This is similar to Clojure get map key by value
However, there is one difference. How would you do the same thing if hm is like
{1 ["bar" "choco"]}
The idea being to get 1 (the key) where the first element if the value list is "bar"? Please feel free to close/merge this question if some other question answers it.
I tried something like this, but it doesn't work.
(def hm {:foo ["bar", "choco"]})
(keep #(when (= ((nth val 0) %) "bar")
(key %))
hm)
You can filter the map and return the first element of the first item in the resulting sequence:
(ffirst (filter (fn [[k [v & _]]] (= "bar" v)) hm))
you can destructure the vector value to access the second and/or third elements e.g.
(ffirst (filter (fn [[k [f s t & _]]] (= "choco" s))
{:foo ["bar", "choco"]}))
past the first few elements you will probably find nth more readable.
Another way to do it using some:
(some (fn [[k [v & _]]] (when (= "bar" v) k)) hm)
Your example was pretty close to working, with some minor changes:
(keep #(when (= (nth (val %) 0) "bar")
(key %))
hm)
keep and some are similar, but some only returns one result.
in addition to all the above (correct) answers, you could also want to reindex your map to desired form, especially if the search operation is called quite frequently and the the initial map is rather big, this would allow you to decrease the search complexity from linear to constant:
(defn map-invert+ [kfn vfn data]
(reduce (fn [acc entry] (assoc acc (kfn entry) (vfn entry)))
{} data))
user> (def data
{1 ["bar" "choco"]
2 ["some" "thing"]})
#'user/data
user> (def inverted (map-invert+ (comp first val) key data))
#'user/inverted
user> inverted
;;=> {"bar" 1, "some" 2}
user> (inverted "bar")
;;=> 1
Related
There is a map containing sequences. The sequences contain items.
I want to remove a given item from any sequence that contains it.
The solution I found does what it should, but I wonder if there is a better
or more elegant way to achieve the same.
my current solution:
(defn remove-item-from-map-value [my-map item]
(apply merge (for [[k v] my-map] {k (remove #(= item %) v)})))
The test describe the expected behaviour:
(require '[clojure.test :as t])
(def my-map {:keyOne ["itemOne"]
:keyTwo ["itemTwo" "itemThree"]
:keyThree ["itemFour" "itemFive" "itemSix"]})
(defn remove-item-from-map-value [my-map item]
(apply merge (for [[k v] my-map] {k (remove #(= item %) v)})))
(t/is (= (remove-item-from-map-value my-map "unknown-item") my-map))
(t/is (= (remove-item-from-map-value my-map "itemFive") {:keyOne ["itemOne"]
:keyTwo ["itemTwo" "itemThree"]
:keyThree ["itemFour" "itemSix"]}))
(t/is (= (remove-item-from-map-value my-map "itemThree") {:keyOne ["itemOne"]
:keyTwo ["itemTwo"]
:keyThree ["itemFour" "itemFive" "itemSix"]}))
(t/is (= (remove-item-from-map-value my-map "itemOne") {:keyOne []
:keyTwo ["itemTwo" "itemThree"]
:keyThree ["itemFour" "itemFive" "itemSix"]}))
I'm fairly new to clojure and am interested in different solutions.
So any input is welcome.
I throw in the specter
version for good measure. It keeps the vectors inside the map
and it's really compact.
(setval [MAP-VALS ALL #{"itemFive"}] NONE my-map)
Example
user=> (use 'com.rpl.specter)
nil
user=> (def my-map {:keyOne ["itemOne"]
#_=> :keyTwo ["itemTwo" "itemThree"]
#_=> :keyThree ["itemFour" "itemFive" "itemSix"]})
#_=>
#'user/my-map
user=> (setval [MAP-VALS ALL #{"itemFive"}] NONE my-map)
{:keyOne ["itemOne"],
:keyThree ["itemFour" "itemSix"],
:keyTwo ["itemTwo" "itemThree"]}
user=> (setval [MAP-VALS ALL #{"unknown"}] NONE my-map)
{:keyOne ["itemOne"],
:keyThree ["itemFour" "itemFive" "itemSix"],
:keyTwo ["itemTwo" "itemThree"]}
i would go with something like this:
user> (defn remove-item [my-map item]
(into {}
(map (fn [[k v]] [k (remove #{item} v)]))
my-map))
#'user/remove-item
user> (remove-item my-map "itemFour")
;;=> {:keyOne ("itemOne"),
;; :keyTwo ("itemTwo" "itemThree"),
;; :keyThree ("itemFive" "itemSix")}
you could also make up a handy function map-val performing mapping on map values:
(defn map-val [f data]
(reduce-kv
(fn [acc k v] (assoc acc k (f v)))
{} data))
or shortly like this:
(defn map-val [f data]
(reduce #(update % %2 f) data (keys data)))
user> (map-val inc {:a 1 :b 2})
;;=> {:a 2, :b 3}
(defn remove-item [my-map item]
(map-val (partial remove #{item}) my-map))
user> (remove-item my-map "itemFour")
;;=> {:keyOne ("itemOne"),
;; :keyTwo ("itemTwo" "itemThree"),
;; :keyThree ("itemFive" "itemSix")}
I think your solution is mostly okay, but I would try to avoid the apply merge part, as you can easily recreate a map from a sequence with into. Also, you could also use map instead of for which I think is a little bit more idiomatic in this case as you don't use any of the list comprehension features of for.
