))
Q1.
The JVM can not optimize the recursion, a recursive function that would directly and stack overflow. Therefore, in Clojure, which uses the loop/recur. So, without using a function that recur deep recursion can not be defined. (which is also used internally to recur as a function trampoline.)
Q2.
a recursive function by recur, must be tail-recursive. If the normal recursive function change to tail-recursive function, so there is a need to carry about the value of a variable is required as the accumulator.
(defn reverse-seq [sss]
(if (not (empty? sss))
(conj (reverse-seq (rest sss)) (first sss))
)
)
(defn recursive-reverse [coll]
(if (empty? coll)
()
(concat (vector (peek coll)) (recursive-reverse (pop coll )))
)
)
and test:
user=> (recursive-reverse [1])
(1)
user=> (recursive-reverse [1 2 3 4 5])
(5 4 3 2 1)
This is my input data:
[[:a 1 2] [:a 3 4] [:a 5 6] [:b \a \b] [:b \c \d] [:b \e \f]]
I would like to map this into the following:
{:a [[1 2] [3 4] [5 6]] :b [[\a \b] [\c \d] [\e \f]]}
This is what I have so far:
(defn- build-annotation-map [annotation & m]
(let [gff (first annotation)
remaining (rest annotation)
seqname (first gff)
current {seqname [(nth gff 3) (nth gff 4)]}]
(if (not (seq remaining))
m
(let [new-m (merge-maps current m)]
(apply build-annotation-map remaining new-m)))))
(defn- merge-maps [m & ms]
(apply merge-with conj
(when (first ms)
(reduce conj ;this is to avoid [1 2 [3 4 ... etc.
(map (fn [k] {k []}) (keys m))))
m ms))
The above produces:
{:a [[1 2] [[3 4] [5 6]]] :b [[\a \b] [[\c \d] [\e \f]]]}
It seems clear to me that the problem is in merge-maps, specifically with the function passed to merge-with (conj), but after banging my head for a while now, I'm about ready for someone to help me out.
I'm new to lisp in general, and clojure in particular, so I also appreciate comments not specifically addressing the problem, but also style, brain-dead constructs on my part, etc. Thanks!
Solution (close enough, anyway):
(group-by first [[:a 1 2] [:a 3 4] [:a 5 6] [:b \a \b] [:b \c \d] [:b \e \f]])
=> {:a [[:a 1 2] [:a 3 4] [:a 5 6]], :b [[:b \a \b] [:b \c \d] [:b \e \f]]}
(defn build-annotations [coll]
(reduce (fn [m [k & vs]]
(assoc m k (conj (m k []) (vec vs))))
{} coll))
Concerning your code, the most significant problem is naming. Firstly, I wouldn't, especially without first understanding your code, have any idea what is meant by annotation, gff, and seqname. current is pretty ambiguous too. In Clojure, remaining would generally be called more, depending on the context, and whether a more specific name should be used.
Within your let statement, gff (first annotation)
remaining (rest annotation), I'd probably take advantage of destructuring, like this:
(let [[first & more] annotation] ...)
If you would rather use (rest annotation) then I'd suggest using next instead, as it will return nil if it's empty, and allow you to write (if-not remaining ...) rather than (if-not (seq remaining) ...).
user> (next [])
nil
user> (rest [])
()
In Clojure, unlike other lisps, the empty list is truthy.
This article shows the standard for idiomatic naming.
Works at least on the given data set.
(defn build-annotations [coll]
(reduce
(fn [result vec]
(let [key (first vec)
val (subvec vec 1)
old-val (get result key [])
conjoined-val (conj old-val val)]
(assoc
result
key
conjoined-val)))
{}
coll))
(build-annotations [[:a 1 2] [:a 3 4] [:a 5 6] [:b \a \b] [:b \c \d] [:b \e \f]])
I am sorry for not offering improvements on your code. I am just learning Clojure and it is easier to solve problems piece by piece instead of understanding a bigger piece of code and finding the problems in it.
Although I have no comments to your code yet, I tried it for my own and came up with this solution:
(defn build-annotations [coll]
(let [anmap (group-by first coll)]
(zipmap (keys anmap) (map #(vec (map (comp vec rest) %)) (vals anmap)))))
Here's my entry leveraging group-by, although several steps in here are really concerned with returning vectors rather than lists. If you drop that requirement, it gets a bit simpler:
(defn f [s]
(let [g (group-by first s)
k (keys g)
v (vals g)
cleaned-v (for [group v]
(into [] (map (comp #(into [] %) rest) group)))]
(zipmap k cleaned-v)))
Depending what you actually want, you might even be able to get by with just doing group-by.
(defn build-annotations [coll]
(apply merge-with concat
(map (fn [[k & vals]] {k [vals]})
coll))
So,
(map (fn [[k & vals]] {k [vals]})
coll))
takes a collection of [keys & values] and returns a list of {key [values]}
(apply merge-with concat ...list of maps...)
takes a list of maps, merges them together, and concats the values if a key already exists.