Store plus and minus in a variable PHP

Question

How can I solve this problem:

if ($variable == 1) {
    $math = "-";
} else {
    $math = "+";
}

$numberOne = 10;
$numberTwo = 10;

$result = $numberOne $math $numberTwo;

This doesn´t work, is there any way to solve this?

You cannot just put variables in a row and expect them to be evaluated as PHP expression. — Felix Kling, Jul 20 '11 at 21:00
Similar to http://stackoverflow.com/questions/1015242/how-to-evaluate-formula-passed-as-string-in-php. Ought to be helpful if you plan on supporting other operations. — simshaun, Jul 20 '11 at 21:02
@Gazler's answer works, but what you're trying to do here suggests something of a misunderstanding of how PHP interprets statements (you're assuming it will concatenate the values of three variables and then correctly evaluate them as a mathematical statement). Is there a more-basic question we can answer about why this doesn't work? — Dan J, Jul 20 '11 at 21:02

score 11 · Accepted Answer · answered Jul 20 '11 at 20:59

This will work for your example. A subtraction is the same as adding a negative. This will be far safer than the alternative of using eval.

if ($variable == 1) {
    $modifier = -1;
} else {
    $modifier = 1;
}

$numberOne = 10;
$numberTwo = 10;

$result = $numberOne + ($numberTwo * $modifier);

Pascal MARTIN · Answer 2 · 2011-07-20T21:03:18.160

I suppose you could use eval() -- but that would be quite a bad idea (it would not be good for performances, it's not "clean", ...)

In this kind of situation, I would generally go with a switch on the operator, and one case per possible operator.

Here, it would mean using something like this :

switch ($math) {
    case '+':
        $result = $numberOne + $numberTwo;
        break;
    case '-':
        $result = $numberOne - $numberTwo;
        break;
}

Which can easily be extends to other operators.

(In your specific situation, if you only have + and -, though, some calculation based on a multiplication by +1 or -1 would be faster to write)

score 2 · Answer 3 · answered Jul 20 '11 at 20:59

2

if ($variable == 1) {
    $math = -1; // subtraction
} else {
    $math = 1; // addition
}

$numberOne = 10;
$numberTwo = 10;

$result = $numberOne + ($math * $numberTwo);

answered Jul 20 '11 at 20:59

Paul DelRe

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score 1 · Answer 4 · edited May 23 '17 at 12:25

1

No love for the ternary operator?
To minify Gazler's answer a bit further:

$modifier = ($variable == 1) : -1 ? 1;

$numberOne = 10;
$numberTwo = 10;

$result = $numberOne + ($numberTwo*$modifier);

edited May 23 '17 at 12:25

Community

1
1

answered Jul 20 '11 at 21:03

barfoon

27,481
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1

I think most people avoid answers using the ternary operator unless the code provided uses it for clarity reasons. There is absolutely nothing wrong with it though, and I would use it if I was using my own solution. – Gazler Jul 20 '11 at 21:06
1

minify hell: `$result = $numberOne + ($numberTwo * (($variable == 1) : -1 ? 1) );` – feeela Jul 20 '11 at 21:12

score 0 · Answer 5 · answered Jul 20 '11 at 21:03

0

If you plan to use more complex mathematics, you can use the eval() function.

answered Jul 20 '11 at 21:03

Maxime Fafard

370
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Store plus and minus in a variable PHP

5 Answers5