Question

I have a large mathematical expression that has to be created dynamically. For example, once I have parsed “something” the result will be a string like: "$foo+$bar/$baz";.

So, for calculating the result of that expression I’m using the eval function… something like this:

eval("\$result = $expresion;");
echo "The result is: $result";

The problem here is that sometimes I get errors that says there was a division by zero, and I don’t know how to catch that Exception. I have tried things like:

eval("try{\$result = $expresion;}catch(Exception \$e){\$result = 0;}");
echo "The result is: $result";

Or:

try{
    eval("\$result = $expresion;");
}
catch(Exception $e){
    $result = 0;
}
echo "The result is: $result";

But it does not work. So, how can I avoid that my application crashes when there is a division by zero?

Edit:

First, I want to clarify something: the expression is built dynamically, so I can’t just eval if the denominator is zero. So… with regards to the Mark Baker’s comment, let me give you an example. My parser could build something like this:

"$foo + $bar * ( $baz / ( $foz - $bak ) )"

The parser build the string step by step without worrying about the value of the vars… so in this case if $foz == $bak there’s in fact a division by zero: $baz / ( 0 ).

On the other hand as Pete suggested, I tried:

<?php
$a = 5;
$b = 0;

if(@eval(" try{ \$res = $a/$b; } catch(Exception \$e){}") === FALSE)
        $res = 0;
echo "$res\n";
?> 

But it does not print anything.

Answer 1

if ($baz == 0.0) {
    echo 'Divisor is 0';
} else {
    ...
}

Rather than use eval, which is highly dangerous if you’re using user-input within the evalled expression, why not use a proper parser such as evalmath on PHPClasses, and which raises a clean exception on divide by zero