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jennyjean's solution

to Pascal's Triangle in the Elixir Track

Published at May 18 2020 · 0 comments
Instructions
Test suite
Solution

Compute Pascal's triangle up to a given number of rows.

In Pascal's Triangle each number is computed by adding the numbers to the right and left of the current position in the previous row.

``````    1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
# ... etc
``````

Running tests

Execute the tests with:

``````\$ mix test
``````

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
``````

If you're stuck on something, it may help to look at some of the available resources out there where answers might be found.

Source

Pascal's Triangle at Wolfram Math World http://mathworld.wolfram.com/PascalsTriangle.html

Submitting Incomplete Solutions

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

pascals_triangle_test.exs

``````defmodule PascalsTriangleTest do
use ExUnit.Case

# @tag pending
test "one row" do
assert PascalsTriangle.rows(1) == [[1]]
end

@tag :pending
test "two rows" do
assert PascalsTriangle.rows(2) == [[1], [1, 1]]
end

@tag :pending
test "three rows" do
assert PascalsTriangle.rows(3) == [[1], [1, 1], [1, 2, 1]]
end

@tag :pending
test "fourth row" do
assert List.last(PascalsTriangle.rows(4)) == [1, 3, 3, 1]
end

@tag :pending
test "fifth row" do
assert List.last(PascalsTriangle.rows(5)) == [1, 4, 6, 4, 1]
end

@tag :pending
test "twentieth row" do
expected = [
1,
19,
171,
969,
3876,
11_628,
27_132,
50_388,
75_582,
92_378,
92_378,
75_582,
50_388,
27_132,
11_628,
3876,
969,
171,
19,
1
]

assert List.last(PascalsTriangle.rows(20)) == expected
end
end``````

test_helper.exs

``````ExUnit.start()
ExUnit.configure(exclude: :pending, trace: true)``````
``````defmodule PascalsTriangle do
@doc """
Calculates the rows of a pascal triangle
with the given height
"""
@spec rows(integer) :: [[integer]]
def rows(num) do
Enum.reduce((num..1), [], fn (i, acc) -> [get_next_row(i) | acc] end)
end

def get_next_row(1), do: [1]
def get_next_row(num) do
prev_row = get_next_row(num - 1)

midpoint = Integer.floor_div(Kernel.length(prev_row), 2)

split_row = Enum.slice(prev_row, 0, midpoint + 1)

half_row = make_half_row(Enum.with_index(split_row))

if rem(num, 2) == 0 do
List.flatten([half_row | Enum.reverse(half_row)])
else
List.flatten([half_row | tl(Enum.reverse(half_row))])
end
end

def make_half_row(split_row) do
Enum.map(split_row, fn ({num, index}) ->
if index == 0 do
1
else
{prev_num, prev_index} = Enum.find(split_row, fn ({prev_num, prev_index}) -> prev_index == index - 1 end)
prev_num + num
end
end)
end
end``````

What can you learn from this solution?

A huge amount can be learned from reading other peopleâ€™s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

• What compromises have been made?