Add all data from file to structure in Elixir

Question

I am very new to Elixir and Functional Programming.

I am reading a large text file that describes a graph, and need to store it in a Adjacency Matrix and/or Adjacency List for calculations.

I am having a hard problem trying to figure out how to keep storing the values in a data structure , since data is immutable in Functional Programming.

The file is such as this:

3      //Number of Nodes
1 2    //Non-directional edge
3 4
4 2

Q1 : How can this be done ? [SOLVED]

Q2 : Can this be done with a Stream instead of a full file read ?

Answer 1

You have pretty much two options in functional languages:

1. recursion
2. reduce (specifically, Enum.reduce )

Reduce is really quick and easy if you just need to accumulate a value from a list.

https://hexdocs.pm/elixir/Enum.html#reduce/2

It almost sounds like recursion is the way to go with the problem you're trying to solve:

File.read!("path/to/file.txt")
|> String.split("\n", trim: true)
|> parse(%{})

def parse([line | rest], matrix) do
  add_line_to_matrix(line, matrix)
  parse(rest, matrix)
end

Answer 2

There's no one fixed way to parse string into a data structure. For simple cases like this, String.split and String.to_integer are good enough. You haven't provided the exact output structure you want so I'll show how to decode this into an integer and a list of pair of integers for the edges.

input = "3
1 2
3 4
4 2"

# or input = File.read!("file.txt")

[head | tail] = String.split(input, "\n")
count = String.to_integer(head)
edges = for line <- tail do
  [from, to] = String.split(line)
  {String.to_integer(from), String.to_integer(to)}
end

IO.inspect count
IO.inspect edges

Output:

3
[{1, 2}, {3, 4}, {4, 2}]

Answer 3

Another example with a reducer:

(1..10)
|> Enum.to_list # or "file.txt" |> File.read! |> String.split("\n")
|> Enum.reduce(%{}, fn i, acc ->
     Map.put acc, <<i + 96 :: utf8>>, i
end)
#⇒ %{
#    "a" => 1,
#    "b" => 2,
#    ...
#    "j" => 10
#  }