Optimization Tips

thomthom

And why is for faster then each?

Looking at the source code for Array.each:


VALUE
rb_ary_each(ary)
    VALUE ary;
{
    long i;

    for (i=0; i<RARRAY(ary)->len; i++) {
        rb_yield(RARRAY(ary)->ptr[i]);
    }
    return ary;
}

It's using for as well, and the whole loop is done in C - so why isn't this C for loop faster than a ruby for loop?

AdamB

OT: Any chance the forum administrator of SCF can fix the [ruby] tag to not remove formatting. Formatting is a big part of understanding code, and while for regular text collapsing whitespace down to a single space might work, for code it does not.

thomthom

@adamb said:

OT: Any chance the forum administrator of SCF can fix the [ruby:38zsi59i]tag to not remove formatting. Formatting is a big part of understanding code, and while for regular text collapsing whitespace down to a single space might work, for code it does not.

I think the ruby is meant for inline code. While you got the code tag for block codes. (Though I wish there was a way to expand it - I loathe internal scrollbars.)

thomthom

Interesting test Adam:

doit 6.474 3.292 nil

Note: I increased the number of iterations ( 10000000.times { ... })

thomthom

Didn't realise Ruby would recreate the variables for each iteration. I'd thought it'd keep them for the duration of the loop...

AdamB

Seems an arbitrary (and wrong) assumption that inline code requires removing whitespace. Why not just leave in what the author wrote rather than trying to second guess? Whatever.

thomthom

Agree - whitespace eating of ruby has bothered me as well. Will ask if it can be changed.

AdamB

@thomthom said:

Didn't realise Ruby would recreate the variables for each iteration. I'd thought it'd keep them for the duration of the loop...

The closure you create with curly braces is handled as a first class object and passed as an argument to the iterator. This means the scope of any variables you mention inside that block is limited to that block - it must create them each time.

thomthom

Is that why each is slow?

Gaieus

Guys, Thom asked if we can do something with these white spaces but I have to say it is most probable that we cannot. I is hard coded somewhere in the php script of the forum software and even if we could tweak that, it would be impossible to keep it through upgrades (which is very due soon anyway).

Is the code tag not good (apart from that scrolling annoyance)?

thomthom

It's ok. I just hoped there was a config UI for BBCode tags on the forum. Thought it was normal. The code tag is ok, just figured if it could be changed...

I don't suppose there are forum plugins that can be installed? having the code block apply syntax highlighting would be a delight for us coders. Such as this:

http://code.google.com/p/syntaxhighlighter/

Edit: what version of phpBB does SCF run? I'm looking at this: http://www.phpbb.com/kb/article/adding-custom-bbcodes-in-phpbb3/ from this it appear to be that it'd be a matter of setting the HTML replacement for the ruby tag to not collapse white space using CSS.

Replacement sample something like this:
<span style="white-space:pre;">{TEXT}</span>

Gaieus

I can imagine you would like that syntax highlight! I use Notepad++ and know what a difference it is!

Coen and Tavi should be spoken to about these things.

thomthom

@adamb said:

I see a lot of SU scripts using some of the more compact iterators Ruby iterators. So they might read nice, but they're often slower than just simple for-loops.

http://blog.shingara.fr/each-vs-for.html

In regard to this should one init the variables used by for in to speed up things? or is that not needed?

Would this
` x = 0
for x in collection

...

endbe faster than for x in collection

...

end`

AdamB

no

Jim

I've always thought for used each under the hood.

http://blog.grayproductions.net/articles/the_evils_of_the_for_loop

for loops do not have their own scope - the loop variable and any variables created in the loop become available (or are over-written) in the current scope.

With .each, variables are local to the block {..}

thomthom

@jim said:

I've always thought for used each under the hood.

http://blog.grayproductions.net/articles/the_evils_of_the_for_loop

for loops do not have their own scope - the loop variable and any variables created in the loop become available (or are over-written) in the current scope.

If you click the method names in the Ruby API manual you get to see the sourcecode:
http://ruby-doc.org/core/classes/Array.html#M002173

Jim

That's showing a for loop in the c language.

thomthom

That's what it's doing under the hood.

Jim

@thomthom said:

That's what it's doing under the hood.

Right, so where is the definition for the for function?

The answer is there isn't one because for is not a function, but is "sugar". The for loop in Ruby really uses the .each method behind the scenes.

Although, I can't recall where I learned that. The link to the blog article mentions it, though.

tbd

speaking of each vs for :

loop1 = []
loop2 = []

calls = ["one", "two", "three"]

calls.each do |c|
  loop1 << Proc.new { puts c }
end

for c in calls
  loop2 << Proc.new { puts c }
end

loop1[1].call #=> "two"
loop2[1].call #=> "three"