Programming Elixir Chapter 7 Notes

Making the best of recursion.

1. Chapter 7 uses recursion where I would use iteration or list comprehension in Python.

2. Exercises 1 thru 4 solved:

   defmodule MyList do
   
     # ListsAndRecursions-1
     def mapper([], _func), do: []
   
     def mapper([head | tail], func), do: [func.(head) | mapper(tail, func)]
   
     def reduce([], value, _func), do: value
   
     def reduce([head | tail], value, func) do
       reduce(tail, func.(head, value), func)
     end
   
     def mapsum(lst, func) do
       # Map the function.
       reduce(mapper(lst, func), 0, &(&1 + &2))
     end
   
   
     # ListsAndRecursions-2
     defp maximum([], maxie), do: maxie
   
     defp maximum([head | tail], maxie) when head <= maxie do
       maximum(tail, maxie)
     end
   
     defp maximum([head | tail], maxie) when head > maxie do
       maximum(tail, head)
     end
   
     def max([]), do: []
   
     def max([head | tail]) do
       maximum([head | tail], head)
     end
   
      @lower_a 97
      @lower_z 122
   
     # ListsAndRecursions-3
     defp caesar_wrap(val, offset) do
       # Caesar works on only one case.
       # Exercise is assuming lowercase letters.
       new_val = val + offset
   
       cond do
         new_val > @lower_z ->  @lower_a + (new_val - @lower_z)
         true -> new_val
       end
     end
   
     def caesar([], _offset), do: []
   
     def caesar(lst, offset) do
       mapper(lst, &caesar_wrap(&1, offset))
     end
   
   end
   
   # ListsAndRecursions-4
   def span(from, to) when from==to, do: [to]
   
   def span(from, to) do
     [from | span(from+1, to)]
   end

   # ListsAndRecursions-1
   MyList.mapsum([1, 2, 3], &(&1 * &1))
   14
   
   # ListsAndRecursions-2
   MyList.max([1, 10, 3, 5, 88, -1])
   88
   
   # ListsAndRecursions-3
   MyList.caesar('abcxyz', 13)
   'noplmn'
   
   # ListsAndRecursions-4
   MyList.span(1,10)
   [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

All notes and comments are my own opinion. Follow me at @rgacote@genserver.social