Outline for May 6, 2009

Reading: §8.4–8.6

  1. Booleans
    1. Values are True, False
    2. Values considered False: None, False, 0 of any type, empty string, empty list, empty sequence; all others True
    3. Variable assignment
    4. Short-circuit evaluation
  2. Operators and truth tables
    1. and, or, not
    2. A and B, A or B, not A
    3. Basic rules of Boolean algebra
      1. A and true == A, A and false == false
      2. A or true == true, A or false == A
      3. not (not A) == A
    4. Distributive laws
      1. A or (B and C) == (A or B) and (A or C)
      2. A and (B or C) == (A and B) or (A and C)
    5. De Morgan’s Laws
      1. not (A or B) == (not A) and (not B)
      2. not (A and B) == (not A) or (not B)
  3. Combining operations
    1. Precedence: who “binds” more tightly
    2. Here, “not” highest precedence; then “and”; then “or”
      1. a or not b and c is (a or ((not b) and c))
    3. c. Relational operators have higher precedence
      1. a == b and c == d is ((a == b) and (c == d))
    4. Danger: response == "Y" or "y" doesn’t do what you think
  4. Other types of loops
    1. break, continue (see loop1.py)
    2. Post-test (repeat ... until); put test at bottom
    3. Loop and a half (see loop2.py)
    4. Example (see sent2.py)