Filtering a map against a list of values

Maxximiliann · February 19, 2020, 2:09am

trades = 
%{
  "3QvxP6YFBKpWJSMAfYtL8Niv8KmmKsnpb9uQwQpg8QN2" => [
    %{
      asset: "3QvxP6YFBKpWJSMAfYtL8Niv8KmmKsnpb9uQwQpg8QN2",
      last_price: 0.09019287,
      priced_in: "WAVES",
      volume: 13918.3182330112
    },
    %{
      asset: "3QvxP6YFBKpWJSMAfYtL8Niv8KmmKsnpb9uQwQpg8QN2",
      last_price: 0.1313214,
      priced_in: "DG2xFkPdDwKUoBkzGAhQtLpSGzfXLiCYPEzeKH2Ad24p",
      volume: 9986.99466236054
    },
    %{
      asset: "3QvxP6YFBKpWJSMAfYtL8Niv8KmmKsnpb9uQwQpg8QN2",
      last_price: 1.268e-5,
      priced_in: "8LQW8f7P5d5PZM7GtZEBgaqRPGSzS3DfPuiXrURJ4AJS",
      volume: 9786.93694331709
    }
  ],
  "474jTeYx2r2Va35794tCScAXWJG9hU2HcgxzMowaZUnu" => [
    %{
      asset: "474jTeYx2r2Va35794tCScAXWJG9hU2HcgxzMowaZUnu",
      last_price: 192.01,
      priced_in: "WAVES",
      volume: 137852.319057268
    },
    %{
      asset: "474jTeYx2r2Va35794tCScAXWJG9hU2HcgxzMowaZUnu",
      last_price: 279.152165,
      priced_in: "DG2xFkPdDwKUoBkzGAhQtLpSGzfXLiCYPEzeKH2Ad24p",
      volume: 77018.2552159809
    },
    %{
      asset: "474jTeYx2r2Va35794tCScAXWJG9hU2HcgxzMowaZUnu",
      last_price: 0.02768054,
      priced_in: "8LQW8f7P5d5PZM7GtZEBgaqRPGSzS3DfPuiXrURJ4AJS",
      volume: 38723.9182969945
    },
    %{
      asset: "474jTeYx2r2Va35794tCScAXWJG9hU2HcgxzMowaZUnu",
      last_price: 459.22,
      priced_in: "Ft8X1v1LTa1ABafufpaCWyVj8KkaxUWE6xBhW6sNFJck",
      volume: 513.413421879477
    }
  ],
  "4LHHvYGNKJUg5hj65aGD5vgScvCBmLpdRFtjokvCjSL8" => [
    %{
      asset: "4LHHvYGNKJUg5hj65aGD5vgScvCBmLpdRFtjokvCjSL8",
      last_price: 0.05171444,
      priced_in: "WAVES",
      volume: 14110.9296573005
    },
    %{
      asset: "4LHHvYGNKJUg5hj65aGD5vgScvCBmLpdRFtjokvCjSL8",
      last_price: 7.5e-6,
      priced_in: "8LQW8f7P5d5PZM7GtZEBgaqRPGSzS3DfPuiXrURJ4AJS",
      volume: 9722.17123718049
    },
    %{
      asset: "4LHHvYGNKJUg5hj65aGD5vgScvCBmLpdRFtjokvCjSL8",
      last_price: 0.075459,
      priced_in: "DG2xFkPdDwKUoBkzGAhQtLpSGzfXLiCYPEzeKH2Ad24p",
      volume: 8482.23199810624
    }
  ]
}

defmodule Remove_Blacklisted_Assets do		

	def filter_excluded_assets(number_of_assets_to_be_excluded, accumulator) when accumulator == number_of_assets_to_be_excluded do

		excluded = ['DG2xFkPdDwKUoBkzGAhQtLpSGzfXLiCYPEzeKH2Ad24p', '8LQW8f7P5d5PZM7GtZEBgaqRPGSzS3DfPuiXrURJ4AJS', '2tR9EKvDPMThfrFSb96CSBGYNwNPtzwkGcdhQjv4dVoj']
		number_of_assets_to_be_excluded = length(excluded)			
				
