Is there a way to mimic a this variable in something like (def foo {:two 2 :three (inc (:two this))})? Even better would be something like (def foo {:two 2 :three (inc ::two)}). I was told that there is a library that does exactly this, but I can't really find anything similar.
Thanks!
If you want a temporary name for something, that's what let is for.
(def foo (let [x {:two 2}]
(assoc x :three (inc (:two x)))))
I don't know of any library that does what you want. Every once in a while, someone suggests a "generalized arrow", like -> but with a magic symbol you can stick in the intermediary expressions which will be replaced by something else. See for example here and here. But this idea tends to be shot down because it's more complex and confusing for little benefit. let is your friend. See Rich's example:
(let [x []
x (conj x 1)
x (into x [2 3])
x (map inc x)]
...)
(Update: Rearranged & reworked. build-map and (a sketch of) -m> macros added.)
You could write this particular example as
(def foo (zipmap [:two :three] (iterate inc 2)))
The easiest general solution which occurs to me at this moment is
user> (-> {} (assoc :two 2) (#(assoc % :three (inc (:two %)))))
{:three 3, :two 2}
It's actually very flexible, although it does require you to write out assoc repeatedly.
To enable syntax similar to that from the question text, you could use something like this:
(defn build-map* [& kvs]
(reduce (fn [m [k v]]
(assoc m k (v m)))
{}
kvs))
(defmacro build-map [& raw-kvs]
(assert (even? (count raw-kvs)))
(let [kvs (map (fn [[k v]] [k `(fn [m#] (let [~'this m#] ~v))])
(partition 2 raw-kvs))]
`(build-map* ~#kvs)))
user> (build-map :two 2 :three (inc (:two this)))
{:three 3, :two 2}
You could easily change this to use a user-supplied symbol rather than the hardcoded this. Or you could switch to %, which is just a regular symbol outside anonymous function literals. Maybe add an explicit initial map argument, call it -m> (for map threading) and you can do
(-m> {} :two 2 :three (inc (:two %)))
for the same result.
Another funky way (mostly for the fun):
;;; from Alex Osborne's debug-repl,
;;; see http://gist.github.com/252421
;;; now changed to use &env
(defmacro local-bindings
"Produces a map of the names of local bindings to their values."
[]
(let [symbols (map key &env)]
(zipmap (map (fn [sym] `(quote ~sym)) symbols) symbols)))
(let [two 2
three (inc two)]
(into {} (map (fn [[k v]] [(keyword k) v]) (local-bindings))))
{:two 2, :three 3}
Note that this will also capture the bindings introduced by any outer let forms...
Related
Is there a convenient way in ClojureScript to pretty print a nested hash-map in the way that the whole tree-structure becomes immediately visible.
For instance a map like this
(def my-map {:a {:b 1 :c 9} :b {:d 8 :e {:f 2 :g 3 :h 4}} :c 10})
should be printed like this:
{:a {:b 1
:c 9}
:b {:d 8
:e {:f 2
:g 3
:h 4}}
:c 10}
EDIT: There might also be vectors in the map. The usecase is just to inspect larger data structures during development.
There is no built-in way to do it. You might come close to what you want by using cljs.pprint and setting cljs.pprint/*print-right-margin* to a low value.
I would recommend to take a look at a small library shodan which provides a very useful inspect function:
(require '[shodan.inspection :refer [inspect]])
(inspect {:aaaaaa 1
:bbbbbb {:ccc 2
:dddddd [1 2 3 4 5]}})
It won't print anything in your CLJS REPL but will provide a handy view in your browser's console:
You can collapse and expand nested datastructures - it basically does what you asked for.
As a personal challenge I wrote the following code:
(enable-console-print!)
(def atomic? (complement coll?))
