operator |=

[··········· sylvain]

hello,

what does the operator |= mean?

I saw it in a script :

$output = "png";

$style = BCS_ALIGN_CENTE R;
$style |= ($output == "png" ) ? BCS_IMAGE_PNG : 0;
$style |= ($output == "jpeg") ? BCS_IMAGE_JPEG : 0;


thanks

[Luke Ross]

Re: operator |=

Hi,

··········· sylvain wrote:[color=blue]
>
> what does the operator |= mean?
>
> I saw it in a script :
>
> $output = "png";
>
> $style = BCS_ALIGN_CENTE R;
> $style |= ($output == "png" ) ? BCS_IMAGE_PNG : 0;
> $style |= ($output == "jpeg") ? BCS_IMAGE_JPEG : 0;[/color]

I assume this is the same as the Perl operator, in which case it means,
if the left hand side isn't true, set it to the right hand side. Useful
if you want to set up defaults.

# If $a isn't set, set it to the string "undefined"
$a |= "undefined" ;

Regards,

Luke

[Zac Hester]

Re: operator |=

Luke Ross wrote:
[color=blue]
> Hi,
>
> ··········· sylvain wrote:
>[color=green]
>>
>> what does the operator |= mean?
>>
>> I saw it in a script :
>>
>> $output = "png";
>>
>> $style = BCS_ALIGN_CENTE R;
>> $style |= ($output == "png" ) ? BCS_IMAGE_PNG : 0;
>> $style |= ($output == "jpeg") ? BCS_IMAGE_JPEG : 0;[/color]
>
>
> I assume this is the same as the Perl operator, in which case it means,
> if the left hand side isn't true, set it to the right hand side. Useful
> if you want to set up defaults.
>
> # If $a isn't set, set it to the string "undefined"
> $a |= "undefined" ;
>
> Regards,
>
> Luke
>[/color]

I think you correctly answered the wrong question. He was asking what
the |= operator is doing, not the ?/: trinary operator. The trinary
operator in question does the same as the Perl conditional assignment
operator.

However, the |= operator is the unary equivalent of the bitwise OR. It
means the same thing as:

$lhs = $lhs | $rhs

So, this code:

<?
$lhs = 6;
$rhs = 13;
$lhs |= $rhs;
echo $lhs;
?>

Would display:

15

(Since 00000110 | 00001101 = 00001111)

The code he found is probably a streamlined way of conditionally setting
a binary sequence (or set of flags) within a preexisting bitfield. The
binary operator doesn't have any _expected_ effect on strings (notice
how the results of the expressions within the trinary operator are
probably both boolean [or a bitfield in the case of the second
operand--a full-out guess based on the fact that the second operand is a
"macroscopi c" definition]).

The trinary operator is also interesting in this example, though.

$style |= ($output == "png" ) ? BCS_IMAGE_PNG : 0;

Says this:

1. Check if the variable "$output" is equivalent to the constant "png"
2. If this expression is true, return the results of the definition
"BCS_IMAGE_ PNG" to the outer expression...
3. If this expression is false, return a constant 0 to the outer
expression...
4. The resulting macro or '0' is sent to the unary operator which
updates the variable "$style" using the bitwise OR.

If $output contains the string "png", and BCS_IMAGE_PNG is set to 1,
then set the low bit of the $style bitfield (makes the number odd and
adds 1 to the value, if it was even, _or_ just leaves the number odd and
maintains its value if it was already odd). The nice thing about this
method is that if $style already has some information in the bitfield, a
bitwise OR with "0" will not affect its contents.

The $style string is probably processed through a bit mask to extract
individual values from the field:

<?
function is_low_bit_set( $bitfield) {
return($bitfiel d & 1);
}
?>

This says that if the low bit is set, return a 1, otherwise, return a 0.
That means that any odd, integer will return a 1, and any even integer
will return a 0.

HTH,
Zac

[Andy Hassall]

Re: operator |=

On Fri, 19 Sep 2003 10:48:18 +0200, "·········· · sylvain"
<sylvainantispa m@sylvax.net> wrote:
[color=blue]
>what does the operator |= mean?
>
>I saw it in a script :
>
>$output = "png";
>
>$style = BCS_ALIGN_CENTE R;
>$style |= ($output == "png" ) ? BCS_IMAGE_PNG : 0;
>$style |= ($output == "jpeg") ? BCS_IMAGE_JPEG : 0;[/color]

$x |= $y is equivalent to $x = $x | $y

| is bitwise-or.

http://uk2.php.net/manual/en/language.operators.bitwise.php

e.g.

<pre>
<?php
printf("1 = %03b\n", 1);
printf("6 = %03b\n", 6);
printf("1 | 6 = %03b\n", 1|6);
?>
</pre>

Outputs:

1 = 001
6 = 110
1 | 6 = 111

--
Andy Hassall (andy@andyh.co. uk) icq(5747695) (http://www.andyh.co.uk)
Space: disk usage analysis tool (http://www.andyhsoftware.co.uk/space)

[BKDotCom]

Re: operator |=

More specifically,
$a |= $b;
is the same as
$a = $a || $b;

http://www.php.net/manual/en/language.operators.php

hey, look at that... a whole section of the manual dedicated to operators.

Luke Ross <lukeross@sys31 75.co.uk> wrote in message news:<vmli6itl5 sfd44@corp.supe rnews.com>...[color=blue]
> Hi,
>
> ··········· sylvain wrote:[color=green]
> >
> > what does the operator |= mean?
> >
> > I saw it in a script :
> >
> > $output = "png";
> >
> > $style = BCS_ALIGN_CENTE R;
> > $style |= ($output == "png" ) ? BCS_IMAGE_PNG : 0;
> > $style |= ($output == "jpeg") ? BCS_IMAGE_JPEG : 0;[/color]
>
> I assume this is the same as the Perl operator, in which case it means,
> if the left hand side isn't true, set it to the right hand side. Useful
> if you want to set up defaults.
>
> # If $a isn't set, set it to the string "undefined"
> $a |= "undefined" ;
>
> Regards,
>
> Luke[/color]

[Andy Hassall]

Re: operator |=

On 19 Sep 2003 13:46:25 -0700, BradKent@swbell .net (BKDotCom) wrote:
[color=blue]
>More specifically,
>$a |= $b;
>is the same as
>$a = $a || $b;[/color]

No, it's not.

$a |= $b is $a = $a | $b

$a | $b is bitwise OR.
$a || $b is logical OR.

<pre>
<?php
var_dump(1 | 4);
var_dump(1 || 4);
?>
</pre>

Outputs:

int(5)
bool(true)

--
Andy Hassall (andy@andyh.co. uk) icq(5747695) (http://www.andyh.co.uk)
Space: disk usage analysis tool (http://www.andyhsoftware.co.uk/space)