Beefy Boxes and Bandwidth Generously Provided by pair Networks
Your skill will accomplish
what the force of many cannot
 
PerlMonks  

Re: Efficiency of map vs. more verbose basic/fundamental code

by BrowserUk (Pope)
on Oct 04, 2012 at 22:49 UTC ( #997339=note: print w/ replies, xml ) Need Help??


in reply to Efficiency of map vs. more verbose basic/fundamental code

Even so, I would like to understand in what ways the map version of the code above would be less efficient than the more verbose foreach loop?

You see the print part of the statement? That means the statement is going to take at least milliseconds regardless of what comes after it, so efficiency -- in terms of time -- is a red-herring. And if the hash is large, the sort is going to dominate the time required.

What the authors are probably clumsily trying to allude to it the memory (in)efficiency if the hash happens to contain a large number of keys. The one-line statement uses a lot of memory because it creates several intermediate stack based lists in order to process. If there are 1 million keys in %h, then this is what that looks like internally:

print <1e6 key/value/newline> map "$_: $h{$_}\n", <1e6 ordered keys> s +ort <1e6 unordered keys>;

You can see it can be memory hungry. Memory usage increases by 163 MB in order to execute that statement.

However, if the hash is (or could be) big enough for that to be a serious concern, then the book's alternative construction is almost as bad:

foreach $key ( <1e6 sorted items> sort <1e6 unordered items> keys %h) +{ print "$key: $h{$key}\n"; }

Here, the memory usage increases by 56 MB to process the loop. Better, but still pretty wasteful, but that cannot be avoided if you use sort.

However, you can avoid the need for spreading the single semantic operation -- display the hash pairs ordered by key -- over three lines, whilst avoiding the increased memory usage of using map, by doing it this way:

print "$_ : $h{ $_ }\n" for sort keys %h;

Clean, concise and a single line representing a single semantic operation; but then there are some books that will tell you that is "difficult (for beginners) to read".

But in the end, unless you are routinely dealing with huge datasets, all three are much of a muchness and it really comes down to what you personally prefer. Trying to guess what the next guy to come along will find most readable is a mug's game. If there are 3 ways to do something, whichever you choose you'll be wrong 2/3rds of the time.


With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

RIP Neil Armstrong


Comment on Re: Efficiency of map vs. more verbose basic/fundamental code
Select or Download Code
Re^2: Efficiency of map vs. more verbose basic/fundamental code
by aaron_baugher (Deacon) on Oct 05, 2012 at 11:58 UTC

    I find statement modifiers to be the clearest way to write code, but maybe I'm just used to them. Still, I'm having trouble seeing why turning a standard for loop into a statement modifier cuts down on the memory usage. Why are examples A and B below different? Is it that the parentheses cause a list to be created in memory? Would example C be as bad as A?

    # A for ( sort keys %h ){ dostuffwith($_); } # B dostuffwith($_) for sort keys %h; # C dostuffwith($_) for (sort keys %h);

    I guess I need to learn to benchmark memory usage.

    Aaron B.
    Available for small or large Perl jobs; see my home node.

      Still, I'm having trouble seeing why turning a standard for loop into a statement modifier cuts down on the memory usage.

      I didn't suggest it did.

      Only that it reduced the memory requirement compared to the map variant; and sourcecode/mindspace compared to the standard for loop.


      With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.

      RIP Neil Armstrong

        Ah, my misunderstanding, I see that you didn't suggest what I was assuming. I agree; the statement modifier form uses less mindspace for me too.

        Aaron B.
        Available for small or large Perl jobs; see my home node.

Log In?
Username:
Password:

What's my password?
Create A New User
Node Status?
node history
Node Type: note [id://997339]
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others chanting in the Monastery: (7)
As of 2014-08-29 08:16 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    The best computer themed movie is:











    Results (277 votes), past polls