(def padding #(apply str (repeat % " ")))
(def tabulate #(apply str (repeat % "\t")))
(def strcat #(->> (apply concat %&) (apply str)))
(defn my-max-key [x] (if (empty? x) [""] (apply (partial max-key count) x)))
(defn longest-key [m] (->> m keys (filter atomic?) (map str) my-max-key))
(def length (comp count str))
(def not-map? (complement map?))
(def nested? #(some coll? %))
(def join #(apply str (interpose % %2)))
(def join-lines (partial join "\n"))
(defn has-atomic? [coll] (some atomic? coll))
(defn diff-key-lengths [key1 key2] (- (length key1) (length key2)))
(defn convert
([thing] (convert -1 thing))
([depth thing]
(defn convert-items []
(defn convert-seq []
(conj []
(map (partial convert (inc depth)) thing)
""))
(defn string-horizontally [[key value]]
(str (tabulate (inc depth))
key
(padding (diff-key-lengths (longest-key thing) key))
" → "
value))
(defn string-vertically [[key value]]
(str (convert (inc depth) key) "\n"
(convert (+ 2 depth) "↓") "\n"
(convert (inc depth) value) "\n"))
(defn convert-kv [[key value]]
(if (nested? [key value])
(string-vertically [key value])
(string-horizontally [key value])))
(cond (atomic? thing)
[(str (tabulate depth) thing)]
(not-map? thing)
(convert-seq)
(map? thing)
(map convert-kv thing)))
(->> (convert-items) flatten join-lines)))
(def sample-input [["the first thing in this nested vector"]
{{"this is a key in a nested map"
"that points to me!!!"}
{"and that entire map points to this map!!!"
"cool!!!"
"but it gets cooler cause..."
"the value's line up!!!"}}])
(->> sample-input convert println)
The terminal output is (psst... the values in a map do line up but I don't think that chrome uses a monospaced font!):
Given a collection"
[{:key "key_1" :value "value_1"}, {:key "key_2" :value "value_2"}]
I would like to convert this to:
{"key_1" "value_1" "key_2" "value_2"}
An function to do this would be:
(defn long->wide [xs]
(apply hash-map (flatten (map vals xs))))
I might simplify this using the threading macro:
(defn long->wide [xs]
(->> xs
(map vals)
(flatten)
(apply hash-map)))
This still requires explicit definition of the function argument which I am not doing anything with other than passing to the first function. I might then rewrite this using comp to remove this:
(def long->wide
(comp (partial apply hash-map) flatten (partial map vals)))
This however requires repeated use of partial which to me is a lot of noise in the function.
Is there a some function in clojure that combines comp and ->> so I can create a higher order function without repeated use of partial, and also which out having to create a new function?
Since many of the answers here already don't answer the original question, but
suggest different approaches, I put that one back up too.
I'd go with reduce and destructuring:
(reduce
(fn [m {:keys [key value]}]
(assoc m key value))
{}
[{:key "key_1" :value "value_1"}, {:key "key_2" :value "value_2"}])
Note, that this will also work with string keys (which you mentioned in the comments) (note :strs):
(reduce
(fn [m {:strs [key value]}]
(assoc m key value))
{}
[{"key" "key_1" "value" "value_1"}, {"key" "key_2" "value" "value_2"}])
Another (point-free) version, when using keywords:
(partial (into {} (map (juxt :key :value))))
Since you mentioned in the comments, that you are using values from a DB, there might also be the chance, that you can switch to just return value tuples. Then the whole operation is just:
(into {} [["key_1" "value_1"]["key_2" "value_2"]])
Also note, that the use of vals on a map and expecting "insertion order" is
dangerous. Small maps are ordered only by accident:
user=> (take 3 (zipmap (range 3) (range 3)))
([0 0] [1 1] [2 2])
user=> (take 3 (zipmap (range 100) (range 100)))
([0 0] [65 65] [70 70])
An other alternative to the nice answers is also:
(apply hash-map (mapcat vals [{:key "key_1" :value "value_1"}, {:key "key_2" :value "value_2"}]))
or:
((comp #(apply hash-map %) #(mapcat vals %)) [{:key "key_1" :value "value_1"}, {:key "key_2" :value "value_2"}])
which are exactly the same.
