> For the complete documentation index, see [llms.txt](https://ploegert.gitbook.io/til/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ploegert.gitbook.io/til/programmy/clojure/aggregation-using-merge-with.md).

# Aggregation Using merge-with

Clojure provides the `merge-with` function as a way of conjoining a series of maps. You must provide `merge-with` a function that it can use to merge two values for matching keys. For instance, imagine you have a bunch of maps that contain counts for entities identified by keywords. You can consolidate the sum of all the counts into a single map using the `merge-with` function combined with the `+` function.

```clojure
> (merge-with + {:a 1 :b 3} {:b 2 :c 3} {:c 1 :d 4})
{:a 1, :b 5, :c 4, :d 4}
```

For different kinds of data, a different function argument may be more appropriate. For instance, aggregating lists instead of integers calls for the `concat` function:

```clojure
> (merge-with concat {:a '(1 2) :b '(3 4)} {:c '(3 4) :a '(5 4 1)})
{:a (1 2 5 4 1), :b (3 4), :c (3 4)}
```