Computer Science 15-110, Spring 2011
Class Notes:  Conditionals

  1. The "if" statement
    1. "if'
    2. "if-else"
    3. "if-elif-...-else"
    4. Nested "if" statements
  2. Incorrect usage
    1. Needless Condition
    2. Negated Condition (with "else" clause)
    3. Using "if" instead of "and"
    4. Avoiding "else"

Conditionals

  1. The "if" statement 
    1. "if'

      n = int(raw_input("Enter an integer: "))
      if (n > 0):
          print n,"is positive"
      print "Amazing"

      Aside:  Same example with a nasty bug:  (try entering a negative number...)

      n = raw_input("Enter an integer: ")
      if (n > 0):
          print n,"is positive"
      print "Amazing"
       
    2. "if-else"

      n = int(raw_input("Enter an integer: "))
      if (n > 0):
          print n,"is positive"
      else:
          print n,"is not positive"
      print "Incredible!"
       
    3. "if-elif-...-else"

      n = int(raw_input("Enter an integer: "))
      if (n > 0):
          print n,"is positive"
      elif (n == 0):
          print n,"is zero"
      else:
          print n,"is negative"
      print "Who knew?"
       
    4. Nested "if" statements 
      if (today == "monday"):
         if (hour < 12):
             print "Alice"
         else:
           print "Bob"
      else:
         if (hour < 10)
             print "Chad"
         else:
             print "Donna"
  2. Incorrect usage
     
    1. Needless Condition
       
      Wrong Right
      if (b)
          return True
      else:
          return False
      		 return b


    2. Negated Condition (with "else" clause)
       
      Wrong Right
      if (not b):
          print "no"
      else:
          print "yes"

      Or:

      if (b == False):
          print "no"
      else:
          print "yes"
      		 if (b):
          print "yes"
      else:
          print "no"

      Or:

      if (b == True):
          print "yes"
      else:
          print "no"

       

    3. Using "if" instead of "and"
       
      Wrong Right
      if (b1):
          if (b2):
              print "both!"
      		 if (b1 and b2):
          print "both!"

    4. Avoiding "else"
       
      Wrong Right
      if (b):
          print "yes"
      if (not b):
          print "no"
      		 if (b):
          print "yes"
      else:
          print "no"

      Another Example:

      Wrong Right
      x = 10
      if (x < 5):
          print "small"
      if ((x >= 5) and (x < 10)):
          print "medium"
      if ((x >= 10) and (x < 15)):
          print "large"
      if (x >= 15):
          print "extra large"
      		 x = 10
      if (x < 5):
          print "small"
      elif (x < 10):
          print "medium"
      elif (x < 15):
          print "large"
      else:
          print "extra large"

      Yet Another Example:

      Wrong Right
      c = 'a'
      if ((c >= 'A') and (c <= 'Z')):
          print "Uppercase!"
      if ((c >= 'a') and (c <= 'z')):
          print "lowercase!"
      if ((c < 'A') or \
          ((c > 'Z') and (c < 'a')) or \
          (c > 'z')):
          print "not a letter!"      		 c = 'a'
      if ((c >= 'A') and (c <= 'Z')):
          print "Uppercase!"
      elif ((c >= 'a') and (c <= 'z')):
          print "lowercase!"
      else:
          print "not a letter!"

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