Incrementing value in Map : variable-key ?

[mraptor]

Why this works :

%{m | :k1 => m[:k1]+1}

but this do not :
x = :k1
%{m | x => m[:k1]+1}

 >>> illegal use of variable x in map key

how do I update map if i dont know the key in advance ?

[José Valim]

You can use the API in the Map module, for example, Map.update/4.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Lead Developer

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[mraptor]

Hmm this starts to get little confusing for multilevel structures :

def inc(map,key) do
   Map.update map, key, 0, fn v -> v + 1 end
#  %{m | k => m[k] + 1 }
 end
 def inc(map,key,sub_key) do
   Map.update map, key, 0, fn k -> inc(k,sub_key)  end
 end

[Ben Wilson]

Checkout http://elixir-lang.org/docs/master/elixir/Kernel.html#put_in/3 and http://elixir-lang.org/docs/master/elixir/Kernel.html#put_in/2 for dealing with sub keys.

[mraptor]

That's nicer, thank you both ....

def inc(map,keys = [ k,sk | _ ], n \\ 1) do
   put_in map, keys, map[k][sk]+n
 end

[mraptor]

Contd... Once I have the update/inc operation, how would you apply a list of increments ... i.e. something like this :
(the sub key is the same on all keys)

apply_inc_updates(map, [k1,k2,k1,k3,k1,k6,k9,k1])

I did something like this :

 #helper list increase
 def hlst_inc([],acc), do: acc
 def hlst_inc([h|t],acc) do
   x = inc acc, [h,:sub_key]
   hlst_inc(t,x)
 end

does it seem like correct way of doing this ?

[Ben Wilson]

I recommend taking some time to browse through the Elixir standard library, there are a lot of functions out there that will make your life a lot easier. The Enum module is likely where you'd want to start. Assuming that you have some function already to increment a key in a map called inc/2 just do ```Enum.reduce(keys, map, &inc/2)```

As an aside, there's no need to abbreviate your function names so severely, it impedes readability. map_increment or something would be a lot clearer than hlst_inc. When you need a comment just to make the function name readable you should probably reconsider the function name.

[Ben Wilson]

Forgot to send you a link! http://elixir-lang.org/docs/master/elixir/

[mraptor]

Thanks... I forgot about reduce, will try it... Probably because I associate it with Lists :), wasnt thinking of reducing-maps ...
I was using short names because I was experimenting around...

On Monday, September 29, 2014 4:40:15 PM UTC-4, Ben Wilson wrote: