Variable Operators in PHP

Question

Given this example, how would I return the result of the equation rather than the equation itself as a string?

$operator = '+';
foreach($resultSet as $item){
    $result = $item[$this->orderField] . $operator . 1;
    echo $result;
}

I needed to swap items in an array but maintain their keys and values. $item[$this->orderField] would be the current item, +/- 1 would be the item it would swap positions with. I have written an array_swap function, and combined with reko_t's method it produces something similar to array([1]=>'1', [0]=>'0', [2]=>'2') from array([0]=>'0', [1]=>'1', [2]=>'2'). — BenTheDesigner, Apr 01 '10 at 09:26

score 17 · Accepted Answer · edited Mar 31 '10 at 13:17

17

You could make functions that wrap the operators, or for simplicity just use the bc extension:

$operator = '+';
$operators = array(
  '+' => 'bcadd',
  '-' => 'bcsub',
  '*' => 'bcmul',
  '/' => 'bcdiv'
);

foreach($resultSet as $item){
    $result = call_user_func($operators[$operator], $item[$this->orderField], 1);
    echo $result;
}

edited Mar 31 '10 at 13:17

John Carter

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answered Mar 31 '10 at 10:35

reko_t

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1

Use an arbitrary precision math library to perform regular math? Looks kind of quirky... – Álvaro González Mar 31 '10 at 14:51
2

Develop with extensibility in mind Alvaro... Who says the same function may not need to perform precision mathematics? – BenTheDesigner Apr 01 '10 at 09:34
The performance in a situation like this is absolutely negligible in most cases. Ever heard of the saying premature optimization is the root of all evil? ;) Write clean, easy-to-understand and maintainable code and optimize if necessary. – reko_t Sep 03 '11 at 12:50

user187291 · Answer 2 · 2010-03-31T10:41:43.947

To achieve exactly that, you can use create_function

$operator = '+';
$func = create_function('$a,$b', "return \$a $operator \$b;");
foreach($resultSet as $item){
    $result = $func($item, 1);
    echo $result;
}

A cleaner solution is possible with lambdas (php5.3 required)

$func = function($a, $b) { return $a + $b; };
foreach($resultSet as $item){
    $result = $func($item, 1);
    echo $result;
}

See also array_sum, array_reduce

Advanced example with array_reduce and lambdas

$ary = array(
    array('foo' => 1, 'bar' => 91),
    array('foo' => 2, 'bar' => 92),
    array('foo' => 3, 'bar' => 93),
    array('foo' => 4, 'bar' => 94),
    array('foo' => 5, 'bar' => 95),
);

$sumOfFoos = array_reduce($ary, 
    function($val, $item) { return $val + $item['foo']; } 
);
$sumOfBars = array_reduce($ary, 
    function($val, $item) { return $val + $item['bar']; } 
);

The main point is, that instead of 'variable operators' (which is not possible in php), you should rather use variable functions (which is possible and much more flexible).

it will eval my _code_ but not _variables_, which is the key difference. — user187291, Mar 31 '10 at 10:49
But either $operator is an arbitrary variable, too, or you don't need create\_function (like in reko\_t's answer), do you? — VolkerK, Mar 31 '10 at 10:57

score 2 · Answer 3 · answered Mar 31 '10 at 10:27

2

You can use eval(), but it's generally a bad idea, since it's a major potential security hole (be careful you're not allowing visitors to run arbitrary code!).

It can also result in hard to maintain code.

answered Mar 31 '10 at 10:27

John Carter

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score 2 · Answer 4 · answered Mar 31 '10 at 10:34

Quick answer is eval(). However, in this precise example I would just hard-code the possible operations:

<?php

$operator = '+';
foreach($resultSet as $item){
    switch($operator){
        case '+':
            $result = $item[$this->orderField] + 1;
            break;
    }
    echo $result;
}

?>

score 1 · Answer 5 · answered Mar 31 '10 at 10:28

1

Use the eval function of PHP: http://php.net/manual/en/function.eval.php

$operator = '+';
foreach($resultSet as $item){
    $result = $item[$this->orderField] . $operator . 1;
    eval("\$result = \"$result\";");
    echo $result;
}

answered Mar 31 '10 at 10:28

Hannes de Jager

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Variable Operators in PHP

5 Answers5

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