# file: lists.py
# author: Bob Muller
#
# CS1101 Computer Science I
#
# Code covered in class on Tuesday February 9, 2016.

# add : int list -> int
#
def add(list):
  sum = 0                              # sum is mutable, called a "variable"
  for n in list:                       # because its value varies. But it isn't
    sum = sum + n                      # a variable in the mathematical sense.
    print "So far sum = " + str(sum)
  return sum

# recAdd : int list -> list
#
# Works recursively. Natural in many PLs but not the native
# idiom in Python.
#
def recAdd(list):
  if len(list) == 0:
    return 0
  else:
    return list[0] + recAdd(list[1:])


# find: A * A list -> int
#
# This is a naive definition of find, requires ~N units of work.
#
def find(item, list):
  for i in range(len(list)):
    if item == list[i]:
      return i
  print "Not found"
  return None

# Redefining find to use binary search, requires only Log_2 N units
# of work but requires that the elements of the input list are ordered.
#
# isAscending : A list -> bool
#
def isAscending(list):
  if len(list) > 1:
    for i in range(len(list) - 1):
      if list[i] > list[i + 1]:
        return False
  return True

# find : A * A list -> bool
#
def find(item, list):
  assert isAscending(list), "Bad input list"

  lo = 0
  hi = len(list) - 1

  while lo <= hi:
    middle = (lo + hi) / 2
    if item == list[middle]:
      return middle
    elif item < list[middle]:
      hi = middle - 1
    else:
      lo = middle + 1
  
  print "Not found"
  return None