Convert an array of tuples into a hash-map in Clojure

Question

I have an array of tuples, where each tuple is a 2 tuple with a key and a value. What would be the cleanest way to convert this array of tuples into a hash-map?

Solution 1

user=> (into {} [[:a 1] [:b 2]])
{:a 1, :b 2}

Solution 2

A map is a sequence of MapEntry elements. Each MapEntry is a vector of a key and value. The tuples in the question are already in the form of a MapEntry, which makes things convenient. (That's also why the into solution is a good one.)

user=> (reduce conj {} [[:a 1] [:b 2]])
{:b 2, :a 1}

Solution 3

Assuming that "tupel" means "two-elememt array":

(reduce 
  (fn [m tupel] 
      (assoc m 
            (aget tupel 0) 
            (aget tupel 1))) 
  {} 
  array-of-tupels)

Solution 4

user=> (def a [[:a 4] [:b 6]])
user=> (apply hash-map (flatten a))
{:a 4, :b 6}

View more solutions

Comments

[Isaac]

Nice. If your (to the OP) "array" is in fact a seq, instead of (aget t n) you can use (t n), which is a bit prettier.

[Michał Marczyk]

Note that this also works for arrays of two-element arrays -- you'd just have to add a (map vec ...) around the array argument: (into {} (map vec an-array-of-two-element-arrays)).

[Isaac]

And again, one of those "duh" moments you so often have while using Clojure. Nice answer.

[kotarak]

(t n) does not work with sequences. It does with vectors. But then you can simply use (into {} array-of-vectors) instead of the reduce.

[kotarak]

But a relatively expensive one.

[miner49r]

flatten is overkill for this problem. You can take advantage of the tuples being in the form of a MapEntry (vector of key and value).

[Yu Shen]

Thanks for good explanation! So only vector of 2 elements is MapEntry, thus (into {} [[:a 1] {:b 2}]) will work, but (into {} ['( :a 1 ) {:b 2}]) will not work.It gets the error of "java.lang.ClassCastException: clojure.lang.Keyword cannot be cast to java.util.Map$Entry". I wonder why can't Clojure treat all 2 elements sequence the same way in this context. It will make it more consistent. What's the reason it does not?

[Radon Rosborough]

Never, ever use flatten for a situation like this: it will happily flatten all of your keys and values too, if they happen to be data structures!

[Dennis Hackethal]

@MichałMarczyk I don't get it... if you already have an array of arrays, why map vec over them? I think vec just leaves arrays as is...

[Greyshack]

Why does it have to be vector, why doesnt a list of two element lists work?