On 31 March 2010 05:45, Matty Sarro <msarro@xxxxxxxxx> wrote:
> That explains it perfectly, thanks you!
>
> On Wed, Mar 31, 2010 at 12:40 AM, Tommy Pham <tommyhp2@xxxxxxxxx> wrote:
>
>> On Tue, Mar 30, 2010 at 9:22 PM, Matty Sarro <msarro@xxxxxxxxx> wrote:
>> > Hey all!
>> > This is probably my second post on the list, so please be gentle. Right
>> now
>> > I am running through the "Heads First PHP and MySQL" book from O'Reilly.
>> > It's been quite enjoyable so far, but I have some questions about some of
>> > the code they're using in one of the chapters.
>> >
>> > Basically the code is retreiving rows from a DB, and I'm just not getting
>> > the explanation of how it works.
>> >
>> > Here's the code:
>> >
>> > $result=mysqli_query($dbc,$query)
>> >
>> > while($row=mysqli_fetch_array($result)){
>> > echo $row['first_name'].' '.$row['last_name'].' : '. $row['email'] . '<br
>> > />';
>> > }
>> >
>> > Now, I know what it does, but I don't understand how the conditional
>> > statement in the while loop works. Isn't an assignment operation always
>> > going to result in a "true" condition? Even if
>> mysqli_fetch_array($result)
>> > returned empty values (or null) wouldn't the actual assignment to $row
>> still
>> > be considered a true statement? I would have sworn that assignment
>> > operations ALWAYS equated to true if used in conditional operations.
>> > Please help explain! :)
>> >
>> > Thanks so much!
>> > -Matty
>> >
>>
>> http://www.php.net/manual/en/function.mysql-fetch-array.php
>>
>> "Returns an array of strings that corresponds to the fetched row, or
>> FALSE if there are no more rows."
>>
>> while($row=mysqli_fetch_array($result)) is equivalent to this:
>>
>> $row=mysqli_fetch_array($result);
>> while ($row){
>>
>> // do something
>>
>> $row=mysqli_fetch_array($result);
>> }
>>
>> So, if $row is not equal to FALSE, the loop will happens.
>>
>
Another part to the answer is the value of the assignment. From the
documentation (http://docs.php.net/manual/en/language.operators.assignment.php),
...
"The value of an assignment expression is the value assigned."
while($row = mysqli_fetch_array($result))
But there is a significant issue here. Whilst not attributable to the
mysqli_fetch_array() function, it is certainly possible to give
yourself a significant WTF moment with it.
May be will be easier to see the problem when the code is written as ...
while(False === ($row = mysqli_fetch_array($result)))
No?
If I say ...
0 == False
does that help?
Without running the 2 stupid scripts below, what output should you get?
<?php while($pos = strpos('abc', 'a')) { echo 'a';} ?>
and
<?php while($pos = strpos('abc', 'z')) { echo 'z';} ?>
Clearly, they should be different, yes?
Unfortunately, they won't be.
The first call returns 0 and the second returns False.
And as 0 == False, the while() does not loop.
But ...
<?php while(False !== ($pos = strpos('abc', 'a'))) { echo 'a';} ?>
and
<?php while(False !== ($pos = strpos('abc', 'z'))) { echo 'z';} ?>
now operate correctly. The first one runs forever, printing a's and
the second one quits straight away.
By always using ...
while(False !== ($var = function($param)))
you can clearly differentiate between functions that return false to
indicate failure and 0 to indicate a position or actual value.
Regards,
Richard.
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