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Comments on How this recursive treewalker works?

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How this recursive treewalker works?

+1
−4

Credit for User:Meriton for developing the following code (first published here).

function replaceIn(e) {
  if (e.nodeType == Node.TEXT_NODE) {
    e.nodeValue = e.nodeValue.replaceAll("a", "");
  } else {
    for (const child of e.childNodes) {
      replaceIn(child);
    }
  }
}

replaceIn(document.body);

How this recursive treewalker works?

As a side question which I grasp as important, can there be an even simpler and more direct version without the else?

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2 comment threads

As explained [here](https://software.codidact.com/posts/286333), there are different types of nodes. ... (8 comments)
What is the goal that you want to achieve? Why do you want to avoid using an if-else? (6 comments)
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+5
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Note: This answer was written for a version of the question that focused on getting rid of the if-else, rather than on an explanation of the workings of the recursive tree-traversal.

One way of getting rid of if-elses to achieve uniform handling of different objects is to hide the distinction in lower levels of abstraction:

Imagine a bowl of apples and bananas. Basically, you just want to eat all of them. However, if you one by one pick the bowl contents, you first have to check if you have a banana - which you need to peel before you bite into it. That is, your algorithm is "for each fruit: if banana, peel then eat, if apple, eat". You can abstract it by introducing a function "prepare_for_eating". Then your algorithm is "for each fruit: prepare_for_eating(fruit) then eat(fruit)". Seems as if the if-else has disappeared. But, if you look inside the "prepare_for_eating" function, you will again have the original if-else statement where you decide to peel or not to peel.

With object oriented techniques you can hide if-elses behind method calls: A Fruit base class would provide a member function "prepare_for_eating". For banana, this would contain the "peel" statement, for the apple, this would be empty. The algorithm would be "for each fruit: fruit.prepare_for_eating then fruit.eat". If fruit happens to be a banana, the banana member function doing the peel would be called. For an apple object, the apple member function would be called that does nothing.

You can use other tricks to get rid of if-elses by computing boolean expressions and use their result as zero or one in multiplications or table lookups. For example:

if x == 42 then x = -1

can be re-written as (assuming true can be used as 1 and false as 0):

x = (x == 42) * -1 + (x != 42) * x;

Or, using such a result for table lookups:

action_table = [peel, do_nothing]
for each fruit: call action_table[fruit is apple] then eat;

Whether any of the above ideas can be applied meaningfully to your problem and is suited to fulfill your expectations is up to you.

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2 comment threads

A question (3 comments)
Hello ! I understood everything in the answer besides this: x = (x == 42) * -1 + (x != 42) * x;... (8 comments)
Hello ! I understood everything in the answer besides this: x = (x == 42) * -1 + (x != 42) * x;...
deleted user wrote almost 2 years ago · edited almost 2 years ago

Hello !

I understood everything in the answer besides this:

x = (x == 42) * -1 + (x != 42) * x;

We put in x the result of -42 and then, given that x is not equal 42 we multiply -42 in itself to get +84?

deleted user wrote almost 2 years ago

Furthermore when I run x = (x == 42) * -1 + (x != 42) * x; in console I get:

Uncaught ReferenceError: x is not defined at :1:1

hkotsubo‭ wrote almost 2 years ago

deleted user You must assign a value to x before running that:

x = 42;
x = (x == 42) * -1 + (x != 42) * x;
deleted user wrote almost 2 years ago

hkotsubo‭ please help me to break it down to two steps a moment

x = 42;
x = (x == 42) * -1

Console result:

-1

Why -1 and not -42?

hkotsubo‭ wrote almost 2 years ago · edited almost 2 years ago

deleted user Note that (x == 42) is a comparison (it's checking if x is equals to 42), so the result is a boolean (it can be true or false).

In this case, the result is true. But when you multiply a boolean and a number, the boolean is automatically converted to a number (JavaScript does a lot of those automatic conversions - the fancy name is "type coercion"). And the boolean true, when converted to a number, results in 1, so the code is multiplying 1 by -1, hence the result is -1

deleted user wrote almost 2 years ago · edited almost 2 years ago

hkotsubo‭ I think I get it,

Since true was translated into a number 1 and we multiply it by -1 we get -1.

That said, x !=42 is false that would give us zero so -1 + 0 * x should give us -42, right?

Skipping 1 deleted comment.

deleted user wrote almost 2 years ago

hkotsubo‭ I don't know why you didn't reply to me here, perhaps you were baffled by some basic math knowledge I lack (this may well be the case as sadly I didn't learn much math in my past but I am always eager to fill in gaps if there are).

For community members in general, I have asked a specified question about this and hkotsubo replied there:

https://software.codidact.com/posts/286372

hkotsubo‭ wrote almost 2 years ago

@kalispero Sorry, I had too many unread notifications and probably missed this one