Re: Newbie Question about Conditionals

On 31 March 2010 05:45, Matty Sarro <msarro@gmail.com> wrote: > That explains it perfectly, thanks you! > > On Wed, Mar 31, 2010 at 12:40 AM, Tommy Pham <tommyhp2@gmail.com> wrote: > >> On Tue, Mar 30, 2010 at 9:22 PM, Matty Sarro <msarro@gmail.com> 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. -- ----- Richard Quadling "Standing on the shoulders of some very clever giants!" EE : http://www.experts-exchange.com/M_248814.html EE4Free : http://www.experts-exchange.com/becomeAnExpert.jsp Zend Certified Engineer : http://zend.com/zce.php?c=ZEND002498&r=213474731 ZOPA : http://uk.zopa.com/member/RQuadling