I have to extract a key from a map using a value. Is there a way to do this other than implementing reverse lookup myself?
I think that map-invert is the right way to do this.
From the docs:
;; Despite being in clojure.set, this has nothing to do with sets.
user=> (map-invert {:a 1, :b 2})
{2 :b, 1 :a}
;; If there are duplicate keys, one is chosen:
user=> (map-invert {:a 1, :b 1})
{1 :b}
;; I suspect it'd be unwise to depend on which key survives the clash.
You can reverse a map really easily with a 2-line function:
(defn reverse-map [m]
(into {} (map (fn [[a b]] [b a]) m)))
(def a {:a 1 :b 2 :c 3})
(reverse-map a)
=> {1 :a, 3 :c, 2 :b}
((reverse-map a) 1)
=> :a
Try
(some #(if (= (val %) your-val) (key %)) your-map)
If you are using ClojureScript or you need one more alternative :)
(zipmap (vals m) (keys m))
Another one:
(defn reverse-map [m]
(apply hash-map (mapcat reverse m)))
(defn reverse-lookup [m k]
(ffirst (filter (comp #{k} second) m)))
if you want to keep the keys, it is better to just invert the map, but collect the old keys in a set / list etc...
(defn map-inverse [m]
(reduce (fn [m' [k v]] (update m' v clojure.set/union #{k})) {} m))
(defn map-inverse [m]
(reduce (fn [m' [k v]] (update m' v conj k)) {} m))
Related
Hope you are well. I am stuck with a recursive program that is suppose to traverse a graph until it finds a path back to the start node. the code is here,
(def graph {:A {:B 5 :D 5 :E 7}
:B {:C 4}
:C {:D 8 :E 2}
:D {:C 8 :E 6}
:E {:B 3}
})
(defn looper [g startnode & args]
(let [[inode] (vec args)
acc []]
(if (= startnode inode)
(conj acc inode)
(conj acc inode (map (partial looper g startnode) (vec (keys (get g inode)))))
)))
(looper graph :C)
there is something wrong with the way I accumulate the result I couldnt find what exactly.
The function should return something like '(CDC CEBC) for the above call.
This did the trick, hope its helpful for someone :)
(defn- dfs
[graph goal]
(fn search
[path visited]
(let [current (peek path)]
(if (= goal current)
[path]
(->> current graph keys
(remove visited)
(mapcat #(search (conj path %) (conj visited %))))))))
(defn findpath
"Returns a lazy sequence of all directed paths from start to goal
within graph."
[graph start goal]
((dfs graph goal) [start] #{start}))
(defn circular-path-count [graph node]
(flatten (map (fn [s]
(map count (findpath graph s node))) (vec (keys (get-in graph [node]))) )))
e.g. usage: (circular-path-count paths :B)
Here's my problem: I want a function helpme that takes a map and replaces the keys :r and :g with empty vectors if and only if those keys exist. For example:
Input:
(helpme {:a "1" :r ["1" "2" "3"] :g ["4" "5"]})
Output:
{:a "1" :r [] :g []}
Input:
(helpme {:a "1" :r ["1" "2" "3"]})
Output:
{:a "1" :r []}
I can define a function "helpme" that does this, but it's overly complicated, and I feel like there must be an easier (more idiomatic) way...
Here's the overly complicated way I've done, as requested below:
(defn c [new-doc k] (if (contains? new-doc k) (assoc new-doc k []) new-doc))
(defn helpme [new-doc] (c (c new-doc :r) :g))
(defn helpme [m]
(into m (for [[k _] (select-keys m [:r :g])]
[k []])))
Short, and only requires editing in one place when the number of items to set to [] changes.
In my search for a version of update-in which only updated the map if the key actually existed, Google insisted that I could find my answer here. For others in search of the same thing I've created the following helper functions:
(defn contains-in?
[m ks]
(not= ::absent (get-in m ks ::absent)))
(defn update-if-contains
[m ks f & args]
(if (contains-in? m ks)
(apply (partial update-in m ks f) args)
m))
That way you could:
> (def my-data {:a {:aa "aaa"}})
> (update-if-contains my-data [:a :aa] clojure.string/upper-case)
{:a {:aa "AAA"}}
> (update-if-contains my-data [:a :aa] clojure.string/split #" ")
{:a {:aa ["a" "aa"]}}
> (update-if-contains my-data [:a :b] clojure.string/upper-case)
{:a {:aa "aaa"}} ; no change because [:a :b] didn't exist in the map
(defn helpme
[mp]
(as-> mp m
(or (and (contains? m :r) (assoc m :r []))
m)
(or (and (contains? m :g) (assoc m :g []))
m)
m))
if there were a third replacement, I would use this function:
(defn replace-contained [m k v] (or (and (contains? m k) (assoc m k v)) m))
as-> is new in clojure 1.5 but the definition is very simple if you are stuck using an older clojure version:
(defmacro as->
"Binds name to expr, evaluates the first form in the lexical context
of that binding, then binds name to that result, repeating for each
successive form, returning the result of the last form."
{:added "1.5"}
[expr name & forms]
`(let [~name ~expr
~#(interleave (repeat name) forms)]
~name))
what about using cond-> for that purpose?
(defn helpme [m]
(cond-> m
(:r m) (assoc :r [])
(:g m) (assoc :g [])))
One option:
(defn helpme [m]
(merge m
(apply hash-map (interleave
(clojure.set/intersection
(set (keys m)) #{:r :g})
(repeat [])))))
If this is really as simple as conditionally setting the value of two fixed keys, I'd just write it out long hand to keep it simple.
