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Well, assume something like this:
if
($tformat =~ /^[ymd]{1,4}([^ymd]{1})[ymd]{1,4}[^ymd]{1}[ymd]{1,4}$/i
or
$tformat =~
/^([ymd]{1,4})[^ymd]{1}([ymd]{1,4})[^ymd]{1}([ymd]{1,4})$/i)
{
}
Actually this is a weird example since I have accomplished with something
different what I needed. However I believe it would be good for me to know for a
future case if the short-circuit will avoid extra processing. As you can see, on
the second part of the OR I have three capturing parentheses which create $1,
$2, and $3 which are more expensive that just $1. The perldocs state that once
used, (in this case $2 and $3) they have to be provided for every match. So if
my program matches the left side OR it would not provide $2 and $3 to every
other regex, thus saving resources. I want to know if this is
true.
Regards,
Javier Moreno
==============
Softtek/GXS
EFS NearShore - TradeWeb 2nd Tier
Phone: (5281)
81-53-2082
Dialcomm: *879-2918
...when you have eliminated the
impossible, whatever remains, however improbable, must be the truth.
- Sherlock Holmes
Oops, meant to send this to the
list...
Why doesn't the list put its own address in the
reply-to field of all messages delivered? Most of the time, we'll want to
reply to the list.
J-S
----- Original Message -----
Sent: Thursday, July 31, 2003 9:06 AM
Subject: Re: performance
Tim,
> A
statement that results in a true or false with a defined directive has to be
completed fully before the directive can be
met.
Not true. As the original poster noted, the
logical operators in almost any language are short-circuit (including Perl,
C/C++, Java, etc). That means that if the _expression_ is sure to return true
after only evaluating, for example, the left side of an OR operation, then the
right side will never be evaluated. (because True OR <anything> = True
and False AND <anything> = False)
Try this simple script :
____________________ start script
____________________
#!/usr/bin/perl -w
use strict;
sub isExecuted {
print "I
was executed!\n";
return 1;
}
if (1 || isExecuted) {
print "First condition True\n";
}
else {
print
"First condition False\n";
}
print "\n";
if (0 || isExecuted) {
print "Second condition True\n";
}
else {
print
"Second condition False\n";
}
print "\n";
if (1 && isExecuted)
{
print "Third condition True\n";
}
else
{
print "Third condition False\n";
}
print "\n";
if (0 && isExecuted)
{
print "Fourth condition True\n";
}
else
{
print "Fourth condition False\n";
}
____________________ end script
____________________
The results should be:
____________________ start output
____________________
First condition True
I was executed!
Second condition True
I was executed!
Third condition True
Fourth condition False
____________________ end output
____________________
The implications of this is that if you have a boolean _expression_ that
depends on a variable and a function call, the result of both being given to
an AND or OR operator to give the final result, you should always put the
variable first, so that Perl can short-circuit without ever calling the
function. The other way around would mean that the function would always be
called, no matter what the variable's value is.
But to return to the original poster's question, could you give us some
example code? I'm not sure what you mean by "This IF captures one parentheses and the 'or' part captures
three on regexes."... With an example we might
be able to answer your question.
J-S
----- Original Message -----
Sent: Wednesday, July 30, 2003 5:17
PM
Subject: RE: performance
I
can't speak for the interpretation of Perl (as an interpretive langauge) in
the processing of a command structure or statement (though the Benchmark module is a useful way to judge code performance).
However, I can speak about the way "all" langauges end up, on any
platform, as a set of memory locations that set or hold values
dependant on flow results (what used to be called machine code and is now
"sadly" missing from parlance - is that that its sad or I am
?).
A statement that
results in a true or false with a defined directive has to be completed
fully before the directive can be met. In other words if you stack a load of
decisions in a pile you can never jump out of the pile with an answer until
all parts of the stack has been processed. The deeper the stack - the deeper
the process (or interupt) has to run to obtain an answer.
Logic.
In the days heady
of Basic interpretation (god I'm old) the stacks used to be very very deep
and we only had 8 bits to play with in our stacks - these days the
registries where these stacks reside are much wider (32bits etc) - so the
interpreted code gets "streamlined" into single sweeps of process (or
interupt). But, I am certain that my initial premis holds true - if you need
to make a decision, and even if you get the answer straight away, you still
have to pass through the stack before you can act on it. I doubt perl does
it different as its based/built on C (I do doubt it is - but please let
me know if you know different - "I would be
impressed").
So in answer - and
remember the overhead of the interpretation will usually be greater than any
of the questions asked within it - the deeper you make a stack the more
process and shuffling of memory locations is going to be needed - the longer
it is going to take to answer the
question.
All this is
sub-bonnet (ie deep in the bowels of your CPU etc) - I would suggest you try
different ways of scripting the questions - run them with Benchmark (or your
stop watch of choice) and answer each specific point
individually.
One thing I have
learned here - there is a billion ways to do everything in Perl - the right
way is your way.
T
Hi all,
Would anyone know this? I am
attempting a data-formatting function. I know the 'or' operator will
short-circuit. So I have an IF that has an 'or' operator. This IF captures
one parentheses and the 'or' part captures three on regexes. I know that
the more captured parentheses the greater penalty on performance. So I am
wondering if my program short-circuits with the or, will there be no
penalty for those non-processed three capturing parentheses?
Regards,
Javier Moreno
==============
Softtek/GXS
EFS NearShore - TradeWeb 2nd
Tier
...when you have eliminated the
impossible, whatever remains, however improbable, must be the
truth.
-
Sherlock Holmes
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