(defn remove-item-from-map-value [m item]
(->> m
(map (fn [[k vs]]
{k (remove #(= item %) vs)}))
(into {})))
Another solution much like #leetwinski:
(defn remove-item [m i]
(zipmap (keys m)
(map (fn [v] (remove #(= % i) v))
(vals m))))
Here's a one-liner which does this in an elegant way. The perfect function for me to use in this scenario is clojure.walk/prewalk. What this fn does is it traverse all of the sub-forms of the form that you pass to it and it transforms them with the provided fn:
(defn remove-item-from-map-value [data item]
(clojure.walk/prewalk #(if (map-entry? %) [(first %) (remove #{item} (second %))] %) data))
What the remove-item-from-map-value fn will do is it will check if current form is a map entry and if so, it will remove specified key from its value (second element of the map entry, which is a vector containing a key and a value, respectively).
The best this about this approach is that is is completely extendable: you could decide to do different things for different types of forms, you can also handle nested forms, etc.
It took me some time to master this fn but once I got it I found it extremely useful!
Today I tried to implement a "R-like" melt function. I use it for Big Data coming from Big Query.
I do not have big constraints about time to compute and this function takes less than 5-10 seconds to work on millions of rows.
I start with this kind of data :
(def sample
'({:list "123,250" :group "a"} {:list "234,260" :group "b"}))
Then I defined a function to put the list into a vector :
(defn split-data-rank [datatab value]
(let [splitted (map (fn[x] (assoc x value (str/split (x value) #","))) datatab)]
(map (fn[y] (let [index (map inc (range (count (y value))))]
(assoc y value (zipmap index (y value)))))
splitted)))
Launch :
(split-data-rank sample :list)
As you can see, it returns the same sequence but it replaces :list by a map giving the position in the list of each item in quoted list.
Then, I want to melt the "dataframe" by creating for each item in a group its own row with its rank in the group.
So that I created this function :
(defn split-melt [datatab value]
(let [splitted (split-data-rank datatab value)]
(map (fn [y] (dissoc y value))
(apply concat
(map
(fn[x]
(map
(fn[[k v]]
(assoc x :item v :Rank k))
(x value)))
splitted)))))
Launch :
(split-melt sample :list)
The problem is that it is heavily indented and use a lot of map. I apply dissoc to drop :list (which is useless now) and I have also to use concat because without that I have a sequence of sequences.
Do you think there is a more efficient/shorter way to design this function ?
I am heavily confused with reduce, does not know whether it can be applied here since there are two arguments in a way.
Thanks a lot !
If you don't need the split-data-rank function, I will go for:
(defn melt [datatab value]
(mapcat (fn [x]
(let [items (str/split (get x value) #",")]
(map-indexed (fn [idx item]
(-> x
(assoc :Rank (inc idx) :item item)
(dissoc value)))
items)))
datatab))
I'm a new clojure programmer.
Given...
{:foo "bar"}
Is there a way to retrieve the name of the key with a value "bar"?
I've looked through the map docs and can see a way to retrieve key and value or just value but not just the key. Help appreciated!
