I then twiddled for the better part of an hour trying to find the clearest way to do weighted random choice with Lisp (loop …). This is the best that I have come up with:

The biggest pedagogical obstacle in the above is the (decf …) which decrements the variable in place by the given amount and returns the new value of the variable. What I really wanted to do was this:

That fails on multiple fronts, however.

• There is no tracking keyword in (loop …). I can sum or maximize or collect items, but I cannot keep track of the last one that I saw. I had hoped to use the finally keyword, but its value isn’t returned from the loop.
• Lisp requires that the decrement amount in the by of a downfrom be greater than zero. As such, I have to filter out any items that have zero weight. Feh. I can do that. I would rather not. But, I can do that.
• Lisp only evaluates the by expression once. It does not evaluate it each time through the loop.

I am still learning all of the ins and outs of (loop …). Today, I learned as many outs as ins. Feh.

I knew that I was going to have to track multiple versions of uniquely identifiable data with different versions stored in different locations.

I had all sorts of UML drawing on whiteboards and graph paper and in OmniGraffle. Everything just kept getting messy.

Earlier this week, I read a great article about Using SQLite on the iPhone. So, today, I tried to hammer out what I would do with this data if I were using SQL to track it. I wrote a bunch of CREATE TABLE statements and then a few SELECT statements.

It worked. Getting the JOINs in the SELECT statements to make sense forced me to make the relations in my data as simple as possible, but no simpler. I had been trying to jam many-to-many relationships into one-to-many or many-to-one relationships. The SQL exercise forced me to get it right.

Will I use SQL in the final project? Maybe, but it doesn’t seem like I will need it. Did I need to use SQL for the design? Absolutely.