Calculate the number of grains of wheat on a chessboard given that the number on each square doubles.
There once was a wise servant who saved the life of a prince. The king promised to pay whatever the servant could dream up. Knowing that the king loved chess, the servant told the king he would like to have grains of wheat. One grain on the first square of a chess board. Two grains on the next. Four on the third, and so on.
There are 64 squares on a chessboard.
Write code that shows:
Did you get the tests passing and the code clean? If you want to, these are some additional things you could try:
Then please share your thoughts in a comment on the submission. Did this experiment make the code better? Worse? Did you learn anything from it?
Execute the tests with:
$ mix test
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.
JavaRanch Cattle Drive, exercise 6 http://www.javaranch.com/grains.jsp
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
defmodule GrainsTest do
use ExUnit.Case
# @tag :pending
test "square 1" do
assert Grains.square(1) === {:ok, 1}
end
@tag :pending
test "square 2" do
assert Grains.square(2) === {:ok, 2}
end
@tag :pending
test "square 3" do
assert Grains.square(3) === {:ok, 4}
end
@tag :pending
test "square 4" do
assert Grains.square(4) === {:ok, 8}
end
@tag :pending
test "square 16" do
assert Grains.square(16) === {:ok, 32768}
end
@tag :pending
test "square 32" do
assert Grains.square(32) === {:ok, 2_147_483_648}
end
@tag :pending
test "square 64" do
assert Grains.square(64) === {:ok, 9_223_372_036_854_775_808}
end
@tag :pending
test "total grains" do
assert Grains.total() === {:ok, 18_446_744_073_709_551_615}
end
@tag :pending
test "square greater than 64 returns an error" do
assert Grains.square(65) ===
{:error, "The requested square must be between 1 and 64 (inclusive)"}
end
@tag :pending
test "negative square returns an error" do
assert Grains.square(-1) ===
{:error, "The requested square must be between 1 and 64 (inclusive)"}
end
@tag :pending
test "square 0 returns an error" do
assert Grains.square(0) ===
{:error, "The requested square must be between 1 and 64 (inclusive)"}
end
end
ExUnit.start()
ExUnit.configure(exclude: :pending, trace: true)
defmodule Grains do
@range 1..64
@spec square(pos_integer) :: pos_integer
def square(number) when number in @range,
do: {:ok, pow(number)}
def square(_),
do: {:error, "The requested square must be between 1 and 64 (inclusive)"}
@spec total :: pos_integer
def total(),
do: {:ok, Enum.reduce(@range, 0, &(pow(&1) + &2))}
defp pow(number, base \\ 2),
do: base |> :math.pow(number - 1) |> round()
end
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