Operator as a Variable in PHP

Question

Im trying to compute a simple expression in PHP, the expression will always be the same the only change is in the operator.

Is there a simple solution to rather than duplicate the formula, just use the single expression and have the operator as a variable.

Something like..

function calc($qty, $qty_price, $max_qty, $operator_value, $operator = '-') 
{

  $operators = array(
   '+' => '+',
   '-' => '-',
   '*' => '*',
  );

  //adjust the qty if max is too large
  $fmq = ($max_qty > $qty)? $qty  : $max_qty ;


  return ( ($qty_price  . $operators[$operator] . $operator_value) * $fmq ) + ($qty - $fmq) * $qty_price;

}

You can use eval() but that's usually a bad code smell

John Conde
– John Conde

2013-07-20 03:21:48 +00:00
Commented Jul 20, 2013 at 3:21 — John Conde
– John Conde, Commented Jul 20, 2013 at 3:21
It has crossed my mind but Id rather not use eval()

IEnumerable
– IEnumerable

2013-07-20 03:41:21 +00:00
Commented Jul 20, 2013 at 3:41 — IEnumerable
– IEnumerable, Commented Jul 20, 2013 at 3:41

stefancarlton · Accepted Answer · 2013-07-20 03:30:42Z

2

The 5.3 answer is nice, however, why not pre-calcaute $qty_price +-* $operator_value rather than repeating the whole function? It'd make the code a little more readable...

E.g.

function calc($qty, $qty_price, $max_qty, $operator_value, $operator = '-') 
{
  $qty_price_orig = $qty_price;

  switch($operator) {
      case '-':
         $qty_price -= $operator_value;
         break;
      case '+':
        $qty_price += $operator_value; 
        break;
      case '*':
         $qty_price = $qty_price * $operator_value; 
         break;


  //adjust the qty  i max is too large
  $fmq = ($max_qty > $qty)? $qty  : $max_qty ;


  return ( $qty_price * $fmq ) + ($qty - $fmq) * $qty_price_orig;

}

answered Jul 20, 2013 at 3:30

stefancarlton

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4 Comments

IEnumerable Over a year ago

Ill probably use this code for compatibility reasons. My software needs to work on both PHP 5.1 and 5.3.

stefancarlton Over a year ago

you might need to adjust your formula by the way - at the moment it is multiplying ($qty - $fmq) * $qty_price_orig and then adding ( $qty_price * $fmq ). If that is desired, add brackets around the multiplier so that when you come back to edit code / someone else comes into maintain it, it's immediately obvious what you wanted it to do.

IEnumerable Over a year ago

do you mean ( $qty_price * $fmq ) + ( ($qty - $fmq) * $qty_price_orig);

stefancarlton Over a year ago

yup! Just means that it's immediately obvious to me what you meant the function to do. I wondered if you meant it to be ( ( $qty_price * $fmq ) + ($qty - $fmq) ) * $qty_price_orig; you see.

prodigitalson · Accepted Answer · 2013-07-20 03:26:07Z

2

If youre using 5.3+ then just use functions as your operator values:

$operators = array(
   '+' => function (a,b) { return a+b; },
   '-' => function (a,b) { return a-b; },
   '*' => function (a,b) { return a*b; },
);

$fmq = ($max_qty > $qty)? $qty  : $max_qty ;


return ( $operators[$operator]($qty_price, $operator_value) * $fmq ) + ($qty - $fmq) * $qty_price;

If youre using < 5.3 then you can use create_function() to do much the same.

answered Jul 20, 2013 at 3:26

prodigitalson

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3 Comments

IEnumerable Over a year ago

I didn't know we could store the function in the array, nice

IEnumerable Over a year ago

I really like this solution, however I think in my case dealing with compatibility is probably more important so @stefancarlton solution may have to be the one for me

prodigitalson Over a year ago

@IEnumerable: Yep, that requirement trumps ease/compactness. As long as you dont use eval youre good :-)

jh314 · Accepted Answer · 2013-07-20 03:29:40Z

0

If you want to use eval() to get the job done:

<?php
$operators = array(
   '+' => '+',
   '-' => '-',
   '*' => '*',
  );

$a = 3;
$b = 4;
foreach ($operators as $op) {
    $expr = "$a ".$op." $b";
    eval("\$result=".$expr.";");
    print_r($expr." = ".$result."\n");
}
?>

However, use it with caution! Here is the official warning:

The eval() language construct is very dangerous because it allows execution of arbitrary PHP code. Its use thus is discouraged. If you have carefully verified that there is no other option than to use this construct, pay special attention not to pass any user provided data into it without properly validating it beforehand.

answered Jul 20, 2013 at 3:29

jh314

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1 Comment

prodigitalson Over a year ago

I know you gave the warning, but eval is probably the least appropriate answer even though it works given how simple the problem is.

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