Here is another tip from "Mistakes I've Made". Don't mix up your conditions and assignments. In other words be careful about using assignment operators (like the eqauls sign '=') inside of conditional statements. Although this happened with PHP, the same problem can occur with many other languages. Let me explain...
Here is a very common way to loop:
<?php
while ($data = mysql_fetch_array($result)) {
// do stuff with $data...
}
?>I was debugging a problem inside of this loop. While I was doing this, I wanted to break the loop early. So this is what I wrote:
<?php
$stop = 0;
while ($data = mysql_fetch_array($result) && $stop++ < 50) {
// do stuff with $data...
}
?>As soon as I changed this code, everything inside the loop that dealt with $data stopped working. Can you see why?
I had forgotten about operator precedence. If you look at the table on the documentation for PHP operators, you will see that the logical AND ('&&') has a higher precedence than the equals sign. This means that the line was not evaluated in left to right order like I was expecting. Instead the first thing that was evaluated was this:
<?php
mysql_fetch_array($result) && $stop++ < 50
?>Which evaluated to either true or false. Then that value was assigned to $data:
<?php
$data = true
?>So when I tried to treat $data like the array I was expecting I immediately ran into problems. Of course, part of the problem is that PHP is a loosely typed language. That means that it didn't care if $data was an array or a boolean. I'll be the first to admit that I take advantage of this, but it is times like these when I regret not being forced to declare my variables and their types, and to do conversion by hand like I did in the good old days.
Anway, here is one method for doing what I wanted:
<?php
$stop = 0;
while (($data = mysql_fetch_array($result)) && $stop++ < 50) {
// do stuff with $data...
}
?>As you can see I have added parentheses which make my intent clear. Next time I will remember to watch out when mixing conditions and assignments, and I hope this article helps you remember too.