Robust-first Computing Wiki

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===== Week One =====
To be copied here

===== Week Two =====

==== Q: Dynamic atom coloring? ====

We can override some properties of an element, such as its color, when we were programming C++ to create elements on the MFM. How can we do the same thing in ULAM? I want to create two types of Res with different colors. (This is a simplification of the Aura and Medicine element. Later I will convert them to real Aura and Medicine)  

**A: 28-May-2015 01:04:05PM-0600** There was a getColor() special function to allow the ULAM programmer to determine atom colors at runtime, but that mechanism is currently broken by the in-progress Site redesign efforts.  Hope for getColor or some replacement to reappear within a week!

==== Q: Access other atom data members? ====
Sometimes we want to get other Atom's data member. In the 'Aura and Medicine' game, a Medicine atom will compare the density that it observed with the density stored in another Aura atom. How can we access the data member of other atoms? I tried to write like this but got compile errors: 
<code>
Atom a = ew[slot];
Int yourDensity = a.density;
</code>   

**A: 29-May-2015 04:37:12AM-0600:** Probably the most straightforward is to use an 'is' expression combined with a cast:
<code>
Atom a = ew[slot];
if (a is Foo) {       
  Foo f = (Foo) a;    // Note that f is a copy of a!
  Int n = f.fooField;
  // ..use n for something..
} // else a is not a Foo
</code>
If you want to //modify// fields of an atom this way, you need to write the changes back:
<code>
element Foo {
  EventWindow ew;
  Int(7) fooField;

  Void behave() {
    if (ew[1] is Foo) {
      Foo f = (Foo) ew[1];  // Makes a copy
      ++f.fooField;         // Modifies the copy
      ew[1] = f;            // Updates eventwindow
    }
  }
}
</code>

==== Q: WindowServices next() returning -1? ====
I have a problem using ''WindowServices'' with a ''for'' loop. I printed out the value of ''ws.next()''. It is '-1'. I think that's why the code inside the for loop is not performed. Please tell me again how to pick a nearby neighbor in my EventWindow so that I can read or modify its fields.

<code>   
WindowServises ws;
ws.reset(0,4);
Signal s;
Int st=au.getType((Atom) s);
if(ws.scan(st)){
  density=ws.getHits(0);
  if(density>0){
    du.print((Int) ws.next());
    for (Int slot = ws.next(); slot >= 0; slot = ws.next()) {
       Atom a = ew[slot];
       if (a is Signal) {
          //compare your density with mine
       }
    }
}    
</code>

**A: 29-May-2015 06:24:01PM-0600:** The ''WindowServices next()'' method returns -1 when there are no more slots to scan in your chosen range -- but the main issue here is: You don't want to use //both// a ''ws.scan()'' call //and// a for loop with ''ws.next'', you want one or the other.  If you use scan, then you use ''getHits'' to see if anything matched, and ''getPick'' to get a randomly-chosen match.  That code has got other issues too (e.g., ''WindowServi__**c**__es'').

==== Q: How to access slot number when using ''WindowServices scan()''? ====
//Thank you!// I fixed the code. This time I use the for loop on ws.next(). I wanted to use ''ws.scan(someType)'' but then if I used ''ws.getPick(0)'' to find a match, how would I know its actual 'slot number'? 
<code>
WindowServices ws;
Random rdm;
ws.reset(1,4);
Request r;
Int rt=au.getType((Atom) r);
for(Int slot=ws.next();slot>=0;slot=ws.next()){
  Atom a= ew[slot];
  if(a is Request){
    Request you=(Request) a;
    if(density>you.density){
      if(rdm.oneIn(2)){
        ew.swap(0,slot);
      }
    }
  }
}
</code>

**A: 29-May-2015 11:08:39PM-0600:** As long as the corresponding ''ws.getHits'' returns greater than zero, then return value of ''ws.getPick'' //is// the slot number!  You can do stuff like:

<code>
WindowServices ws;
ws.reset(1,4);
Request r;
Int rt=au.getType((Atom) r);
if (ws.scan(rt)) {            // Any matches for rt?
  Atom a = ew[ws.getPick(0)]; // Yes! Access chosen slot
  if(a is Request){           // This if will always succeed..
    r = (Request) a;
    .. use r.density etc ..
  }
}
</code>

or even this:
<code>
WindowServices ws;
ws.reset(1,4);
Request r;
Int rt=au.getType((Atom) r);  // Get type of element Request
if (ws.scan(rt)) {            // Any matches for that?
  r = (Request) ew[ws.getPick(0)]; // Yes! Access slot of chosen one
  .. use r.density etc ..
}
</code>
because that particular ''ws.scan(rt)'' can only return true if ''ws.getPick(0)'' is going to return the slot number of a ''Request'' atom.  (Though it wouldn't be that easy if you were scanning for multiple things simultaneously.)