# control statements in Python
#    for some students, Python's block structure
#    (using indentation instead of { .. } to define a block)
#    is the most difficult part of learning Python

def f():
  print "one"
  print "two"

# notice that the body of function f is indented
# recommendation:  do not use tabs to indent

# wrong example:

def g():
  print "one"
     print "two"

# don't make irregular indentation:  line statements up, please

# the if/else statement

def h(t):
  ' t is supposed to be tuple and h(t) returns True if t[0]>t[1]'
  if type(t) != type((1,)):  # test for tuple 
     return False
  if len(t) != 2:  return False 
  (a,b) = t
  if a>b:
     return True
  else:
     return False

# notice (1) that "if" doesn't need (...) around condition,
#           (2) that "if" statement needs ":" after the condition
#           (3) new block of statements needs more indentation
#           (4) for a one-liner, can put on same line as "if"

# other forms of "if"

def k(a,b,c): 
   if a>b: 
      if c>a:
         d =b-a
      else:
         d = b
   elif a==b:
      d = b**2  # Python uses ** for exponentiation (instead of ^)
   else:
      d = b+a
   if d>1000: return -1
   return d  

# recommendation:  don't use complicated if/elif/else logic
# (it makes code harder to read, and small indentation errors
#  could be hard to find unless you have some debugging tool)

# the "for" statement

def sum(sX):
   sum = 0
   for e in sX:  sum += e
   return e

# using range(...)

def sum2(sY):
    sum = 0
    for i in range(0,len(sY)):
      sum = sum + sY[i]
    return sum

# using while(...)

def sum3(sY):
    sum = 0
    while len(sY)>0:
       sum += sY[0]
       sY = sY[1:] 
    return sum

# another way to do the same thing (because + is commutative)

def sum4(sY):
  sum = 0
  while len(sY)>0:
    sum += sY[-1] # add last element of sY
    sY = sY[:-1]
  return sum

# note:  be careful modifying a list and using "while" as above,
# because Python's implementation may test the "while" condition 
# in a different way than what you expect, for example:

def elimNeg(a):
   i = 0
   while i<len(a):
     if a[i] < 0:  del a[i]
     i += 1
   return a

>>> elimNeg([2,2,2,-3,-3,-3])
[2, 2, 2, -3]

# same can happen with "for" statements, for instance

def elimPos(a):
  for x in a:
    if x > 0: a.remove(x)
  return a

>>> elimPos([2,2,2,-3,-3,-3])
[2, -3, -3, -3]

#  other control statements:  
#      continue, break, pass, try (to catch exceptions), raise, yield
#  you can read about these as needed

#  missing from Python:  "switch", "select", "goto"

#  Python has other exotic ways of looping:  
#        lambdas, list comprehensions
#  -- probably not useful in this course (of course, you can use
#  recursion if you like)