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POCan I rely on PHP php.ini precision workaround for floating point issue
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copied!<p>I've found some workaround for <a href="http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems" rel="noreferrer">floating point problem</a> in PHP:</p> <p>php.ini setting <code>precision = 14</code></p> <pre><code>342349.23 - 341765.07 = 584.15999999992 // floating point problem </code></pre> <p>php.ini setting, let's say <code>precision = 8</code> </p> <pre><code>342349.23 - 341765.07 = 584.16 // voila! </code></pre> <p>Demo: <a href="http://codepad.org/r7o086sS" rel="noreferrer">http://codepad.org/r7o086sS</a></p> <p>How bad is that?</p> <p><strong>1. Can I rely on this solution if I need just precise 2 digits calculations (money)?</strong></p> <p><strong>2. If not can you provide me a clear example when this solutions fails?</strong> </p> <p><strong>Edit: 3. Which php.ini.precision value suits best two digits, money calculations</strong> </p> <hr> <ul> <li>Please mind I can't use integer calculations (float*100 = cents), it's far too late for that.</li> <li>I am not going to work on numbers higher than 10^6</li> <li>I don't need to compare numbers</li> </ul> <h1>UPDATE</h1> <p>@Baba answer is good, but he used <code>precision=20</code>, <code>precision=6</code> in his tests... So still i am not sure is it gonna work or not. </p> <p>Please consider following:</p> <p>Let's say <code>precision = 8</code> and only thing I do is addition <code>+</code> and subtraction <code>-</code></p> <p><code>A + B = C</code></p> <p><code>A - B = C</code></p> <p><strong>Question 1:</strong> Is precision workaround gonna fail for numbers between 0..999999.99, where A and B is a number with decimal places? If so please provide me an example. </p> <p>Simple test would do the job:</p> <pre><code>// if it fails what if I use 9,10,11 ??? // **how to find when it fails??? ** ini_set('precision', 8); for($a=0;$a<999999.99;$a+=0.01) { for($b=0;$b<999999.99;$b+=0.01) { // mind I don't need to test comparision (round($a-$b,2) == ($a-$b)) echo ($a + $b).','.($a - $b)." vs "; echo round($a + $b, 2).','.round($a - $b, 2)."\n"; } } </code></pre> <p><a href="http://codepad.viper-7.com/wra15R" rel="noreferrer">but obviously <code>99999999 * 2</code> is too big job so I can't run this test</a></p> <p><strong>Question 2:</strong> How to estimate/calculate when precision workaround fails? Without such crazy tests? <strong>Is there any mathematicial*, straight answer for it?</strong> How to calculate is gonna to fail or not?</p> <p>*i don't need to know floating point calculations works, but when workaround fails if you know precision, and range of A and B</p> <hr> <p>Please mind <strong>I really know</strong> cents and bcmath are best solution. But still I am not sure is workaround gonna fails or not for substraction and addition</p>

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