 # davearonson's solution

## to Matrix in the Elixir Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

#### Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Given a string representing a matrix of numbers, return the rows and columns of that matrix.

So given a string with embedded newlines like:

``````9 8 7
5 3 2
6 6 7
``````

representing this matrix:

``````    0  1  2
|---------
0 | 9  8  7
1 | 5  3  2
2 | 6  6  7
``````

your code should be able to spit out:

• A list of the rows, reading each row left-to-right while moving top-to-bottom across the rows,
• A list of the columns, reading each column top-to-bottom while moving from left-to-right.

The rows for our example matrix:

• 9, 8, 7
• 5, 3, 2
• 6, 6, 7

And its columns:

• 9, 5, 6
• 8, 3, 6
• 7, 2, 7

## Running tests

Execute the tests with:

``````\$ elixir matrix_test.exs
``````

### Pending tests

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by commenting out the relevant `@tag :pending` with a `#` symbol.

For example:

``````# @tag :pending
test "shouting" do
assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"
end
``````

Or, you can enable all the tests by commenting out the `ExUnit.configure` line in the test suite.

``````# ExUnit.configure exclude: :pending, trace: true
``````

For more detailed information about the Elixir track, please see the help page.

## Source

Warmup to the `saddle-points` warmup. http://jumpstartlab.com

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

### matrix_test.exs

``````if !System.get_env("EXERCISM_TEST_EXAMPLES") do
end

ExUnit.start()
ExUnit.configure(trace: true, exclude: :pending)

defmodule MatrixTest do
use ExUnit.Case

@input "1 2 3\n4 5 6\n7 8 9"

# @tag :pending
test "reading from and writing to string" do
matrix = Matrix.from_string(@input)
assert Matrix.to_string(matrix) == @input
end

@tag :pending
test "rows should return nested lists regardless of internal structure" do
matrix = Matrix.from_string(@input)

assert Matrix.rows(matrix) == [
[1, 2, 3],
[4, 5, 6],
[7, 8, 9]
]
end

@tag :pending
test "row should return list at index" do
matrix = Matrix.from_string(@input)

assert Matrix.row(matrix, 0) == [1, 2, 3]
assert Matrix.row(matrix, 1) == [4, 5, 6]
assert Matrix.row(matrix, 2) == [7, 8, 9]
end

@tag :pending
test "columns should return nested lists regardless of internal structure" do
matrix = Matrix.from_string(@input)

assert Matrix.columns(matrix) == [
[1, 4, 7],
[2, 5, 8],
[3, 6, 9]
]
end

@tag :pending
test "column should return list at index" do
matrix = Matrix.from_string(@input)

assert Matrix.column(matrix, 0) == [1, 4, 7]
assert Matrix.column(matrix, 1) == [2, 5, 8]
assert Matrix.column(matrix, 2) == [3, 6, 9]
end
end``````
``````defmodule Matrix do
defstruct matrix: nil

@doc """
Convert an `input` string, with rows separated by newlines and values
separated by single spaces, into a `Matrix` struct.
"""
@spec from_string(input :: String.t()) :: %Matrix{}
def from_string(input) do
input
|> String.split("\n")
|> Enum.map(&(String.split(&1, " ")))
|> Enum.map(&make_elements_integers/1)
end

# need this to be a separate function
# cuz elixir won't let us nest captures
defp make_elements_integers(row) do
row |> Enum.map(&String.to_integer/1)
end

@doc """
Write the `matrix` out as a string, with rows separated by newlines and
values separated by single spaces.
"""
@spec to_string(matrix :: %Matrix{}) :: String.t()
def to_string(matrix) do
matrix
|> Enum.map(&(Enum.join(&1, " ")))
|> Enum.join("\n")
end

@doc """
Given a `matrix`, return its rows as a list of lists of integers.
"""
@spec rows(matrix :: %Matrix{}) :: list(list(integer))
def rows(matrix) do
matrix
end

@doc """
Given a `matrix` and `index`, return the row at `index`.
"""
@spec row(matrix :: %Matrix{}, index :: integer) :: list(integer)
def row([_head| tail], index), do: row(tail, index-1)

@doc """
Given a `matrix`, return its columns as a list of lists of integers.
"""
@spec columns(matrix :: %Matrix{}) :: list(list(integer))
def columns(matrix) do
matrix |> transpose |> rows
end

defp transpose(matrix) do
matrix |> List.zip |> Enum.map(&Tuple.to_list/1)
end

@doc """
Given a `matrix` and `index`, return the column at `index`.
"""
@spec column(matrix :: %Matrix{}, index :: integer) :: list(integer)
def column(matrix, index) do
matrix |> transpose |> row(index)
end
end``````