It's not dependent on whether it's a literal but on the size of the map, 8
key-value pairs is the threshold.
This results in a PersistentHashMap
(class {1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9}) =>
clojure.lang.PersistentHashMap
This gets you a PersistentArrayMap
(class {1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8}) => clojure.lang.PersistentHashMap
You can see where this happens in the source here:
https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/PersistentArrayMap.java#L115
HASHTABLE_THRESHOLD is a constant set to 16, 8 keys and 8 values. So when
you assoc onto an arraymap with 8 key-value pairs it returns a hashmap.
The reason for this, as far as I understand it, is that with small hashmaps
it's more efficient to do simple copy-on-write. In other words when you
assoc onto it, it copies the entire map, adds the new key-value pair to the
copy, and then returns the copy. With larger hashmaps, it becomes more
useful to do use a more complicated tree structure which uses structural
sharing so that assoc doesn't copy the entire map. Copying a small 5
element map isn't a big deal, but copying one with several thousand
elements is.
On Fri, Aug 10, 2012 at 5:43 PM, Hussein B. <hubaghd...@gmail.com> wrote:
> Hi,
> Why Clojure map literal creates an instance of array map but not hash map?
> What are the advantages of array map over hash map?
> Thanks.
>
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