Class 2: expressions, flow control

Expressions

Make use of operators and variables to form small statements..


// overall evaluation happens right to left
int $i = $j + $k;
float $f = 2.718;
int $ang = 375 % 360; // modulus
int $k = ($j == $r); // always 0 or 1
int $v = ( ($f == $a) && ($g < $s) );
$i = 45; // OK
45 = $i; // *not* OK!
$i = ($f==3.1415); // OK
$i = (3.1415==$f); // also OK

Flow control - two (or three, including "recursion") ways

Looping

Two (three) ways:

'for' statement: two versions
'while' statement

Branching

Three variations:

'if-else' – binary branching
'case' – multiway branches
conditional assignment, using the '?' operator (and the ':' operator)
```
// if ($a==6)$k=5;else $k=556;
int $k = ($a==6)?5:556;
```

Fun

"Should I stay or should I go?"

Courtesy of Eric Keller:

if ($go) 
{
  $trouble = 1;
}
else
{
  $trouble = $trouble*2;
}

Recursion

Another powerful, elegant way to manipulate program flow is recursion, which MEL supports. Here's an example:

proc int iterativeFactorial(int $n)
{
   int $result = 1, $i;

   for($i=1;$i<=$n;$i++)
   {
     $result *= $i;
   }

   return $result;
}// iterativeFactorial(()

// run the non-recursive (loop-based) version:
int $rslt = iterativeFactorial(6);

proc int recursiveFactorial(int $n)
{
   if($n==0) 
     return 1; // 0!=1, by definition
   else
     return ($n*recursiveFactorial($n-1)); // recursion!!
}// recursiveFactorial(()

// test the recursive version
int $rslt = recursiveFactorial(6);

Recursion is very good for creating certain kinds of snowflake curves, tiling patterns, cloud/fireball shapes, plants/trees, etc.

Flow control - summary


To recap, ‘flow’ refers to flow of program statements,
which really reflect flow of logic.

Before you write a single line of MEL, make sure that
you can write your program out in pseudocode
on index cards, run through the main parts in your head
and explain it to your non-technical neighbor.

DESIGN, **then** CODE.
Coding is the “easy” part..

Extras

// "horn"
int $theta;
for($theta=0;$theta<360;$theta+=10)
{
  float $r=1.0;
  float $x = $r*cos($theta*3.14159278/180);
  float $y = $r*sin($theta*3.14159278/180);
  sphere -p $x $y 0.0 -r (.05+($theta*.001));
}

// grid of spheres
for($i=0;$i<5;$i++)
{
 for($j=0;$j<5;$j++)
 {
   sphere -r .1 -p $i $j 0.0;
 }
}

// box of spheres
proc boxOfSpheres(int $nS, float $rMin, float $rMax,int $size)
{
 int $i;
 for($i=0;$i<$nS;$i++)
 {
   float $x = rand(-$size,$size);
   float $y = rand(-$size,$size);
   float $z = rand(-$size,$size);
   float $r = rand($rMin,$rMax);
   print("rad is " + $r + "\n");
   sphere -p $x $y $z -r $r;
 }
}// boxOfSpheres

// usage:
boxOfSpheres(100,1,2,10);

Class 2: Expressions, flow control