How do you convert a list of numbers into a map?

Question

I have a list of keys: (1 2 3 4)

I want a map with the values set to 0 like this: {1 0, 2 0, 3 0, 4 0}. How do I do that?

Answer 1

You could do something like this with the zipmap function:

(zipmap '(1 2 3 4) (repeat 0))
=> {4 0, 3 0, 2 0, 1 0}

zipmap takes a list of keys and a list of values and converts them into a map. The repeat function creates an infinite sequence of 0s. zipmap stops when it reaches the end of the shorter list, so just don't pass it two infinite sequences :)

Answer 2

You can also create a function with James' zipmap:

Clojure=> (defn map-to-n [n] (zipmap (range 1 n) (repeat 0)))
#'user/map-to-n
Clojure=> (map-to-n 10)
{9 0, 8 0, 7 0, 6 0, 5 0, 4 0, 3 0, 2 0, 1 0}

Answer 3

The more general pattern for this is to use (apply collection list to create the collection. The Clojure collections all have "constructors" or creation functions that take a variable number of arguments and return thows arguments bundled up in the collection. if your arguments are already wrapped up in another collection then apply is a convenient way to take them out of the collection and pass them to the creation function as arguments.

This is a lot more work. which is why we have wrapper functions like zipmap.

Answer 4

Wow I didn't know about zipmap, thats useful

I would have done it like this

(apply hash-map (interleave '(1 2 3 4)  (repeat 0)))