CSC152 2006S, Class 11: Conditionals

Admin:
* Sorry about the readings for today
* Homework revised: One-line calculator
* Readings for Friday available some time Thursday
* Feel free to ask questions today!  (As always)

Overview:
* Form of conditionals
* Form of tests in conditionals
* Lab

Most languages have one or more control structures that let
you say
"If this condition holds then do this thing, otherwise do this other thing"

In Scheme
* (cond
    ((this condition) (this thing))
    ((another condition) (another thing))
    ...
    (else (something-to-do-if-none-of-the-conditions-hold)))
* (if (this condition)
      (this thing)
      (this other thing))
* Java provides only if (and an ugly thing called "switch")

        if (TEST) {
          CODE_TO_RUN_IF_TEST_SUCCEEDS;
        }
        else {
          CODE_TO_RUN_IF_TEST_FAILS;
        }

* When you want to do multiple tests, as in a cond
        if (TEST1) {
          CODE_TO_RUN_IF_TEST1_SUCCEEDS;
        }
        else if (TEST2) {
          CODE_TO_RUN_IF_TEST1_FAILS_AND_TEST2_SUCCEEDS;
        }
        else if (TEST3) {
          ....
        }
        ...
        else {
          CODE_TO_RUN_IF_NO_TEST_SUCCEEDS;

Form of tests:

* Arithmetic comparison
        x < y
        x <= y
        x == y ;        Equality uses two equal signs
        x > y
        x >= y
        x != y ;        x is not equal to y

* Combine tests
        test1 || test2  ; OR, pronounced "bar bar" or "pipe pipe" or "or or"
        test1 && test2  ; AND , pronounced "and and"
        !test1          ; NOT , pronounced "bang"

* Example, is i between 1 and 10?
        if ((i >= 1) && (i <= 10)) {
                ...
        }

* In dealing with objects, you need to use methods rather than those cool one-or-two-character operations

      * obj.equals(other)
        if (textuserentered.equals(password)) {
                pen.println("Welcome, you have entered the correct password";
        }
        else {
                toaster.lasergun.zap(user);
        }

      * obj.compareTo(other)
        * Returns a negative number if obj "naturally precedes" other
        * Returns 0 if obj is equal to other
        * Returns a positive number of obj "naturally follows" other
      * compareTo is supposed to be transitive and reflexive
        * Transitive if a naturally precedes b and b naturally precedes c then a naturally precedes c
        * Reflexive: if a nabutrally precedes b then b natrually follows a
        * Misspelled: This eboard