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PHP compare floats and print +/- before difference prints both + and –

After reading about comparing float numbers, I wrote the following comparison:

$profitMarginPercent = (($clientTotal - $vendorTotal)/$vendorTotal) * 100;

if(abs($clientTotal - $vendorTotal) > PHP_FLOAT_EPSILON) {
    $profitMarginPercent = "+".round($profitMarginPercent, 2);
}
elseif(abs($clientTotal - $vendorTotal) < PHP_FLOAT_EPSILON) {
    $profitMarginPercent = "-".round($profitMarginPercent, 2);
}
elseif(abs($clientTotal - $vendorTotal) == PHP_FLOAT_EPSILON) {
    $profitMarginPercent = round($profitMarginPercent, 2);
}

I want to print, for example, -50% if $vendorTotal is greater than $clientTotal or +50% if the other way round.

This works but with some exceptions:

  1. If $clientTotal is 0.00000 and $vendorTotal is any positive number, e.g. 14.72, my code prints +-100.

  2. If the two variables are equal, e.g. 89.64110, my code prints -0.

Can you explain why this is happening and how I can fix it?

Tags:

2

Answers


  1. 0

    Here is a solution based on sprintf():

    function format($val)
{
    if(abs($val) < 1e-6)
        return '0%';
    else
        return sprintf('%+.2f%%', $val);
}

echo format(1.234), PHP_EOL;
echo format(1e-8), PHP_EOL;
echo format(-1.234), PHP_EOL;

    Output:

    +1.23%
0%
-1.23%

    Small values are considered equal to zero.

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  2. 0

    I would go with something like this:

    $profitMarginPercent = round((($clientTotal - $vendorTotal + PHP_FLOAT_EPSILON)/$vendorTotal) * 100, 2);
$profitMarginPercent = ($profitMarginPercent > 0.0 ? "+" : "") . $profitMarginPercent;

    and then most likely put it in a function or method.

    See: https://3v4l.org/j5BR7

    What does it do?

    As you can see I add PHP_FLOAT_EPSILON to the numerator of the division, this is to prevent a negative result when the numerator and denominator have the same value. It’s a very small value, and you round to two decimals, so it never changes the outcome.

    Then it adds a + character in front of the number whenever $profitMarginPercent is positive. The - character is already there when the value is negative.

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