Lecture Notes on 18 Feb 2013

# Flip pancakes to get them in sorted order

# Find the index of the max value in a sub-list
def findMaxIdx (top):
  size = len(top)
  idx = 0
  maxVal = top[0]
  for i in range (size):
    if (top[i] > maxVal):
      maxVal = top[i]
      idx = i
  return idx
  
# Flip pancakes to get one of the pancakes in 
# its right place
def flip (a, idx):
  # Get the stack of pancakes form the top
  top = a[:idx]

  # Find where the biggest pancake is in 
  # that portion
  maxIdx = findMaxIdx (top)

  # Get slice that has the biggest pancake 
  # at the bottom
  slice = a[:maxIdx + 1]

  # Flip so that the biggest is on the top
  slice.reverse()

  # Now add the flipped portion and the 
  # unflipped portion
  top = slice + top[(maxIdx + 1):]

  # Flip again so that the biggest is 
  # in the right position
  top.reverse()

  # Copy the changes back to the original array
  for i in range (idx):
    a[i] = top[i]
  
def pancake (a):
  size = len(a)
  for i in range (size, 0, -1):
    flip (a, i)

def main ():
  a = [9, 1, 8, 2, 7, 3, 6, 4, 5]
  print (a)
  pancake (a)
  print (a)

main()