Beware Pseudo-Arrays

I'm always fascinated by the kinds of programming mistakes that novices make, and what they say about that student's state of mind. I particularly enjoy mistakes that aren't really mistakes, in the sense that the student has actually gotten the code to work, but where the code clearly demostrates some flaw in that student's mental model of programming. For example, see my previous post on Boolean confusion.

Here's another example involving arrays, or rather the lack of them. I was reminded of this example just yesterday when multiple teams tried to use this idea in a local programming contest.

Suppose you are rolling some standard six-sided dice, and keeping track of how many times each number has appeared. The most natural way to do this would be to use an array indexed from 1-6. In a language with zero-based arrays, you might use indices 0-5 and subtract 1 from each die value, or you might use indices 0-6 and simply ignore index 0.

However, many novices faced with this decision will reject arrays altogether and use six separate variables, perhaps named count1, count2, count3, count4, count5, and count6. (Or, if they are really gluttons for punishment, ones, twos, threes, fours, fives, and sixes.) I call these kinds of variables pseudo-arrays.

Why use pseudo-arrays? Because real arrays are scary. Plain variables and if-statements are much friendlier! After all, why write

   count[dieValue]++;

when you could write

   if (dieValue == 1) {
      count1++;
   }
   else if (dieValue == 2) {
      count2++;
   }
   else if (dieValue == 3) {
      count3++;
   }
   else if (dieValue == 4) {
      count4++;
   }
   else if (dieValue == 5) {
      count5++;
   }
   else {
      count6++;
   }

Oh, and don't bother suggesting a switch-statement — if-statements are good enough, thank you very much.

Why do so many novices do this? In many cases, I think it is because they were first exposed to if-statements, and found them easy, and then later were exposed to loops, and found them difficult. Arrays are usually processed using loops, and are usually introduced while the beginner is still reeling from the trauma of loops. Thus, arrays become tainted and suspect. In such a novice's mind, the approach with variables and if-statements truly is easier, especially because so many of the if-statements can be programmed using cut-and-paste — the favorite editing commands of most novice programmers!

Every once in a while, I see code by somebody who has gotten over their fear of loops, but has not yet become reconciled with arrays. This can lead to gems where the student tries to loop through a pseudo-array, as in

   for (int d = 1; d <= 6; d++) {
      if (d == 1) {
         System.out.println(count1);
      }
      else if (d == 2) {
         System.out.println(count2);
      }
      else if (d == 3) {
         System.out.println(count3);
      }
      else if (d == 4) {
         System.out.println(count4);
      }
      else if (d == 5) {
         System.out.println(count5);
      }
      else {
         System.out.println(count6);
      }
   }

As Dave Barry would say, I am not making this up.