gskll gskll - 17 days ago 4
Javascript Question

Javascript IIFE changes result

I'm looking at projecteuler.net's 4th problem, and have come across a curious feature that I'm wondering if anyone could explain.

The following code returns 10001

var n = 999 * 999; //biggest product with 3 digit numbers
var x;

while (n>10000) { //smallest product of 3 digit numbers
if (n.toString() === n.toString().split('').reverse().join('')) {
x = Math.floor(Math.sqrt(n));

while (n % x !== 0 && x >= 100 && n/x <= 999) {
x--;
}

if (n % x === 0 && x>= 100 && n/x <= 999) {
n;
}
}
n--;
}


whereas when wrapped in an IIFE, it returns 906609 which is the correct answer.

(function euler4() {
var n = 999 * 999; //biggest product with 3 digit numbers
var x;

while (n>10000) { //smallest product of 3 digit numbers
if (n.toString() === n.toString().split('').reverse().join('')) {
x = Math.floor(Math.sqrt(n));

while (n % x !== 0 && x >= 100 && n/x <= 999) {
x--;
}

if (n % x === 0 && x>= 100 && n/x <= 999) {
return n;
}

}
n--;
}
}());


Does anybody know why? I can't find an explanation online. Cheers!

Answer

The lone n in the first does not terminate the algorithm, whereas the return n in the second does. This can be fixed by replacing n in the first with a simple break

var n = 999 * 999; //biggest product with 3 digit numbers
var x;

while (n>10000) { //smallest product of 3 digit numbers
  if (n.toString() === n.toString().split('').reverse().join('')) {
    x = Math.floor(Math.sqrt(n));

    while (n % x !== 0 && x >= 100 && n/x <= 999) {
      x--;
    }

    if (n % x === 0 && x>= 100 && n/x <= 999) {
     break;
    }   
  }
  n--;
}

console.log(n);