There can be multiple key/value pairs with value "bar". The values are not hashed for lookup, contrarily to their keys. Depending on what you want to achieve, you can look up the key with a linear algorithm like:
(def hm {:foo "bar"})
(keep #(when (= (val %) "bar")
(key %)) hm)
Or
(filter (comp #{"bar"} hm) (keys hm))
Or
(reduce-kv (fn [acc k v]
(if (= v "bar")
(conj acc k)
acc))
#{} hm)
which will return a seq of keys. If you know that your vals are distinct from each other, you can also create a reverse-lookup hash-map with
(clojure.set/map-invert hm)
user> (->> {:a "bar" :b "foo" :c "bar" :d "baz"} ; initial map
(group-by val) ; sorted into a new map based on value of each key
(#(get % "bar")) ; extract the entries that had value "bar"
(map key)) ; get the keys that had value bar
(:a :c)
As in many other cases you can use for:
(def hm {:foo "bar"})
(for [[k v] hm :when (= v "bar")] k)
And with "some" you can return the first matching item instead of a list (as probably the original question implied):
(some (fn [[k v]] (if (= v "bar") k)) hm)
How can I search and dissoc multiple descendent keys.
Example:
(def d {:foo 123
:bar {
:baz 456
:bam {
:whiz 789}}})
(dissoc-descendents d [:foo :bam])
;->> {:bar {:baz 456}}
clojure.walk is useful in this kind of situations:
(use 'clojure.walk)
(postwalk #(if (map? %) (dissoc % :foo :bam) %) d)
If you wanted to implement it directly then I'd suggest something like this:
(defn dissoc-descendents [coll descendents]
(let [descendents (if (set? descendents) descendents (set descendents))]
(if (associative? coll)
(reduce
(fn [m [k v]] (if (descendents k)
(dissoc m k)
(let [new-val (dissoc-descendents v descendents)]
(if (identical? new-val v) m (assoc m k new-val)))))
coll
coll)
coll)))
Key things to note about the implementation:
It makes sense to convert descendents into a set: this will allow quick membership tests if the set of keys to remove is large
There is some logic to ensure that if a value doesn't change, you don't need to alter that part of the map. This is quite a big performance win if large areas of the map are unchanged.
In clojure, I want to aggregate this data:
(def data [[:morning :pear][:morning :mango][:evening :mango][:evening :pear]])
(group-by first data)
;{:morning [[:morning :pear][:morning :mango]],:evening [[:evening :mango][:evening :pear]]}
My problem is that :evening and :morning are redundant.
Instead, I would like to create the following collection:
([:morning (:pear :mango)] [:evening (:mango :pear)])
I came up with:
(for [[moment moment-fruit-vec] (group-by first data)] [moment (map second moment-fruit-vec)])
Is there a more idiomatic solution?
I've come across similar grouping problems. Usually I end up plugging merge-with or update-in into some seq processing step:
(apply merge-with list (map (partial apply hash-map) data))
You get a map, but this is just a seq of key-value pairs:
user> (apply merge-with list (map (partial apply hash-map) data))
{:morning (:pear :mango), :evening (:mango :pear)}
user> (seq *1)
([:morning (:pear :mango)] [:evening (:mango :pear)])
This solution only gets what you want if each key appears twice, however. This might be better:
(reduce (fn [map [x y]] (update-in map [x] #(cons y %))) {} data)
Both of these feel "more functional" but also feel a little convoluted. Don't be too quick to dismiss your solution, it's easy-to-understand and functional enough.
Don't be too quick to dismiss group-by, it has aggregated your data by the desired key and it hasn't changed the data. Any other function expecting a sequence of moment-fruit pairs will accept any value looked up in the map returned by group-by.
In terms of computing the summary my inclination was to reach for merge-with but for that I had to transform the input data into a sequence of maps and construct a "base-map" with the required keys and empty-vectors as values.
(let [i-maps (for [[moment fruit] data] {moment fruit})
base-map (into {}
(for [key (into #{} (map first data))]
[key []]))]
(apply merge-with conj base-map i-maps))
{:morning [:pear :mango], :evening [:mango :pear]}
Meditating on #mike t's answer, I've come up with:
(defn agg[x y] (if (coll? x) (cons y x) (list y x)))
(apply merge-with agg (map (partial apply hash-map) data))
This solution works also when the keys appear more than twice on data:
(apply merge-with agg (map (partial apply hash-map)
[[:morning :pear][:morning :mango][:evening :mango] [:evening :pear] [:evening :kiwi]]))
;{:morning (:mango :pear), :evening (:kiwi :pear :mango)}
maybe just modify the standard group-by a little bit:
(defn my-group-by
[fk fv coll]
(persistent!
(reduce
(fn [ret x]
(let [k (fk x)]
(assoc! ret k (conj (get ret k []) (fv x)))))
(transient {}) coll)))
then use it as:
(my-group-by first second data)