As with clojure, so many ways to solve most problems.
(partial #(reduce (fn [r m] (assoc r (m :key) (m :value)))
{}
%)))
Not sure if the creation of anonymous functions violates your condition or not but this isn't adding functions to the namespace so I thought I'd throw it out there. This also has the benefit of not requiring the keys in the input maps to be keywords as :key and :value can be replaced with values of any type since the map is in the function position. For example:
(partial #(reduce (fn [r m] (assoc r (m "key") (m "value")))
{}
%)))
I want to be able to loop through a collection but maintain a counter with that loop. I managed to create e.g.
(for [[email i] (map vector emails (range))]
...)
to do this, but ideally I'd prefer a fori macro (unless a function is possible but my gut says it's gotta be a macro) that lets me do equivalently
(fori [email i emails]
...)
or
(fori [[email i] emails]
...)
I've no experience with macros, and looking at the source for the for macro it looks pretty intimidating. Any help?
Super-cool would be a macro that follows the for syntax and allows e.g.
(fori [email i emails
line j (lines email)]
...)
You can write a macro that expands to for, but I'm not really convinced there's much point. There are numerous built-ins for exactly this purpose, and the fori syntax doesn't really read very well. At the very least it should look like
(fori [[email i] emails,
[line j] (lines email)]
...)
But if you really wanted to write it exactly as you said, it's certainly not impossible:
(defmacro fori [bindings body]
(letfn [(clauses [bindings]
(lazy-seq
(cond (empty? bindings) ()
(keyword? (first bindings)) (concat (take 2 bindings)
(clauses (drop 2 bindings)))
:else (let [[name index value] (take 3 bindings)]
(list* [index name] `(map-indexed vector ~value)
(clauses (drop 3 bindings)))))))]
`(for [~#(clauses bindings)]
~body)))
user> (macroexpand-1 '(fori [email i xs,
:when (even? i)
line j (lines email)]
[i line]))
(for [[i email] (map-indexed vector xs)
:when (even? i)
[j line] (map-indexed vector (lines email))]
[i line])
(defn fori [coll]
(map-indexed (fn [idx itm] [idx itm]) coll))
Is probably what you want. e.g.
(fori [:a :b :c :d])
produces
([0 :a] [1 :b] [2 :c] [3 :d])
BTW, for is not a loop but creates a list comprehension. If you're looking to do something with side effects inside a loop you should look into doseq.
I wish to generate all subsets of a set except empty set
ie
(all-subsets #{1 2 3}) => #{#{1},#{2},#{3},#{1,2},#{2,3},#{3,1},#{1,2,3}}
How can this be done in clojure?
In your :dependencies in project.clj:
[org.clojure/math.combinatorics "0.0.7"]
At the REPL:
(require '[clojure.math.combinatorics :as combinatorics])
(->> #{1 2 3}
(combinatorics/subsets)
(remove empty?)
(map set)
(set))
;= #{#{1} #{2} #{3} #{1 2} #{1 3} #{2 3} #{1 2 3}}
clojure.math.combinatorics/subsets sensibly returns a seq of seqs, hence the extra transformations to match your desired output.
Here's a concise, tail-recursive version with dependencies only on clojure.core.
(defn power [s]
(loop [[f & r] (seq s) p '(())]
(if f (recur r (concat p (map (partial cons f) p)))
p)))
If you want the results in a set of sets, use the following.
(defn power-set [s] (set (map set (power s))))
#zcaudate: For completeness, here is a recursive implementation:
(defn subsets
[s]
(if (empty? s)
#{#{}}
(let [ts (subsets (rest s))]
(->> ts
(map #(conj % (first s)))
(clojure.set/union ts)))))
;; (subsets #{1 2 3})
;; => #{#{} #{1} #{2} #{3} #{1 2} #{1 3} #{2 3} #{1 2 3}} (which is correct).
