Hi @Maxximiliann! Since you said your were starting out, I decided to show you a couple of things about Elixir! Hopefully this helps you in your journey. 
I created a repo here: https://github.com/pdgonzalez872/maxximiliann that has an acceptance_test file. There, I show a couple of things about your problem, how to use some tools to your advantage (tests, pry) and ultimately help you with your question. For completeness, I added the content of the file below. Please read the code and hopefully this helps you in your journey.
PD
defmodule AcceptanceTest do
use ExUnit.Case
test "should work" do
# Since you mentioned you are starting out, here are a couple of things you
# may not know. If you do, disregard them.
# You can use `require IEx; IEx.pry` to start a debugging session within a test.
# You can then run the tests like so: `iex -S mix test`
# more about Pry here: https://medium.com/@diamondgfx/debugging-phoenix-with-iex-pry-5417256e1d11
# Uncomment line 20 below to allow pry to run in this test and when the
# debugger asks you if you want to "Allow" Pry, press "Y" to jump in the
# session. It is an IEx session, but through tests. Helpful to debug things
# that you need to go back to later. If you need more than 60 seconds in
# your debugging session (very common), pass the `trace` flag to the mix
# command like so: `iex -S mix test --trace` to make the test have a
# timeout of `infinity`.
# require IEx; IEx.pry
# I'm going to use the code you posted to keep things the same. Apples to
# apples. :)
res = HTTPoison.get!("https://api.wavesplatform.com/v0/pairs")
body = Poison.decode!(res.body, keys: :atoms)
# `body` is a `map` with the following keys:
# We prove this by having the assertion below pass. If the keys were not
# the same, this test would fail.
assert Map.keys(body) == [:__type, :data]
# we can see that `body.data` is a `list`:
assert is_list(body.data) == true
# you can also write the same thing above like this:
assert is_list(body.data)
# This is what the first element in your collection looks like:
first_element = Enum.at(body.data, 0)
# Using IO.inspect() is a great way to look at the shape your data has:
msg = "This will print out to the terminal when you run the test, this is what first_element looks like"
IO.inspect(first_element, label: msg)
# These are the keys you have in `first_element`
assert Map.keys(first_element) == [:__type, :amountAsset, :data, :priceAsset]
# You mentioned you wanted `volumeWaves` for each element. You can see that
# you don't have that key in that first map.
# This is what you have inside `data`:
assert Map.keys(first_element.data) == [:firstPrice, :high, :lastPrice, :low, :quoteVolume, :txsCount, :volume,
:volumeWaves, :weightedAveragePrice]
# There is a little hiccup with your data, you are getting floats back from
# your api call, which is fine, but I suggest using a library when
# dealing with float values. A great one is this one:
# https://github.com/ericmj/decimal
# Here is proof you have a float:
assert is_float(first_element.data.firstPrice)
# To answer your original question: When dealing with collections
# (enumerables), look at the functions in `Enum`:
# https://github.com/elixir-lang/elixir/blob/master/lib/elixir/lib/enum.ex
# Here is `map/2`. The number by the function name is the number of
# arguments it takes (called arity). `Map` iterates through the enumerable
# you pass and keeps the result of a function you pass in as a second
# argument:
all_volume_waves_prices = Enum.map(body.data, fn element -> element.data.volumeWaves end)
IO.inspect(all_volume_waves_prices)
# To answer your question, we can also filter enumerables: `Enum.filter/2`
# will return the element that returns `true` in the function you pass in
# as a second argument:
volume_waves_greater_than_500 = Enum.filter(body.data, fn element -> element.data.volumeWaves > 500.0 end)
IO.inspect(volume_waves_greater_than_500)
# Hope this helps. Thanks for asking the question and I hope you keep using Elixir.
end
end
On a side note, here is a guide to markdown: Basic Syntax | Markdown Guide. It’s nice to be able to format things in these forums, helps you get your point across faster.
