CSC161 2010F, Class 18: Control Flow: Conditionals (2)

Overview:
* If statements, revisited.
* Conditional expressions.
* Case statements.
* lab

Admin:
* Reading for Tuesday: K&R 3.5-3.6 (loops).
* New reading requirement: By 9 p.m. the night before the reading is due, send me a note about what you found most difficult.  I will try to cover those issues in class the next day.
* I will reserve time at the start of class for questions on the exam.

Questions on the Exam
* For Problem 3, 
  *When do we stop?
    * EOF
    * Or you can choose exit
  * Is each line like the command line?
    * No, you still have values on the stack
  * Do you need a p at the end of each line?
    * No, Sam just wasn't very creative in writing the example
    * But you won't get output if you don't have a p.
* For the triangles problem, how do I find out if a scalene triangle is
  obtuse or acute?  Do we have to use trig?  (Not really.)
  * Thing about the angle opposite the longest side.
    * In a right triangle, a^2 + b^2 = c^2
    * In an acute triangle, a^2 + b^2 ? c^2
    * In an obtuse triangle, a^2 + b^2 ? c^2
  * Or use Wolfram alpha
* For Problem 5, 
  * Why does 128.6 show up as 128.6000006?
    * Because of the way that C represents 128.6; it can't represent
      it exactly.

Sam's stupid question of the day
* What, exactly, does "Consistency is the hobgoblin of small minds" mean?
  * We use it when we want to say "Who cares?" to someone who wants us
    to be more consistent (say, about capitalization after bullet points)

Conditional Statements in C

if (TEST)
  CONSEQUENT;
else
  ALTERNATE;

If you want multiple consequents or alternates, put 'em in braces

if (TEST) 
  {
    CONSEQUENT1;
    CONSEQ@UENT2;
  }
else 
  { 
    ALTERNATE1;
    ALTERNATE2;
  }

You don't need the alternate (e.g., the alternate is "nothing")

if (TEST)
  CONSEQUENT;

E.g., if we're only looking at *this* (e.g., if we're only counting
  carriage returns)

if (ch == '\n')
  ++newlines;

You can nest ifs, in which case it tests each condition as it goes,
and "stops" when you get the first successful condition

if (TEST1)
  CONS1;
else if (TEST2)
  CONS2;
else if (TEST3)
  CONS3;
else
  ALTERNATE; // Note: We know that !TEST1 and !TEST2 and !TEST3

For testing ranges

if (x >= 90)
  printf ("A-like");
else if ((x >= 80) && (x < 90))
  printf ("B-like");
  ...

vs.

if (x >= 90)
  printf ("A-like");
else if (x >= 80) 
  printf ("B-like");
  ...

So, how do conditionals in C differ from conditionals in Scheme (other than
the new syntax)

* In Scheme, we use conditionals in (at least) two ways
  * To select between operations to do
    (if (equal? (car object) 'circle)
        (image-select-ellipse! image 10 10 50 50)
        (image-select-rectangle! image 10 10 50 50))
  * To choose between two values to use in an expression
    (square (if (> x y) x y))
* In C, these two versions require different syntaxes
  For the expression form, we use the legendary ?: syntax
  To compute the square root of the larger of x and y
  sqrt (x > y ? x : y)
  Form
  TEST ? CONSEQUENT : ALTERNATE
  3 + (x > y ? x : y)
  It is usable *anywhere* you can use an expression
  * There is no alternate-free version of ?:

Scheme provides one other form of conditional, the switch statement

switch (EXPRESSION) 
  {
    case VAL1:
    case VAL2:
      STATEMENTS;
    case VAL3:
      STATEMENTS;
      break;
    case VAL4:
      STATEMENTS;
      break;
    default:
      STATEMENTS;
      break;
  } 

Meaning:
1. Evaluate the expression
2. Find a matching case value (or default if no one matches)
3. Execute all subsequent code until you hit the first break or return