js/src/perfect.js

   1 // Some simple testing of new, eval and some string stuff.
   2
   3 // constructor -- expression array initialization
   4 function ExprArray(n,v)
   5 {
   6     // Initializes n values to v coerced to a string.
   7     for (var i = 0; i < n; i++) {
   8         this[i] = "" + v;
   9     }
  10 }
  11
  12
  13 // Print the perfect numbers up to n and the sum expression for n's divisors.
  14 function perfect(n)
  15 {
  16     print("The perfect numbers up to " +  n + " are:");
  17
  18     // We build sumOfDivisors[i] to hold a string expression for
  19     // the sum of the divisors of i, excluding i itself.
  20     var sumOfDivisors = new ExprArray(n+1,1);
  21     for (var divisor = 2; divisor <= n; divisor++) {
  22         for (var j = divisor + divisor; j <= n; j += divisor) {
  23             sumOfDivisors[j] += " + " + divisor;
  24         }
  25         // At this point everything up to 'divisor' has its sumOfDivisors
  26         // expression calculated, so we can determine whether it's perfect
  27         // already by evaluating.
  28         if (eval(sumOfDivisors[divisor]) == divisor) {
  29             print("" + divisor + " = " + sumOfDivisors[divisor]);
  30         }
  31     }
  32     print("That's all.");
  33 }
  34
  35
  36 print("\nA number is 'perfect' if it is equal to the sum of its")
  37 print("divisors (excluding itself).\n");
  38 perfect(500);
  39