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Q&A

Add single element for multiple keys to map

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I have a map and a new item to add to the map and a list of keys I want to attach the new item to:

=> m1
{:a 1 :b 2}
=> itm
3
=> ks
[:c :d :e]
=> m2
{:a 1 :b 2 :c 3 :d 3 :e 3}

I thought of using reduce, but that seems overly wordy. How best do I do this?

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2 answers

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(apply assoc m1 (interleave ks (repeat itm)))

Demonstration:

$ clj
Clojure 1.10.3
user=> (def m1 {:a 1 :b 2})
#'user/m1
user=> (def itm 3)
#'user/itm
user=> (def ks [:c :d :e])
#'user/ks
user=> (apply assoc m1 (interleave ks (repeat itm)))
{:a 1, :b 2, :c 3, :d 3, :e 3}

A variation on the other answer, this one uses interleave and apply:

  • interleave returns a lazy sequence of the first element from each argument and then the second element from each argument and then the third, etc, until one runs out.
  • apply calls the given function by treating the last argument (which must be a sequence) as if the elements it contains were directly in the function call. (Works similarly to spread syntax in Javascript and iterable unpacking in Python 3.)
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(merge m1 (zipmap ks (repeat itm)))

Demonstration:

$ clj
Clojure 1.10.3
user=> (def m1 {:a 1 :b 2})
#'user/m1
user=> (def itm 3)
#'user/itm
user=> (def ks [:c :d :e])
#'user/ks
user=> (merge m1 (zipmap ks (repeat itm)))
{:a 1, :b 2, :c 3, :d 3, :e 3}
  • repeat returns a lazy sequence of the given item.
  • zipmap returns a map with the first input (keys) mapped to the second input (vals), and it stops building the map when either input runs out of elements. In this case, it will be the given vector of keys.
  • merge merges all of the input maps into a single map, left-to-right (so any repeated keys will overwrite previous keys).
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