		Enum.filter(trades, fn map -> map.priced_in != Enum.fetch!(excluded, 0) end)
			|> Enum.into(%{})
			|> IO.inspect
	end



	def filter_excluded_assets(number_of_assets_to_be_excluded, accumulator) do

		excluded = ['DG2xFkPdDwKUoBkzGAhQtLpSGzfXLiCYPEzeKH2Ad24p', '8LQW8f7P5d5PZM7GtZEBgaqRPGSzS3DfPuiXrURJ4AJS', '2tR9EKvDPMThfrFSb96CSBGYNwNPtzwkGcdhQjv4dVoj']
		number_of_assets_to_be_excluded = length(excluded)

		Enum.filter(trades, fn map -> map.priced_in != Enum.fetch!(excluded,(number_of_assets_to_be_excluded - accumulator)) end)
			|> IO.inspect
			
		filter_excluded_assets(number_of_assets_to_be_excluded, accumulator + 1)
	end
	
			
end

Desired output:

trades = 
%{
  "3QvxP6YFBKpWJSMAfYtL8Niv8KmmKsnpb9uQwQpg8QN2" => [
    %{
      asset: "3QvxP6YFBKpWJSMAfYtL8Niv8KmmKsnpb9uQwQpg8QN2",
      last_price: 0.09019287,
      priced_in: "WAVES",
      volume: 13918.3182330112
    }
  ],
  "474jTeYx2r2Va35794tCScAXWJG9hU2HcgxzMowaZUnu" => [
    %{
      asset: "474jTeYx2r2Va35794tCScAXWJG9hU2HcgxzMowaZUnu",
      last_price: 192.01,
      priced_in: "WAVES",
      volume: 137852.319057268
    },
    %{
      asset: "474jTeYx2r2Va35794tCScAXWJG9hU2HcgxzMowaZUnu",
      last_price: 459.22,
      priced_in: "Ft8X1v1LTa1ABafufpaCWyVj8KkaxUWE6xBhW6sNFJck",
      volume: 513.413421879477
    }
  ],
  "4LHHvYGNKJUg5hj65aGD5vgScvCBmLpdRFtjokvCjSL8" => [
    %{
      asset: "4LHHvYGNKJUg5hj65aGD5vgScvCBmLpdRFtjokvCjSL8",
      last_price: 0.05171444,
      priced_in: "WAVES",
      volume: 14110.9296573005
    }
    }
  ]
}

iex(1)> Remove_Blacklisted_Assets.filter_excluded_assets(3,0)
** (ArgumentError) you attempted to apply :priced_in on {"3QvxP6YFBKpWJSMAfYtL8Niv8KmmKsnpb9uQwQpg8QN2", [%{asset: "3QvxP6YFBKpWJSMAfYtL8Niv8KmmKsnpb9uQwQpg8QN2", last_price: 0.09019287, priced_in: "WAVES", volume: 13888.8199642112}, %{asset: "3QvxP6YFBKpWJSMAfYtL8Niv8KmmKsnpb9uQwQpg8QN2", last_price: 1.268e-5, priced_in: "8LQW8f7P5d5PZM7GtZEBgaqRPGSzS3DfPuiXrURJ4AJS", volume: 9792.16173281305}, %{asset: "3QvxP6YFBKpWJSMAfYtL8Niv8KmmKsnpb9uQwQpg8QN2", last_price: 0.1313214, priced_in: "DG2xFkPdDwKUoBkzGAhQtLpSGzfXLiCYPEzeKH2Ad24p", volume: 9610.63424570971}]}.

How do I get Enum.filter to drill down to the map where priced_in is located?

Also, is the repetition of the excluded lists and the number_of_asssets_to_be_excluded functions in each function best practice?

kokolegorille · February 19, 2020, 5:25am

There are many errors…

When You use Enum on a map, You receive a tuple of key and value…

Enum.filter(trades, fn {_key, map} -> map.priced_in

It’s what the error message tells You…

Not only this, but the map, is not a map, it’s a list of map.

You are not using number_of_assets_to_be_excluded, then You set it to a new value.

What are You trying to do? If You want to test if a value is in a list, use Enum.member? or in.

