Eloquent Javascript's findSequence clarification

Question

I am stuck on the following function which appears in a few other posts I've also reviewed.

function findSequence(goal) {
  function find(start, history) {
    if (start == goal)
      return history;
    else if (start > goal)
      return null;
   else
      return find(start + 5, "(" + history + " + 5)") ||
             find(start * 3, "(" + history + " * 3)");
 }
  return find(1, "1");
}

print(findSequence(24));

Also given in this link.

Javascript..totally lost in this tutorial

In the above explanation, the answer instead tried to set a goal of 11. They have a start of 1, which is first tested against 11, and then a start of 6 which is tested against 11.

I understand these first two steps. However, I do not understand the leap from the 2nd step (comparing start:6 to goal:11) to the third step (comparing start:3 to goal:11).

How does start go from 6, back down to 3, and then back up to 11 (fourth bullet)?

This explanation in the post is very useful, which is why I provided the link instead. — KMcA
– KMcA, Commented Oct 27, 2012 at 3:09
check this answer - maybe it can clarify. stackoverflow.com/questions/7540111/… — c69
– c69, Commented Oct 27, 2012 at 3:17
c69, can you try to explain my question. I've also reviewed that post. — KMcA
– KMcA, Commented Oct 27, 2012 at 3:19
the *3 is only evaluated if the left-hand side of || evaluates to false. For example type "" || "aaa" in your JS console and then type "bbb" || "ccc" — pedrofurla
– pedrofurla, Commented Oct 27, 2012 at 3:39

c69 · Accepted Answer · 2012-10-27 03:39:00Z

7

Ok, here is a version of the code which was enhanced with console log statements. Open Chrome/Opera/Firefox eveloper tools and execute this code there:

function findSequence (goal) {
  function find (start, history, depth) {
    depth = depth || 0;
    console.log( Array( ++depth ).join('--> '), start, goal, history );
    if (start == goal) {
      console.warn( 'history' );
      return history;
    } else if (start > goal) {
      console.error( 'null' );
      return null;
    } else {
      console.info('recursion!');
      return find(start + 5, "(" + history + " + 5)", depth) ||
             find(start * 3, "(" + history + " * 3)", depth);
    }
  }
  return find(1, "1");
}

console.info( findSequence(24) );

You will get a call trace of this program, and hopefully will grasp the concept of recursion visually, by looking on the trace.

answered Oct 27, 2012 at 3:39

c69

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3 Comments

pedrofurla Over a year ago

I agree this will help him understand concept but by his comments I'd say his confusion lies in the ||.

KMcA Over a year ago

That is exactly what I needed to see and answers my question perfectly. Thank you very much.

KMcA Over a year ago

pedrofurla, so, since the left side eventually returns null, it's is false, and triggers the right side to begin. Correct?

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