RE: [PHP] Unnecessary if statement? Programming technique
Hi,
Why not
for ($i = 0; $i 100/100; ++$i)
{
for ($j = 0; $j 100; ++$j)
{
// do something standard
}
// do something special for this occurence
}
Jared
-Original Message-
From: Steve McGill [mailto:[EMAIL PROTECTED]
Sent: 06 December 2005 10:18
To: php-general@lists.php.net
Subject: [PHP] Unnecessary if statement? Programming technique
Hi everyone
Quick question:
If I have such a loop:
?
for($i=0;$i100;$i++) {
if($i==100) {
}
// do something standard
}
?
In this case it seems such a waste that the if() statement is
done 99 times when it's not needed. Is there any obvious
trick that I am missing?
I'm not sure how taxing a simple if() statement is on a
server, maybe it's negligible, or is it something to worry about?
Something which I'd prefer NOT to do:
?
for($i=0;$i100;$i++) {
// do something standard
}
// do something special for $i = 100
for($i=101;$i100;$i++) {
// do something standard
}
?
as I would have have to either keep two copies of the code or
write a function just for this purpose, which hardly seems worth it.
Thanks to anyone who takes the time to think about my
question and/or respond.
Best wishes,
Steve McGill
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Re: [PHP] Unnecessary if statement? Programming technique
Jared Williams wrote: Why not for ($i = 0; $i 100/100; ++$i) This involves dividing 100 by 100 for each iteration of the loop. It would be better to test against 1. There is also the unwanted side-effect of executing the code on each hundredth iteration, which is unwanted (as far as I understand the problem). :) It would be interesting if Steve could divulge the greater problem that he is seeking a solution to. Cheers, David -- David Grant http://www.grant.org.uk/ -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
Re: [PHP] Unnecessary if statement? Programming technique
Hi,
David is right about the unwanted side-effect. Thanks for the idea though.
Unfortunately the 'greater problem' is not so great, I've just been doing
this for a while now and find myself programming loops like these so often
and I've never got round to testing if a simple IF statement is a major
drain on the CPU. Somehow I doubt it.
I got this reply from someone direct to my mail address, which seems to sum
it up:
--
In truth you are not evaluating the whole if block just the condition
and since its such a simple condition I can't see how it would be at
all taxing on the server. In your specific case I can't think of a
better way to do it either.
--
I'll try and think of a better example:
?
$bool = true; // this is set dynamically and not known in advance
while(true) {
if($bool) { // this condition tested in every single loop
// do first code
} else {
// do second code
}
}
?
and I am wondering if the compiler is smart enough to turn this into:
?
$bool = true; // this is set dynamically and not known in advance
if($bool) { // this condition only tested once
while(true) {
// do first code
}
} else {
while(true) {
// do second code
}
}
?
I realise this might be hard to follow without giving specific examples and
code.
In this case, the coding style of the 2nd example seems far better, but
sometimes the 2 blocks of code are practically identical and it's a
programmer's nightmare to have the blocks of code in 2 places and to
remember to keep them both updated.
I'm also assuming that using function calls is also much slower than
evaluating a very simple IF statement.
Thanks for your interest.
Best wishes,
Steve
David Grant [EMAIL PROTECTED] schreef in bericht
news:[EMAIL PROTECTED]
Jared Williams wrote:
Why not
for ($i = 0; $i 100/100; ++$i)
This involves dividing 100 by 100 for each iteration of the loop.
It would be better to test against 1.
There is also the unwanted side-effect of executing the code on each
hundredth iteration, which is unwanted (as far as I understand the
problem). :)
It would be interesting if Steve could divulge the greater problem that
he is seeking a solution to.
Cheers,
David
--
David Grant
http://www.grant.org.uk/
--
PHP General Mailing List (http://www.php.net/)
To unsubscribe, visit: http://www.php.net/unsub.php
Re: [PHP] Unnecessary if statement? Programming technique
Hi,
I imagine this kind of thing is not especially taxing on the processor,
especially if the condition is a fairly simple comparison. That said, I
have very little understanding aside from my own limited experience of
what runs slowly!
