How to resolve the algorithm 100 doors step by step in the Elixir programming language
How to resolve the algorithm 100 doors step by step in the Elixir programming language
Table of Contents
Problem Statement
There are 100 doors in a row that are all initially closed.
You make 100 passes by the doors.
The first time through, visit every door and toggle the door (if the door is closed, open it; if it is open, close it).
The second time, only visit every 2nd door (door #2, #4, #6, ...), and toggle it.
The third time, visit every 3rd door (door #3, #6, #9, ...), etc, until you only visit the 100th door.
Answer the question: what state are the doors in after the last pass? Which are open, which are closed?
Alternate:
As noted in this page's discussion page, the only doors that remain open are those whose numbers are perfect squares.
Opening only those doors is an optimization that may also be expressed;
however, as should be obvious, this defeats the intent of comparing implementations across programming languages.
Let's start with the solution:
Step by Step solution about How to resolve the algorithm 100 doors step by step in the Elixir programming language
Source code in the elixir programming language
defmodule HundredDoors do
def doors(n \\ 100) do
List.duplicate(false, n)
end
def toggle(doors, n) do
List.update_at(doors, n, &(!&1))
end
def toggle_every(doors, n) do
Enum.reduce( Enum.take_every((n-1)..99, n), doors, fn(n, acc) -> toggle(acc, n) end )
end
end
# unoptimized
final_state = Enum.reduce(1..100, HundredDoors.doors, fn(n, acc) -> HundredDoors.toggle_every(acc, n) end)
open_doors = Enum.with_index(final_state)
|> Enum.filter_map(fn {door,_} -> door end, fn {_,index} -> index+1 end)
IO.puts "All doors are closed except these: #{inspect open_doors}"
# optimized
final_state = Enum.reduce(1..10, HundredDoors.doors, fn(n, acc) -> HundredDoors.toggle(acc, n*n-1) end)
open_doors = Enum.with_index(final_state)
|> Enum.filter_map(fn {door,_} -> door end, fn {_,index} -> index+1 end)
IO.puts "All doors are closed except these: #{inspect open_doors}"
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