speed: by reference or by value?

[Chameleon]

-------
$a = array(............);
$b = $a;
-------
php says that $b points to $a until $b or $a changes.
this means that the code above wants the same time with the code below:
-------
$a = $a = array(............);
$b = & $a;
-------

but when I check this with a script, I found this:
when I use & (ref) the script is faster.

in the code below: (php 5. WinXP. command line script)
first part needs 2.9 secs
second part (in "this line" & exists) needs 3.0 secs
second part (in "this line" & not exist) needs 6.5 secs but not $a nor
$b changes. So I believe that php creates a whole copy of $a in $b.

THE QUESTION:
If I want a FAST script, I must use & everywhere? (e.g. foreach ($b as &
$v); )

---------THE CODE------------------------------------------
<?
$a =
array(42,345,3245,3245,2345,4564,51346,3457234,137 62,4561,3465,23,451,345,32,45,23,45,23,4,76,3,45,1 4,6,45632,46,32,4);

$a['ddd'] = array(345,34,532,45,3245,3,45,2345,7643345,2645612 ,6423,562);

//------------------first
$t = microtime(true);
for($z = 0; $z < 1000000; $z++) {
foreach($a['ddd'] as $v);
}
$t = microtime(true) - $t;
echo "$t\n";
//------------------second
$t = microtime(true);
for($z = 0; $z < 1000000; $z++) {
$b = & $a['ddd']; // <--------- THIS LINE
foreach($b as $v);
}
$t = microtime(true) - $t;
echo "$t\n";
?>
----------------------------------------------------

[Chameleon]

Chameleon wrote:

-------
$a = array(............);
$b = $a;
-------
php says that $b points to $a until $b or $a changes.
this means that the code above wants the same time with the code below:
------- $a = $a = array(............);
error: I mean
$a = array(............);
$b = & $a;
-------

[Berimor]

On Sat, 03 Dec 2005 19:22:29 +0200, Chameleon
<ch******@hotmail.NOSPAM.com> wrote:

-------
$a = array(............);
$b = $a;
-------
php says that $b points to $a until $b or $a changes.
this means that the code above wants the same time with the code below:
-------
$a = $a = array(............);
$b = & $a;
-------

but when I check this with a script, I found this:
when I use & (ref) the script is faster.

in the code below: (php 5. WinXP. command line script)
first part needs 2.9 secs
second part (in "this line" & exists) needs 3.0 secs
second part (in "this line" & not exist) needs 6.5 secs but not $a nor
$b changes. So I believe that php creates a whole copy of $a in $b.

THE QUESTION:
If I want a FAST script, I must use & everywhere? (e.g. foreach ($b as &
$v); )

---------THE CODE------------------------------------------
<?
$a =
array(42,345,3245,3245,2345,4564,51346,3457234,137 62,4561,3465,23,451,345,32,45,23,45,23,4,76,3,45,1 4,6,45632,46,32,4);

$a['ddd'] = array(345,34,532,45,3245,3,45,2345,7643345,2645612 ,6423,562);

//------------------first
$t = microtime(true);
for($z = 0; $z < 1000000; $z++) {
foreach($a['ddd'] as $v);
}
$t = microtime(true) - $t;
echo "$t\n";
//------------------second
$t = microtime(true);
for($z = 0; $z < 1000000; $z++) {
$b = & $a['ddd']; // <--------- THIS LINE
foreach($b as $v);
}
$t = microtime(true) - $t;
echo "$t\n";
?>
----------------------------------------------------

No, you're wrong in your research. The reson is very unnoing though. You
treat the microtime() function wrong. Please read PHP manual - beacuse it
can hurt you once. My fixings shown that second part is really faster -
but only about 8% then first one.
Here is code:

<?
function getmicrotime(){
list($usec, $sec) = explode(" ",microtime());
return ((float)$usec + (float)$sec);
}

$a =
array(42,345,3245,3245,2345,4564,51346,3457234,137 62,4561,3465,23,451,345,32,45,23,45,23,4,76,3,45,1 4,6,45632,46,32,4);

$a['ddd'] = array(345,34,532,45,3245,3,45,2345,7643345,2645612 ,6423,562);

//------------------first
$time_start = getmicrotime();
for($z = 0; $z < 1000000; $z++) {
$b = & $a['ddd'];// <--------- THIS LINE
foreach($b as $v);
}
$time_end = getmicrotime();
$time = $time_end - $time_start;

echo "$time\n ";
//------------------second
$time_start = getmicrotime();
for($z = 0; $z < 1000000; $z++) {
foreach($a['ddd'] as $v);
}

$time_end = getmicrotime();
$time = $time_end - $time_start;

echo "$time\n";


?>

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[Chung Leong]

Chameleon wrote:

THE QUESTION:
If I want a FAST script, I must use & everywhere? (e.g. foreach ($b as &
$v); )

You should only use references when necessary, i.e. when you want the
referenced variable to be modifed somewhere else down the line.
Improper use of references will slow down your code considerably. For
example, if you time the following loops, you'll find that the second
one is much slower:

//------------------first
$t = microtime(true);
for($z = 0; $z < 1000000; $z++) {
$b = $a;
$c = $b;
}

//------------------second
$t = microtime(true);
for($z = 0; $z < 1000000; $z++) {
$b = $a;
$c =& $b;
}

Why is the second loop slow? That's because the reference assignment to
$c forces a copy of the array to be made. The by-reference and by-value
mechanism in PHP is quite unlike that in C/C++. By-value in PHP means
share-read while by-reference means share-read-write. A variable cannot
do both at the same time. If $c and $b are sharing read-write access,
then $a and $b cannot be sharing just read access, as $a and $c are not
sharing read-write access. A good analogy is Jack lending Jill a
picture book. She is supposed to just read it. If she wants to give it
to her little brother to draw on, she had better get her your copy,
since that wasn't the agreement between her and Jack.

