> -----Original Message-----
> From: Chris Knipe [mailto:[EMAIL PROTECTED]]
> Sent: 10 May 2002 04:38
> Just to clarify myself above...
> if ($var1 = $var2) { .....
> In my crazy head, I see $var1 and $var2 to be in a sort of 
> "read-only" state
> inside the () of the if statement.  For a if statement to be 
> able to change
> the values in that instance, would be wrong for me.  
> Afterall, the logic of
> a if statement does go something in the lines of "if value 
> equals value then
> execute this, else, execute that" etc etc etc.... There's 
> nothing in that
> logic of setting or changing any values.... ?

That's not really correct.  Strictly speaking, what's inside the () of an if (or 
while, or switch, ...) is an expression  -- and in PHP (as in many C-like languages) 
an assignment is actually an expression the result of which is the value assigned.  
This is why constructs such as

    a = b = c = 0

work: the assignment expression "c = 0" is first evaluated, giving the result 0; this 
result is then assigned to b, also giving the result 0, which is then assigned to a.

But back to the if problem; consider:

    if (func($y)):
       // lots of code using func($y) several times

Well, if func($y) is some big function that is costly to execute, you really don't 
want to be re-evaluating it each time you want the result, so you might think of 
recoding this as:

    $x = func($y);
    if ($x):
        // lots of code using $x several times

which only evaluates func($y) once.  But using the assignment-as-expression technique, 
you can shorthand this as:

    if ($x = func($y)):
        // lots of code using $x several times

Now, this might no seem like much of an advantage, but look at this while loop:

    while ($x = func($y)):
        // lots of code using $x several times

Without the assignment-as-expression in the while (), you'd have to code this as:

    $x = func($y);
    while ($x):
        // lots of code using $x several times
        $x = func($y);

Not only are you having to duplicate the assignment, the duplicates may be several 
hundred lines of code apart; the shorthand is clearer, less error-prone, and probably 
fractionally more efficient to boot!

Of course, this can also lead to strange-looking constructs such as:

    if ( ($x = func($y)) == 1):

which, when you break it down, actually makes perfect sense (exercise for the reader!)

Finally, a quick reference for =, ==, and ===:

    =    assignment operator
    ==   equality test (values must be equivalent)
    ===  "strict equality" test (values AND TYPES must match)

NOTE: the == equality test checks for equivalent values, using automatic type 
conversion if necessary; this can sometimes lead to unexpected results, since 0 == 
"abc" and "0" == FALSE.  Use the === operator if you want to be sure that two values 
are absolutely identical.

Hope this little lot helps a bit!



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
Tel: +44 113 283 2600 extn 4730      Fax:  +44 113 283 3211 

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