This is a slight variation of #Brent M. Spell's solution in order to seek enlightenment on performance consideration in idiomatic Clojure.
I just wonder if having the construction of the subset in the loop instead of another iteration through (map set ...) would save some overhead, especially, when the set is very large?
(defn power [s]
(set (loop [[f & r] (seq s) p '(#{})]
(if f (recur r (concat p (map #(conj % f) p)))
p))))
(power [1 2 3])
;; => #{#{} #{3} #{2} #{1} #{1 3 2} #{1 3} #{1 2} #{3 2}}
It seems to me loop and recuris not lazy.
It would be nice to have a lazy evaluation version like Brent's, to keep the expression elegancy, while using laziness to achieve efficiency at the sametime.
This version as a framework has another advantage to easily support pruning of candidates for subsets, when there are too many subsets to compute. One can add the logic of pruning at position of conj. I used it to implement the prior algorithm for "Frequent Item Set".
refer to: Algorithm to return all combinations of k elements from n
(defn comb [k l]
(if (= 1 k) (map vector l)
(apply concat
(map-indexed
#(map (fn [x] (conj x %2))
(comb (dec k) (drop (inc %1) l)))
l))))
(defn all-subsets [s]
(apply concat
(for [x (range 1 (inc (count s)))]
(map #(into #{} %) (comb x s)))))
; (all-subsets #{1 2 3})
; (#{1} #{2} #{3} #{1 2} #{1 3} #{2 3} #{1 2 3})
This version is loosely modeled after the ES5 version on Rosetta Code. I know this question seems reasonably solved already... but here you go, anyways.
(fn [s]
(reduce
(fn [a b] (clojure.set/union a
(set (map (fn [y] (clojure.set/union #{b} y)) a))))
#{#{}} s))
I have a function that returns the indexes in seq s at which value v exists:
(defn indexes-of [v s]
(map first (filter #(= v (last %)) (zipmap (range) s))))
What I'd like to do is extend this to apply any arbitrary function for the existence test. My idea is to use a multimethod, but I'm not sure exactly how to detect a function. I want to do this:
(defmulti indexes-of ???)
(defmethod indexes-of ??? [v s] ;; v is a function
(map first (filter v (zipmap (range) s))))
(defmethod indexes-of ??? [v s] ;; v is not a function
(indexes-of #(= v %) s))
Is a multimethod the way to go here? If so, how can I accomplish what I'm trying to do?
If you want to use a multimethod it should be on the filter function, which is the one changing according to the existence test type.
So
(defmulti filter-test (fn [value element]
(cond
(fn? value) :function
:else :value)))
(defmethod filter-test :function
[value element]
(apply value [element]))
(defmethod filter-test :value
[value element]
(= value element))
(defn indexes-of [v s]
(map first (filter #(filter-test v (last %)) (zipmap (range) s))))
Consider the JVM doesn't support first-class functions, or lambdas, out of the box, so there's no "function" data type to dispatch on, that's the reason the fn? test.
None the less the predicate solution proposed by noisesmith is the proper way to go in this situation IMO.
(defmulti indexes-of (fn [v _]
(if (fn? v)
:function
:value)))
(defmethod indexes-of :function
[f coll]
(keep-indexed (fn [i v] (when (f v) i)) coll))
(defmethod indexes-of :value
[v coll]
(indexes-of (partial = v) coll))
How about something simpler and more general:
(defn index-matches [predicate s]
(map first (filter (comp predicate second) (map vector (range) s))))
user> (index-matches even? (reverse (range 10)))
(1 3 5 7 9)
user> (index-matches #{3} [0 1 2 3 1 3 44 3 1 3])
(3 5 7 9)
thanks to a suggestion from lgrapenthin, this function is also now effective for lazy input:
user> (take 1 (index-matches #{300000} (range)))
(300000)