Lectures 5 & 6: Functions
=========================

Based on the lecture and the readings (R&S Functions) you should be
able to:

- Identify how to define and call functions
- Understand scoping.

Below, please find some practice problems for this topic. These
problems should be completed by hand.  You can check your answers
using the files in ``lpp/lec5-6`` in your repository.  (Run ``git pull
upstream master`` to pick up these files.)

1. Write the code necessary to complete the following function.

.. code-block:: python

  def check_number_order(num_list):
    '''
    Purpose: given a list of three numbers (n1, n2, and n3), check
      that n1 < n2 < n3. Return the boolean for whether this order is
      preserved.

    Inputs:
      num_list (list): a list of three numbers (int or float)

    Returns (boolean): return True if the values are in order, False
    otherwise.
    '''

2. Explain what this function does in plain English.

.. code-block:: python

  def mystery(number_list):
      ''' Why knows ... '''
      count = 0
      for num in number_list:
          if num < 0:
              count += 1
      return count

3. What is the output of ``call_mystery``:

.. code-block:: python

  def mystery(l):
      ''' Why knows ... '''
      rv = []
      for x in l:
          rv = [x] + rv
      return rv

  def call_mystery():
      ''' Runs mystery function ... '''
      print("Warmup exercise 2:")
      l = [0, 1, 2, 3, 4]
      nl = mystery(l)
      print("l: ", l)
      print("nl: ", nl)

4. Explain what this function does in plain English.

.. code-block:: python

  def mystery(list1):
      start_val = 0
      rl = []
      i = -1
      for i in list1:
          if i > start_val:
              i = start_val
      for j in list1:
          if j >= i - 100:
              rl.append(j)
      return rl


5. What is the output of the following code:

.. code-block:: python

   def F1(i, j) :
       print("F1({}, {})".format(i, j))
       F2(j, i+j)
       print("F1: i = {}, j = {}".format(i, j))

  
   def F2(i, j) :
       print("    F2({}, {})".format(i, j))
       k = F3(i, j)
       print("    F2: i = {}, j = {}, k = {}".format(i, j, k))
  

   def F3(i, j) :
       print("        F3({}, {})".format(i, j))
       i = i+j
       j = i+2*j
       k = 2*i+3*j
       print("        F3: i = {}, j = {}, k = {}".format(i, j, k))
       return k

    print("Warmup exercise 1:")
    F1(1, 1)
    print()

6. (Challenge exercise): What is the output of the following code?

.. code-block:: python

   def mystery3(l):
       n = len(l)
       for i in range(n // 2):
           t = l[i]
           l[i] = l[n-i-1]
           l[n-i-1] = t

   print("Challenge exercise:")
   l = [0, 1, 2, 3, 4]
   mystery3(l)
   print("l: ", l)