(defn clean [m]
(let [m (if (:r m) (assoc m :r []) m)
m (if (:g m) (assoc m :g []) m)]
m))
If you want something more general and reusable, here's an option:
(defn cond-assoc [m & kvs]
(reduce
(fn [acc [k v]]
(if (get acc k)
(assoc acc k v)
acc))
m
(partition 2 kvs)))
(cond-assoc {:a "1" :r ["1" "2" "3"] :g ["4" "5"]}
:r []
:g []) ; {:r [] :a "1" :g []}
(cond-assoc {:a "1" :r ["1" "2" "3"]}
:r []
:g []) ; {:r [] :a "1"}
By testing for the expected key in the compare function
(sort-by #(if (number? (:priority %))
(:priority %)
(java.lang.Integer/MAX_VALUE))
<
[{:priority 100} {:priority 10} {:test 1}])
>({:priority 10} {:priority 100} {:test 1})
I really like the helpme API to include the keys to reset and the default value to reset to:
(defn helpme [m & {:keys [ks v]
:or {ks #{:r :g}
v []}}]
(apply assoc m (-> m
;; select only existing keys by selecting from original map
(select-keys ks)
keys
;; generate defaults for each (handled by applying `assoc`)
(interleave (repeat v)))))
This uses assocs variadic form by production the arguments to it.
If you give up the general API it can be as short as:
(defn helpme [m]
(apply assoc m (-> m
(select-keys #{:r :g})
keys
(interleave (repeat [])))))
I'm trying to apply a function to all elements in a map that match a certain key.
(def mymap {:a "a" :b "b" :c "c"})
(update-in mymap [:a :b] #(str "X-" %))
I'm expecting
{:a "X-a", :c "c", :b "X-b"}
But I get
ClassCastException java.lang.String cannot be cast to clojure.lang.Associative clojure.lang.RT.assoc (RT.java:702)
Anyone can help me with this?
update-in is to update a single key in the map (at a particular nesting level, [:a :b] means update key :b inside the map value of key :a.
What you want can be done using reduce:
(reduce #(assoc %1 %2 (str "X-" (%1 %2)))
mymap
[:a :b])
Here's a generalized function:
(defn update-each
"Updates each keyword listed in ks on associative structure m using fn."
[m ks fn]
(reduce #(update-in %1 [%2] fn) m ks))
(update-each mymap [:a :b] #(str "X-" %))
In the solution below, the haspmap if first filtered, then it is mapped to the str function, and then merged with the original hashmap -
(def m {:a "a" :b "b" :c "c"})
(def keys #{:a :b})
(->> m
(filter (fn [[k v]] (k keys)))
(map (fn [[k v]] [k (str "X-" v)]))
(into {})
(merge m))
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.
What function can I put as FOO here to yield true at the end? I played with hash-set (only correct for first 2 values), conj, and concat but I know I'm not handling the single-element vs set condition properly with just any of those.
(defn mergeMatches [propertyMapList]
"Take a list of maps and merges them combining values into a set"
(reduce #(merge-with FOO %1 %2) {} propertyMapList))
(def in
(list
{:a 1}
{:a 2}
{:a 3}
{:b 4}
{:b 5}
{:b 6} ))
(def out
{ :a #{ 1 2 3}
:b #{ 4 5 6} })
; this should return true
(= (mergeMatches in) out)
What is the most idiomatic way to handle this?
This'll do:
(let [set #(if (set? %) % #{%})]
#(clojure.set/union (set %) (set %2)))
Rewritten more directly for the example (Alex):
(defn to-set [s]
(if (set? s) s #{s}))
(defn set-union [s1 s2]
(clojure.set/union (to-set s1) (to-set s2)))
(defn mergeMatches [propertyMapList]
(reduce #(merge-with set-union %1 %2) {} propertyMapList))
I didn't write this but it was contributed by #amitrathore on Twitter:
(defn kv [bag [k v]]
(update-in bag [k] conj v))
(defn mergeMatches [propertyMapList]
(reduce #(reduce kv %1 %2) {} propertyMapList))
I wouldn't use merge-with for this,
(defn fnil [f not-found]
(fn [x y] (f (if (nil? x) not-found x) y)))
(defn conj-in [m map-entry]
(update-in m [(key map-entry)] (fnil conj #{}) (val map-entry)))
(defn merge-matches [property-map-list]
(reduce conj-in {} (apply concat property-map-list)))
user=> (merge-matches in)
{:b #{4 5 6}, :a #{1 2 3}}
fnil will be part of core soon so you can ignore the implementation... but it just creates a version of another function that can handle nil arguments. In this case conj will substitute #{} for nil.
So the reduction conjoining to a set for every key/value in the list of maps supplied.
Another solution contributed by #wmacgyver on Twitter based on multimaps:
(defn add
"Adds key-value pairs the multimap."
([mm k v]
(assoc mm k (conj (get mm k #{}) v)))
([mm k v & kvs]
(apply add (add mm k v) kvs)))
(defn mm-merge
"Merges the multimaps, taking the union of values."
[& mms]
(apply (partial merge-with union) mms))
(defn mergeMatches [property-map-list]
(reduce mm-merge (map #(add {} (key (first %)) (val (first %))) property-map-list)))
This seems to work:
(defn FOO [v1 v2]
(if (set? v1)
(apply hash-set v2 v1)
(hash-set v1 v2)))
Not super pretty but it works.
(defn mergeMatches [propertyMapList]
(for [k (set (for [pp propertyMapList] (key (first pp))))]
{k (set (remove nil? (for [pp propertyMapList] (k pp))))}))