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23 lines
829 B
Markdown
23 lines
829 B
Markdown
# Aggregation Using merge-with
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Clojure provides the `merge-with` function as a way of conjoining a series
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of maps. You must provide `merge-with` a function that it can use to merge
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two values for matching keys. For instance, imagine you have a bunch of maps
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that contain counts for entities identified by keywords. You can consolidate
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the sum of all the counts into a single map using the `merge-with` function
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combined with the `+` function.
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```clojure
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> (merge-with + {:a 1 :b 3} {:b 2 :c 3} {:c 1 :d 4})
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{:a 1, :b 5, :c 4, :d 4}
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```
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For different kinds of data, a different function argument may be more
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appropriate. For instance, aggregating lists instead of integers calls for
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the `concat` function:
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```clojure
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> (merge-with concat {:a '(1 2) :b '(3 4)} {:c '(3 4) :a '(5 4 1)})
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{:a (1 2 5 4 1), :b (3 4), :c (3 4)}
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```
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