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How to use a formula stored in mysql, eks: (A + B + C)/(E + F)

Posted on 2006-11-04
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Last Modified: 2010-04-06
How to use several formulas stored in a db?

I have several formulas, it's basic math operations, (AB + AC + AD + AE)/7 etc, the AB' is data has to replaced by values stored in different tables.

I tried to set a rule that each value has to be stored with a space and then split the function up in to a array and the use a loop and then create something like the c++ postfix expression and get the end value,

maybe this can be done easier? I don't like that the formula has to be entered with spaces, it could easily become problem.. I guess that the space thing can be solved by parsing it with regular expressions,

I have tried a bit, here's the start of that code:

sample Formula:( ( A1 + A2 ) + A3 + A4 + A5 + A6 + A7 )

$seperators = array('A1', 'A2','A3','A4','A5','A6', 'A7');

//just replacing the string with a sample number: 1
$str = str_replace($seperators,'1',$str);

$str = (preg_split('# #',$str));

for($i=0; $i<count($str); $i++){

        if( $str[$i] == '+' || $str[$i] == '-' || $str[$i] == '*' || $str[$i] == '/' ){
            //do something
       
        }else if( $str[$i] == ')' ){
            //do something
       
        }else
            //do something
    }
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Question by:kkarumu
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5 Comments
 
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Accepted Solution

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VoteyDisciple earned 250 total points
ID: 17874328
I'm wary of trying to parse expressions by hand, when expressions like that are Context-Free (so a regular expression is insufficient to parse them).  Here's one approach that may be valid if you can carefully control formulas as they're entered:

$variables = array('A1' => 3, 'A2' => 3748.12, ....);
$expression = '((A1 +

foreach($variables as $var => $value) {
     str_replace($var, $value, $expression);
}
eval('$result = ' . $expression);
echo $result;


Instead of str_replace() you'd really have to do a preg_replace() to make sure you don't accidentally fill in the value for A1 in place of A11 and A111 and A1111 and leave a whole huge mess, but the principle is sound.

It's also a nasty way to expose the system if you allow through a formula that actually contains PHP code.  


Just an "out of the box" line of thinking to consider.
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by:kkarumu
ID: 17876885
ok, thanx, that works,

Is it ok to use eval?

have problem as you said to se the differen between A1 and A11..
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Expert Comment

by:VoteyDisciple
ID: 17876902
I'd suggest using preg_replace() instead of str_replace() so you could build a regexp like, perhaps '/\bA1\b/'

Perhaps
$expression = preg_replace('/\b' . $var . '\b/', $value, $expression);

Here note that \b is a "word boundary" in a regular expression, so "A1" followed by another "1" will not be considered an entire word, and will not get matched.  A1 followed by a + or a space or any other non-word character will get replaced.
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Author Comment

by:kkarumu
ID: 17877441
That worked! thanx.

Is it possible to expand the formula with >< (bigger or smaller)?
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Expert Comment

by:VoteyDisciple
ID: 17877661
Absolutely; that's why I suggested eval() -- it'll run absolutely any valid PHP code.
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