lecture 1

lectures/01/lec1.ex

@@ -0,0 +1,84 @@
+defmodule Lec1 do
+  def fact(n) do
+    if n <= 0 do
+      1
+    else
+      n * fact(n - 1)
+    end
+  end
+
+  def fact2(0) do 1 end # multi-clause function
+  def fact2(n) do n * fact2(n - 1) end
+
+  def fact3(n) when n <= 0 do 1 end # using guard
+  def fact3(n) do  n * fact3(n - 1) end
+
+  # What on earth is going on?
+  # do: with commas eliminates end?
+  def fact4(n) when n <= 0, do: 1
+  def fact4(n), do: n * fact3(n - 1)
+
+  # private functions to this module, i.e. no Lec1.fact5 on these
+  defp fact5(n, acc) when n <= 0, do: acc
+  defp fact5(n, acc), do: fact5(n - 1, n * acc)
+
+  # we can call this function internally though
+  def fact5(n), do: fact5(n, 1)
+
+  # ------- pattern matching -------
+
+  def dedup(l) do
+    case l do
+      [x, x | rest] -> dedup([x | rest])
+      [x | rest] -> [x | dedup(rest)]
+      _ -> l
+    end
+  end
+
+  # or matching within pattern matching
+  def dedup1(l) do
+    case l do
+      [x | rest = [x | _]] -> dedup(rest)
+      [x | rest] -> [x | dedup(rest)]
+      _ -> l
+    end
+  end
+
+  # -------
+  defp fizzbuzz_print(n) do
+    cond do
+      rem(n, 15) == 0 -> IO.puts("fizzbuzz")
+      rem(n, 5) == 0 -> IO.puts("buzz")
+      rem(n, 3) == 0 -> IO.puts("fizz")
+      true -> IO.puts(n)
+    end
+  end
+
+  defp fizzbuzz(i, n) when i > n, do: :ok
+  defp fizzbuzz(i, n) do
+    fizzbuzz_print(i)
+    fizzbuzz(i + 1, n)
+  end
+
+  def fizzbuzz(n), do: fizzbuzz(1, n)
+
+  # --------
+  def qsort([]), do: []
+  def qsort([h | t]), do: qsort( Enum.filter(t, &(&1 < h)) ) ++
+    [h] ++ qsort ( Enum.filter(t, &(&1 > h)) )
+end
+
+# why the above works
+# imagine the following
+if 1 < 2 do "OK" else "BAD" end
+
+# is similar to
+if(1 < 2, do: "OK", else: "BAD")
+
+# consider a key-word list
+[{:a, 1}, {:b, 2}]
+
+# so do: ok is esentially a key-word list
+if(1 < 2, [do: "OK", else: "BAD"])
+
+# somehow this works