convert user inputed math equation to php expression

jamesm113

Let's say the user inputs 3t^2+y. I want to convert that to 3$t^2+$y. How would I go about doing that?

Thanks!

James

MarkR

Nontrivial certainly. You could write a recursive-descent parser then have it output the code as a PHP expression - you'd need to be very careful to ensure (as you'd have to use eval() to evaluate it) that the user could not insert arbritary code into the expression.

Also, shouldn't it be:

(3 * $t) ^2 + $y

Mark

silent

That's definitely a job for a parser.. implementing one in PHP could prove to be PAINFULLY slow... but one POSSIBLE option would be to use preg_replace to replace the occurence of a letter with $ + the letter...
but as pointed out above, 3$t wouldn't evaluate in PHP... it'd need to be 3 * $t

jamesm113

Well, I've gotten it so that i have something like 3*5 stored in a variable $equation[$i]. I just need php to output the value of that.

bogu

This regular expresion should add $ in front of all letters:

$var = "3*t^2+y";
$tmp = preg_replace("/([a-zA-Z])/",'$\1',$var);
echo $tmp;

jamesm113

Yes, that gives me a string with 3*t^2+y, but how do I compute the value?

$t = 3;
$y = 2;
$var = "3*t^2+y";
$tmp = preg_replace("/([a-zA-Z])/",'$\1',$var);

eval("\$temp = \"$tmp\";");
echo $temp; // outputs 3*3^2+2

silent

$t = 3;
$y = 2;
$var = "3*t^2+y";
$tmp = preg_replace("/([a-zA-Z])/",'$\1',$var);
eval('$result = '.$tmp.';');

That should work

Weedpacket

I don't think ^ in this context is the bitwise xor operator...

I'd go along with MarkR on this one: you'll want to write yourself a parser (in no small part because eval() is dangerous). But hey, it's a core exercise in programming ability, so it's worth trying. I'd be tempted to be lazy about it and use regexps to do the lifting and carrying:

evaluate_parens($paren_expression)
{
	return evaluate($paren_expression[1]);
}
evaluate_exponent($exponentiation)
{
	return pow($exponentiation[1],$exponentiation[2]);
}
evaluate($expression,$substitutions=null)
{
	// Subsitutions is an array('varname'=>value, ...)
	if(!is_null($substitutions)
		$expression = str_replace(array_keys($substitutions), array_values($substitutions), $expression);
	$expression = preg_replace_callback('/\(([^)]+)\)/', 'evaluate_parens', $expression);
	$expression = preg_replace_callback('/(\d+)^(\d+)/', 'evaluate_exponent', $expression);
	//...
	return $expression;
}

That's a sketch of one off-the-cuff idea for an approach, anyway.