Computer Science 15-112, Spring 2012
Class Notes:  Conditionals

1. Required Reading
2. The "if" statement
   1. "if'
   2. "if-else"
   3. "if-elif-...-else"
   4. Nested "if" statements
   5. The "dangling-else" problem
3. Complex Conditionals and Boolean Logic
4. The pass statement
5. Conditional Expressions
6. Incorrect usage
   1. Negated Condition (with "else" clause)
   2. Empty "if" Clause
   3. Using "if" instead of "and"
   4. Avoiding "else"
   5. Using boolean logic instead of if
   6. Using boolean arithmetic instead of if
7. Practice problems

Conditionals

1. Required Reading
   1. Python Language Reference, Ch 7.0 (Compound statements introduction)
      Be sure to understand the modified BNF and the syntax it describes!
   2. Python Language Reference, Ch 7.1 (The if statement)
      Ditto!
       
2. The "if" statement 
1. "if'
   
   if (1 < 2):
       print "yes!"
   if (5 < 4):
       print "really?"
   print "you bet!"
    
2. "if-else"
   
   if (5 < 4):
       print "really?"
   else:
       print "not really!"
   print "fascinating!"
    
3. "if-elif-...-else"
   
   if (5 < 4):
       print "really?"
   elif (4 < 3):
       print "curious!"
   elif (3 > 2):
       print "perhaps..."
   else:
       print "why not?"
   print "yippee!"
   
   A Better Example:
   
   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"
   
   Aside:  Same example with a nasty bug:
   
   n = raw_input("Enter an integer: ")
   if (n > 0):
       print n,"is positive"
   elif (n == 0):
       print n,"is zero"
   else:
       print n,"is negative"
    
4. Nested "if" statements

   if (2 < 1):
       if (3 == 3):
           print "a"
       else:
           print "b"
   else:
       if (4 == 4):
           print "c"
       else:
           print "d"

5. The "dangling-else" problem

   print "first:"
   if (2 < 1):
       if (3 == 3):
           print "a"
       else:
           print "b"
           
   print "second:"
   if (2 < 1):
       if (3 == 3):
           print "c"
   else:
           print "d"
   

3. Complex Conditionals and Boolean Logic
   

   age = 42
   if ((age >= 6) and (age <= 21)):
       print "student"
   elif (not ((age < 6) or (age > 75))):
       print "professional"

   Same logic, after applying DeMorgan's Law:

   age = 42
   if ((age >= 6) and (age <= 21)):
       print "student"
   elif ((age >= 6) and (age <= 75)):
       print "professional"

   Same logic again, sharing a common condition:

   age = 42
   if (age >= 6):
       if (age <= 21):
           print "student"
       elif (age <= 75):
           print "professional"

   And why we parenthesize...:
   

   print not True and False
   print not (True and False)
   

4. The pass statement
   

   if (2 < 1):
       pass
   else:
       print "sometimes you may need pass, but this is not one of them!"
   

5. Conditional Expressions 

   x = 3 if (1 < 2) else 4
   print x
   x = 5 if (1 > 2) else 6
   print x
   

   A Somewhat Better Example:

   p = 5
   print "I saw",p,"people" if (p>1) else "person"
   p = 1
   print "I saw",p,"people" if (p>1) else "person"
   
   

6. Incorrect usage
    
   1. Negated Condition (with "else" clause)
       

      Wrong	Right
      b = True if (not b):     print "no" else:     print "yes" Or: b = True if (b == False):     print "no" else:     print "yes"	b = True if (b):     print "yes" else:     print "no" Or: b = True if (b == True):     print "yes" else:     print "no"

       

   2. Empty "if" Clause
       

      Wrong	Right
      b = False if (b):     pass else:     print "no"	b = False if (not b):     print "no" Or: b = False if (b == False):     print "no"

       

   3. Using "if" instead of "and"
       

      Wrong	Right
      b1 = True b2 = True if (b1):     if (b2):         print "both!"	b1 = True b2 = True if (b1 and b2):     print "both!"

       

   4. Avoiding "else"
       

      Wrong	Right
      b = True if (b):     print "yes" if (not b):     print "no"	b = True 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!"

       
   5. Using boolean logic instead of if 

      Wrong	Right
      x = 42 y = ((x > 0) and 99)	x = 42 if (x > 0):     y = 99

      Another example:

      Wrong	Right
      x = 42 y = (((x > 0) and 99) or      ((x < 0) and 88) or      77)	x = 42 if (x > 0):     y = 99 elif (x < 0):     y = 88 else:     y = 77

       

   6. Using boolean arithmetic instead of if
       

      Wrong	Right
      x = 42 y = ((x > 0) * 99)	x = 42 if (x > 0):     y = 99 else:     y = 0 # or, if you prefer... y = 99 if (x > 0) else 0

       

7. Practice problems
   
   Write the following methods:   isEven, isOdd, isPositive, ...  Maybe solve hw1 problems using conditionals....

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