If you're worried about code maintenance, then move the code out to a
function and pass the parts that vary as parameters.
Cheers,
David Grant
Steve McGill wrote:
Hi,
David is right about the unwanted side-effect. Thanks for the idea though.
Unfortunately the 'greater problem' is not so great, I've just been doing
this for a while now and find myself programming loops like these so often
and I've never got round to testing if a simple IF statement is a major
drain on the CPU. Somehow I doubt it.
I got this reply from someone direct to my mail address, which seems to sum
it up:
--
In truth you are not evaluating the whole if block just the condition
and since its such a simple condition I can't see how it would be at
all taxing on the server. In your specific case I can't think of a
better way to do it either.
--
I'll try and think of a better example:
?
$bool = true; // this is set dynamically and not known in advance
while(true) {
if($bool) { // this condition tested in every single loop
// do first code
} else {
// do second code
}
}
?
and I am wondering if the compiler is smart enough to turn this into:
?
$bool = true; // this is set dynamically and not known in advance
if($bool) { // this condition only tested once
while(true) {
// do first code
}
} else {
while(true) {
// do second code
}
}
?
I realise this might be hard to follow without giving specific examples and
code.
In this case, the coding style of the 2nd example seems far better, but
sometimes the 2 blocks of code are practically identical and it's a
programmer's nightmare to have the blocks of code in 2 places and to
remember to keep them both updated.
I'm also assuming that using function calls is also much slower than
evaluating a very simple IF statement.
Thanks for your interest.
Best wishes,
Steve
David Grant [EMAIL PROTECTED] schreef in bericht
news:[EMAIL PROTECTED]
Jared Williams wrote:
Why not
for ($i = 0; $i 100/100; ++$i)
This involves dividing 100 by 100 for each iteration of the loop.
It would be better to test against 1.
There is also the unwanted side-effect of executing the code on each
hundredth iteration, which is unwanted (as far as I understand the
problem). :)
It would be interesting if Steve could divulge the greater problem that
he is seeking a solution to.
Cheers,
David
--
David Grant
http://www.grant.org.uk/
--
David Grant
http://www.grant.org.uk/
--
PHP General Mailing List (http://www.php.net/)
To unsubscribe, visit: http://www.php.net/unsub.php
Re: [PHP] Unnecessary if statement? Programming technique
This is probably going to sound strange, but I like to try to think outside the
box (buzzphrase!) and hit things at odd angles.
Would someone care to test (or already know) the performance difference between
a for loop and a foreach loop?
Or the performance difference over many iterations of if (something ==
somethingelse) versus if (true)?
Two examples to test:
?php
$arr = array_fill(0, 99, 'echo \test standard\;');
$arr[100] = 'echo \test special\;';
foreach ($arr as $value) {
eval($value);
}
?
or
?php
$arr = array_fill(0, 99, true);
$arr[100] = false;
foreach ($arr as $value) {
if ($value) {
// do standard
} else {
// do special
}
}
?
I'd be curious to see the benchmarks. I wonder if a if() versus doing the
value check in a for statement are different speed-wise.. I'm wondering how
much eval() slows it down.. and I'm wondering if if (value == value2) is
slower than if (true).
Probably other offbeat ways of doing something like this too.
-TG
= = = Original message = = =
Hi everyone
Quick question:
If I have such a loop:
?
for($i=0;$i100;$i++)
if($i==100)
// do something special for this occurence
// do something standard
?
In this case it seems such a waste that the if() statement is done 99
times when it's not needed. Is there any obvious trick that I am missing?
I'm not sure how taxing a simple if() statement is on a server, maybe it's
negligible, or is it something to worry about?
Something which I'd prefer NOT to do:
?
for($i=0;$i100;$i++)
// do something standard
// do something special for $i = 100
for($i=101;$i100;$i++)
// do something standard
?
as I would have have to either keep two copies of the code or write a
function just for this purpose, which hardly seems worth it.
Thanks to anyone who takes the time to think about my question and/or
respond.
Best wishes,
Steve McGill
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