Question

To find all the types of events that are present in the data set, we simply process each entry in turn and then loop over the events in that entry. We keep an object phis that has correlation coefficients for all the event types we have seen so far. Whenever we run across a type that isn’t in the phis object yet, we compute its correlation and add it to the object.

function gatherCorrelations(journal) {
  var phis = {};
  for (var entry = 0; entry < journal.length; entry++) {
    var events = journal[entry].events;
    for (var i = 0; i < events.length; i++) {
      var event = events[i];
      if (!(event in phis))
        phis[event] = phi(tableFor(event, journal));
    }
  }
  return phis;
}

var correlations = gatherCorrelations(JOURNAL);
console.log(correlations.pizza);
// → 0.068599434

Let’s see what came out.

for (var event in correlations)
  console.log(event + ": " + correlations[event]);
// → carrot:   0.0140970969
// → exercise: 0.0685994341
// → weekend:  0.1371988681
// → bread:   -0.0757554019
// → pudding: -0.0648203724
// and so on...

Most correlations seem to lie close to zero. Eating carrots, bread, or pudding apparently does not trigger squirrel-lycanthropy. It does seem to occur somewhat more often on weekends, however. Let’s filter the results to show only correlations greater than 0.1 or less than -0.1.

for (var event in correlations) {
  var correlation = correlations[event];
  if (correlation > 0.1 || correlation < -0.1)
    console.log(event + ": " + correlation);
}
// → weekend:        0.1371988681
// → brushed teeth: -0.3805211953
// → candy:          0.1296407447
// → work:          -0.1371988681
// → spaghetti:      0.2425356250
// → reading:        0.1106828054
// → peanuts:        0.5902679812

A-ha! There are two factors whose correlation is clearly stronger than the others. Eating peanuts has a strong positive effect on the chance of turning into a squirrel, whereas brushing his teeth has a significant negative effect.

Interesting. Let’s try something.

for (var i = 0; i < JOURNAL.length; i++) {
  var entry = JOURNAL[i];
  if (hasEvent("peanuts", entry) &&
     !hasEvent("brushed teeth", entry))
    entry.events.push("peanut teeth");
}
console.log(phi(tableFor("peanut teeth", JOURNAL)));
// → 1

Well, that’s unmistakable! The phenomenon occurs precisely when Jacques eats peanuts and fails to brush his teeth. If only he weren’t such a slob about dental hygiene, he’d have never even noticed his affliction.

Knowing this, Jacques simply stops eating peanuts altogether and finds that this completely puts an end to his transformations.

All is well with Jacques for a while. But a few years later, he loses his job and is eventually forced to take employment with a circus, where he performs as The Incredible Squirrelman by stuffing his mouth with peanut butter before every show. One day, fed up with this pitiful existence, Jacques fails to change back into his human form, hops through a crack in the circus tent, and vanishes into the forest. He is never seen again.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.