Maxximiliann · February 19, 2020, 6:23am

Thanks for your insights so far.

I’m attempting to iterate throughexcluded, removing each from trades.

kokolegorille · February 19, 2020, 7:38am

It is wrong to think ‘A’ is equal to “A”, they are not. You might encounter some bad surprises if You think so

Please try to understand the solution, because You should be able to write it, as descriptive as it is.

iex> excluded = ["DG2xFkPdDwKUoBkzGAhQtLpSGzfXLiCYPEzeKH2Ad24p", "8LQW8f7P5d5PZM7GtZEBgaqRPGSzS3DfPuiXrURJ4AJS", "2tR9EKvDPMThfrFSb96CSBGYNwNPtzwkGcdhQjv4dVoj"]
iex> trades 
|> Enum.reduce(%{}, fn {k, l}, acc -> 
  Map.put(acc, k, Enum.reject(l, & &1.priced_in in excluded)) 
end)
%{
  "3QvxP6YFBKpWJSMAfYtL8Niv8KmmKsnpb9uQwQpg8QN2" => [
    %{
      asset: "3QvxP6YFBKpWJSMAfYtL8Niv8KmmKsnpb9uQwQpg8QN2",
      last_price: 0.09019287,
      priced_in: "WAVES",
      volume: 13918.3182330112
    }
  ],
  "474jTeYx2r2Va35794tCScAXWJG9hU2HcgxzMowaZUnu" => [
    %{
      asset: "474jTeYx2r2Va35794tCScAXWJG9hU2HcgxzMowaZUnu",
      last_price: 192.01,
      priced_in: "WAVES",
      volume: 137852.319057268
    },
    %{
      asset: "474jTeYx2r2Va35794tCScAXWJG9hU2HcgxzMowaZUnu",
      last_price: 459.22,
      priced_in: "Ft8X1v1LTa1ABafufpaCWyVj8KkaxUWE6xBhW6sNFJck",
      volume: 513.413421879477
    }
  ],
  "4LHHvYGNKJUg5hj65aGD5vgScvCBmLpdRFtjokvCjSL8" => [
    %{
      asset: "4LHHvYGNKJUg5hj65aGD5vgScvCBmLpdRFtjokvCjSL8",
      last_price: 0.05171444,
      priced_in: "WAVES",
      volume: 14110.9296573005
    }
  ]
}

Often You will realize You can write it as a one line command.

Maxximiliann · February 20, 2020, 1:42am

If you would kindly step me through your solution, why is Enum.reduce needed? What precisely is it doing?

Thank you for all your help

kokolegorille · February 20, 2020, 5:26am

I prefer to use Enum.reduce when I would otherwise use Enum.map followed by Enum.into. It’s an iteration less.

Enum.reduce is worth learning how to use it… It takes an enumerable, and reduce it to a single value.

In this case, the value is a map.

All of your module recursive code could be replaced by

Enum.reject(l, & &1.priced_in in excluded)

It takes a list, and rejects the items whose price_in is in excluded. In this case reject is more appropriate than filter.

Map.put(acc, k, Enum.reject(l, & &1.priced_in in excluded))

It takes a map, and returns a map with k set to the new list.

trades |> Enum.reduce(%{}, fn {k, l}, acc ->

It takes a map as enumerable (trades), and set acc to the initial value of an empty map. It loops through trades, each item is a tuple of key and value. In this case value is a list.

Each iteration will return a map, which is the updated accumulator, with the key set to the new list.

Finally, it returns the final accumulator, which is what You want

The Enum module is probably the most used module, and it’s worth knowing all its functions.

ericgray · February 20, 2020, 3:47pm

Just to add what @kokolegorille has already explained usually when you need an accumulator or you need to pass around some state Enum.reduce is usually the right tool for the job.

Matter of fact everything in the Enum module is implemented with Enum.reduce except for zip. Jose Valim explains why and when you should use Enum.reduce in one of his Twitch Videos.
When you get a chance you should watch them, you’ll learn a lot.

Maxximiliann · February 20, 2020, 11:40pm

Thank you so very much for the detailed walk-through

Maxximiliann · February 20, 2020, 11:40pm

Thanks for the recommendation. His videos are great!