From a different post today i used the example of list assignment and scalar assignment as being less and more efficient respectively.
use Benchmark 'cmpthese';
my ($x,$y)=(1,2);
cmpthese(-2,
{list =>sub {
($x,$y)=($y,$x) #list assign, slow
},
scalar=>sub{
#three lines. fast.
my $tmp=$x;
$x=$y;
$y=$tmp
}
}
);
__END__
Benchmark: running list, scalar, each for at least 2 CPU seconds...
list: ( 2.34 usr + 0.00 sys = 2.34 CPU) @ 550148.10/s (n=1288
+997)
scalar: ( 2.11 usr + 0.00 sys = 2.11 CPU) @ 878828.44/s (n=1854
+328)
Rate list scalar
list 550148/s -- -37%
scalar 878828/s 60% --
At bare minimum what you are doing with a hash slice is using list context. As the above shows list assignment is slower than the equivelent scalar assignments. The reason, once you think about how list assignment works is easy to understand. The right side of the assignment is evaluated and the results are placed into a temporary array/list. Then these results are poured into the variables on the left. This means that three temp variables (the list, and the two values) are created. In the scalar version only one temp variable is created. Thus the speed difference.
It wouldn't suprise me that there isn't really a good way to make list assignment as fast as scalar assignment is, irrelevent of the implementation language used, because of the semantic implications of the operation and the wide number of circumstances it needs to work within.
Yves
--
You are not ready to use symrefs unless you already know why they are bad. -- tadmc (CLPM)
-
Are you posting in the right place? Check out Where do I post X? to know for sure.
-
Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
<code> <a> <b> <big>
<blockquote> <br /> <dd>
<dl> <dt> <em> <font>
<h1> <h2> <h3> <h4>
<h5> <h6> <hr /> <i>
<li> <nbsp> <ol> <p>
<small> <strike> <strong>
<sub> <sup> <table>
<td> <th> <tr> <tt>
<u> <ul>
-
Snippets of code should be wrapped in
<code> tags not
<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
intervention).
-
Want more info? How to link
or How to display code and escape characters
are good places to start.