On 08 October 2003 16:13, Chris Shiflett contributed these pearls of wisdom:
> --- "Ford, Mike [LSS]" <[EMAIL PROTECTED]>" <[EMAIL PROTECTED]>
> wrote:
>> Firstly, "or", as a Boolean operator requires two operands,
>> both of which must have an actual value.
>
> [snip]
>
>> statements don't have a value (and can't even be coerced to
>> have one), so "return" can't be valid as one of the operands
>> to "or".
>
> Based on yesterday's discussion, this seems to be a very
> common misconception.
No, it's not -- the misconception appears to be yours.
> I will try to clear it up.
And I will try to clear up your clearing up.
>
> Let's take a common use:
>
> mysql_query($sql) or die(mysql_error());
>
> There is a conditional here, but it has nothing to do with
> the die() part. The
> "or" is not the same as "||" in a conditional expression.
Only because it's got a different precedence -- in all other respects, ||
and or are identical (Boolean-or operator with lazy, or "short-circuit",
evaluation).
> The
> return of die() is not being evaluated.
That's because of the lazy evaluation of Boolean operators in PHP, nothing
to do with whether it's "or" or ||.
> Only the return of
> mysql_query() is.
> This can be
> rewritten as follows:
>
> if (!mysql_query($sql))
> {
> die(mysql_error());
> }
>
> If it helps, you can possibly replace "or" with "else" when
> you read these
> types of statements to clarify the use. It makes sense to me
> like it is.
>
> This really just boils down to language semantics, and it
> probably makes more
> sense to native English speakers than others. The word "or"
> means different
> things in the following two examples:
>
> 1. If you find some Coke or Pepsi, buy some.
>
> This suggests that either Coke or Pepsi will suffice. In PHP:
>
> if ($type == 'coke' || $type == 'pepsi')
OR: if ($type == 'coke' or $type == 'pepsi')
(Note that lazy evaluation applies in this case as well -- if $type is, in
fact, == 'coke', the $type == 'pepsi' condition will never be evaluated.)
> {
> buy_some();
> }
>
> 2. Bring me the antidote, or I will die.
>
> This suggests that if the antidote is not brought, the
> speaker will die. If it
> is, the "or I will die" part never happens. In PHP:
>
> bring_antidote() or die;
OR: bring_antidote() || die;
There really is no difference between the "or" and || operators apart from
their precedence (see, for example,
http://uk.php.net/manual/en/language.operators.php#language.operators.preced
ence and http://uk.php.net/manual/en/language.operators.logical.php), and
because of their ordering in relation to other operators it's really only in
combination with assignment operators that they can cause problems. For
example, consider the following:
$y = validate($x) or print "Invalid!";
This is evaluated as
($y = validate($x)) or print "Invalid!";
which is probably what was intended -- $y is assigned the the value of
validate($x), and then the print is only executed if that is false.
However:
$y = validate($x) || print "Invalid!";
is evaluated as:
$y = (validate($x) || print "Invalid!");
which will assign to $y the value of validate($x) if it is non-empty,
otherwise will execute the print and assign 1 (the return value of print) to
$y. This is probably not what the writer intended.
Cheers!
Mike
---------------------------------------------------------------------
Mike Ford, Electronic Information Services Adviser,
Learning Support Services, Learning & Information Services,
JG125, James Graham Building, Leeds Metropolitan University,
Beckett Park, LEEDS, LS6 3QS, United Kingdom
Email: [EMAIL PROTECTED]
Tel: +44 113 283 2600 extn 4730 Fax: +44 113 283 3211
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