Exercise 2.5

Author: Ben Schmeckpeper

exercise_2.5.exs

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+defmodule MathCons do
+  def cons(a, b), do: round(:math.pow(2, a) * :math.pow(3, b))
+
+  def car(z), do: div_count(z, 2, 0)
+  def cdr(z), do: div_count(z, 3, 0)
+
+  defp div_count(z, r, i) when rem(z, r) != 0, do: i
+  defp div_count(z, r, i), do: div_count(div(z, r), r, i + 1)
+  
+  #defp car_iter(z, i) when rem(z, 2) != 0, do: i
+  #defp car_iter(z, i), do: car_iter(div(z, 2), i + 1)
+
+  #defp cdr_iter(z, i) when rem(z, 3) != 0, do: i
+  #defp cdr_iter(z, i), do: cdr_iter(div(z, 3), i + 1)
+end
+
+ExUnit.start
+
+defmodule MathConsTest do
+  use ExUnit.Case, async: true
+
+  test "car returns the first element of the cons" do
+    z = MathCons.cons 4, 9
+    assert MathCons.car(z) == 4
+  end
+  
+  test "car handles 0" do
+    z = MathCons.cons 0, 9
+    assert MathCons.car(z) == 0
+  end
+  
+  test "cdr returns the last element of the cons" do
+    z = MathCons.cons 4, 9
+    assert MathCons.cdr(z) == 9
+  end
+
+  test "cdr handles 0" do
+    z = MathCons.cons 4, 0
+    assert MathCons.cdr(z) == 0
+  end
+end