If you can ensure that given piece of data is always passed
by-reference, then you might gain some performance benefits. If
somewhere along the line though, it was passed by-value, then you take
a performance hit. That's one of the reason why in PHP 5 objects are
always passed by-reference. Because invoking a method implies passing
the object by reference ($this), objects should never be passed
by-value.

[Chung Leong]

It's oft imprudent to tell others to read the manual without reading it
first ;-)

[Berimor]

On 3 Dec 2005 13:41:09 -0800, Chung Leong <ch***********@hotmail.com>
wrote:

It's oft imprudent to tell others to read the manual without reading it
first ;-)

Explain please.
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[Chung Leong]

See http://fi.php.net/microtime. Note the new parameter available for
PHP 5.

[Berimor]

On 3 Dec 2005 13:28:55 -0800, Chung Leong <ch***********@hotmail.com>
wrote:

Chameleon wrote:

THE QUESTION:
If I want a FAST script, I must use & everywhere? (e.g. foreach ($b as &
$v); )

You should only use references when necessary, i.e. when you want the
referenced variable to be modifed somewhere else down the line.
Improper use of references will slow down your code considerably. For
example, if you time the following loops, you'll find that the second
one is much slower:

//------------------first
$t = microtime(true);
for($z = 0; $z < 1000000; $z++) {
$b = $a;
$c = $b;
}

//------------------second
$t = microtime(true);
for($z = 0; $z < 1000000; $z++) {
$b = $a;
$c =& $b;
}

Why is the second loop slow?

To teach someone try it first by yourself!.
The first part (twice!!!!) slower then second one.
And do not compare C and PHP - its like to compare jet and aerostat.

Do try this code (but under php translator :) ) What will you say then?
<?
function getmicrotime(){
list($usec, $sec) = explode(" ",microtime());
return ((float)$usec + (float)$sec);
}


$a =
array(42,345,3245,3245,2345,4564,51346,3457234,137 62,4561,3465,23,451,345,32,45,23,45,23,4,76,3,45,1 4,6,45632,46,32,4);

$a['ddd'] = array(345,34,532,45,3245,3,45,2345,7643345,2645612 ,6423,562);

//------------------first
$time_start = getmicrotime();
for($z = 0; $z < 1000000; $z++) {
$b = & $a['ddd'];// <--------- THIS LINE
$b = $a;
$c = $b;
}
$time_end = getmicrotime();
$time = $time_end - $time_start;

echo "$time\n ";

//------------------second
$time_start = getmicrotime();
for($z = 0; $z < 1000000; $z++) {
$b = $a;
$c = &$b;
}

$time_end = getmicrotime();
$time = $time_end - $time_start;

echo "$time\n";

?>

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[Berimor]

On 3 Dec 2005 14:28:21 -0800, Chung Leong <ch***********@hotmail.com>
wrote:

See http://fi.php.net/microtime. Note the new parameter available for
PHP 5.

hmmm... Really. Well it's always hard to leave lovely things.



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[Berimor]

ah sorry,
shame on me :(



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[Chameleon]

> example, if you time the following loops, you'll find that the second

one is much slower:

//------------------first
$t = microtime(true);
for($z = 0; $z < 1000000; $z++) {
$b = $a;
$c = $b;
}

//------------------second
$t = microtime(true);
for($z = 0; $z < 1000000; $z++) {
$b = $a;
$c =& $b;
}

Why is the second loop slow? That's because the reference assignment to
$c forces a copy of the array to be made. The by-reference and by-value
mechanism in PHP is quite unlike that in C/C++. By-value in PHP means
share-read while by-reference means share-read-write. A variable cannot
do both at the same time. If $c and $b are sharing read-write access,
then $a and $b cannot be sharing just read access, as $a and $c are not
sharing read-write access. A good analogy is Jack lending Jill a
picture book. She is supposed to just read it. If she wants to give it
to her little brother to draw on, she had better get her your copy,
since that wasn't the agreement between her and Jack.

If you can ensure that given piece of data is always passed
by-reference, then you might gain some performance benefits. If
somewhere along the line though, it was passed by-value, then you take
a performance hit. That's one of the reason why in PHP 5 objects are
always passed by-reference. Because invoking a method implies passing
the object by reference ($this), objects should never be passed
by-value.

indeed.
your explanation sounds logical.

but I believe this is wrong in php core.
why php must copy the object (in our script: array) even if the pointed
data don't change?

I believe it is simple to implement this in C (php core) and I have
opinion but its hard to my to explain in english:

object $a: array
object $b: read pointer to $a
object $c: pointer to object $b

$b is always in the same place in heap, so even if data of $b is a
pointer to $a or copied data of $a has no sense for $c. $c always points
to $b object.
And objects which point to another objects have a flag that means: read
or read-write.

I never look for php core code. I imagine about the structure of it. So
maybe I am totally wrong.

Simply I think it is a bad architecture of php core if without changing
data we have data copy.

[Chung Leong]

Chameleon wrote:

but I believe this is wrong in php core.
why php must copy the object (in our script: array) even if the pointed
data don't change?

Perhaps it's possible to defer the copying until the write operation
actually, but I suspect the logic would be quite complicated. It's like
Jill trying into slip a new picture book under the hand of her little
brother start as he starts drawing.

But as I said, the use of reference usually denotes that changes will
occur. If you don't want variable separation to occur, then don't use
references.