Your fitness function should have a good gradient over the entire search space. Some impossible schedules are more impossible than others. Take off X points for each instructor conflict, Y points for each room conflict, etc. You'll get continually closer to a valid schedule.
And that's also the "aha!" that I had when I wrote the code I pasted earlier. I was thinking too literally in terms of genetics. I would have considered "impossible" solutions as "stillborn" offspring that should never exist to roam around and mate, so I was stumped trying to figure out how to prevent such an individual from ever ending up in the gene pool.
Once I realized that I could just put them in the "undesirable" category by dinging them for unworkabilities, the GA did the right thing, and kept mixing up the "least undesirable", and within a dozen generations, I had a breeding population that was "in the black" (all hard constraints satisified, and thus now working on optimizing the soft constraints).
Well, I learned something. This is always a good thing.
Are you posting in the right place? Check out Where do I post X? to know for sure.
Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
<code> <a> <b> <big>
<blockquote> <br /> <dd>
<dl> <dt> <em> <font>
<h1> <h2> <h3> <h4>
<h5> <h6> <hr /> <i>
<li> <nbsp> <ol> <p>
<small> <strike> <strong>
<sub> <sup> <table>
<td> <th> <tr> <tt>
Snippets of code should be wrapped in
<code> tags not
<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
Want more info? How to link
or How to display code and escape characters
are good places to start.