Using IF condtion with comparison operator stored in variable

Question

I have set of rules which involves comparison operators. I want to do some task based on the value of comparison operator stored in the rule. I am doing it in the following way but it is not working. Check the following code

if($benRules[$i]['amountCriteria']=='Greater than')
    $comparison='>';
if($benRules[$i]['amountCriteria']=='Equal to')
    $comparison='==';
if($benRules[$i]['amountCriteria']=='Less than')
    $comparison='<';

if($value['1'].$comparison.$value[$i]['2']){
    debug('Condtion checked');
}

problem is it always checks the condition to be true. it takes whole parameter inside IF condition to be string so as long as that string is not empty is executes the code inside the parenthesis. Please help me here.

Firstly, Equal to is == not just one =

m1k1o
– m1k1o

2013-08-12 07:02:19 +00:00
Commented Aug 12, 2013 at 7:02 — m1k1o
– m1k1o, Commented Aug 12, 2013 at 7:02

Alma Do · Accepted Answer · 2013-08-12 07:06:25Z

6

Your problem is that you're trying to use string as a code without evaluating it. You can use create_function() like this:

$sCondition = '<';
$mData0     = 4;
$mData1     = 5.5;
$fnCallback=create_function('$x, $y', 'return $x'.$sCondition.'$y;');
var_dump($fnCallback($mData0, $mData1)); // true

Alternatively, there is an eval() function, but it seems direct returning will be more useful in your case.

answered Aug 12, 2013 at 7:06

Alma Do

37.4k10 gold badges81 silver badges106 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

John Parker Over a year ago

This is a much better approach than using eval.

Ja͢ck Over a year ago

@middaparka Though, create_function() behaves pretty much the same as eval() :)

Alma Do Over a year ago

Yes, it is. The only advantage that we can use direct call (no need to create temporary variable).

John Parker · Accepted Answer · 2013-08-12 07:11:13Z

5

It always evaluates to true as you're simply evaluating the existence of a string (you can't construct logical statements in this manner in PHP).

To achieve what you're attempting, you'd need to either use a switch statement (where you simply have a one to one match for each potential comparison operator) like so...

switch ($comparison) {
    case '<': { if($value['1'] < $value[$i]['2']) { ... } break; }
    case '==': { if($value['1'] == $value[$i]['2']) { ... } break; }
    case '>': { if($value['1'] > $value[$i]['2']) { ... } break; }
}

...or use eval to parse the logic. If might seem like a no-brainer to use the eval approach, but as the PHP manual admits:

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.

As such, if you don't understand the risks involved, please stick with the switch approach. That said, a basic solution using eval would resemble the following:

eval('return ' . $value['1'] . $comparison . $value[$i]['2'] . ';');

edited Aug 12, 2013 at 7:11

answered Aug 12, 2013 at 7:05

John Parker

54.5k11 gold badges133 silver badges131 bronze badges

1 Comment

Talha Malik Over a year ago

Of all the answers given, i think switch approach is most appropriate and basic. Thanks to all

m1k1o · Accepted Answer · 2013-08-12 07:19:38Z

1

$str = "$result = (bool)($value['1'] $comparison $value[$i]['2']);";
eval($str);
if($result){
  //true
}

You can use eval(), but this also works:

            if($benRules[$i]['amountCriteria']=='Greater than'){
              $comparison = (bool)($value['1'] > $value[$i]['2']);
            }

            if($benRules[$i]['amountCriteria']=='Equal to'){
              $comparison = (bool)($value['1'] == $value[$i]['2']);
            }

            if($benRules[$i]['amountCriteria']=='Less than'){
              $comparison = (bool)($value['1'] < $value[$i]['2']);
            }

            if($comparison){
                debug('Condtion checked');
            }

EDIT

This looks better

edited Aug 12, 2013 at 7:19

answered Aug 12, 2013 at 7:08

m1k1o

2,36416 silver badges27 bronze badges

1 Comment

lejlot Over a year ago

code in the form if ($x) { $y=true; } else { $y=false; } is highly redundant, simply write $y=(bool)$x;

Ja͢ck · Accepted Answer · 2013-08-12 07:23:08Z

1

This kind of construct is not possible without some kind of magic, either create_function() or eval().

However, you can also use a closure to achieve what you want; return a comparison function based on the operator that you want to use. You could consider this a cheap version of the Strategy pattern:

function getComparisonFunction($operator)
{
    // comparison function is returned
    return function($a, $b) use ($operator) {
        if ($operator == 'Greater than') {
            return $a > $b;
        } elseif ($operator == 'Equal to') {
            return $a == $b;
        } else { // Less than
            return $a < $b;
        }
    }
}
// get comparison function
$fn = getComparisonFunction($benRules[$i]['amountCriteria']);

// use comparison function
if ($fn($value['1'], $value[$i]['2'])) {
    debug('Condtion checked');
}

edited Aug 12, 2013 at 7:23

answered Aug 12, 2013 at 7:12

Ja͢ck

174k39 gold badges269 silver badges316 bronze badges

2 Comments

John Parker Over a year ago

Probably the perfect solution, if you're running PHP 5.3 and above.

Ja͢ck Over a year ago

@middaparka Yeah, but 5.3 has been around for 4+ years :)

mccakici · Accepted Answer · 2013-08-12 07:07:39Z

0

if($benRules[$i]['amountCriteria']=='Greater than')
    $comparison='>';
if($benRules[$i]['amountCriteria']=='Equal to')
    $comparison='==';
if($benRules[$i]['amountCriteria']=='Less than')
    $comparison='<';

eval('$bool = $value[\'1\']'.$comparison.'$value[$i][\'2\'];');

if($bool){
    debug('Condtion checked');
}

answered Aug 12, 2013 at 7:07

mccakici

5503 silver badges7 bronze badges

Comments

lejlot · Accepted Answer · 2013-08-12 07:08:12Z

0

You could use the eval function

if (eval('return '.$value['1'].$comparison.$value[$i]['2'].';')){

}

answered Aug 12, 2013 at 7:08

lejlot

67k9 gold badges138 silver badges168 bronze badges

2 Comments

웃웃웃웃웃 Over a year ago

The eval() language construct is very dangerous because it allows execution of arbitrary PHP code..See documentation php.net/manual/en/function.eval.php

lejlot Over a year ago

knives are also dangerous yet you use them, don't you? If one uses it reasonably, it is a powerfull and interesting tool. Here, we have a $comparison taken from switch, if one also casts $value[$i] so it has to contain just numbers it is a 100% safe code.

Collectives™ on Stack Overflow

Using IF condtion with comparison operator stored in variable

6 Answers 6

3 Comments

1 Comment

1 Comment

2 Comments

Comments

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

3 Comments

1 Comment

1 Comment

2 Comments

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related