Can I rely on PHP php.ini precision workaround for floating point issue(我可以依靠 PHP php.ini 精度解决方法来解决浮点问题吗)

问题描述

我在 PHP 中找到了一些 浮点问题的解决方法:

php.ini 设置 precision = 14

342349.23 - 341765.07 = 584.15999999992//浮点问题

php.ini 设置，比如说 precision = 8

342349.23 - 341765.07 = 584.16//瞧！

演示:http://codepad.org/r7o086sS

这有多糟糕?

1.如果我只需要精确的 2 位数计算(钱)，我可以依靠这个解决方案吗?

<强>2.如果不能，当这个解决方案失败时，你能给我一个明确的例子吗?

3. 哪个 php.ini.precision 值最适合两位数，金钱计算

//如果失败了，如果我使用 9,10,11 怎么办???//**失败时如何查找???**ini_set('精度', 8);for($a=0;$a<999999.99;$a+=0.01) {for($b=0;$b<999999.99;$b+=0.01) {//请注意我不需要测试比较 (round($a-$b,2) == ($a-$b))echo ($a + $b).','.($a - $b)." vs ";echo round($a + $b, 2).','.round($a - $b, 2)."
";}}

ini_set('precision', 8);//你的精度$a = 5.88 ;//1公斤的成本$q = 2.49 ;//用户购买 2.49 公斤$b = $a * 0.01 ;//10% 折扣仅在第一公斤;回声 ($a * $q) - $b;

14.5824 <---- 不精确的 2 位计算，即使精度为 8

$a = 342349.23;$b = 341765.07;

ini_set('precision', 20);//设置为20回声 $a - $b, PHP_EOL;echo floatval(round($a - $b, 2)), PHP_EOL;echo number_format($a - $b, 2), PHP_EOL;回声 bcsub($a, $b, 2), PHP_EOL;

584.15999999997438863584.15999999999996817 <----- 开派对584.16584.15 <-------- 这里是 15 因为精度值是 20

ini_set('precision', 14);//改为 14回声 $a - $b, PHP_EOL;echo floatval(round($a - $b, 2)), PHP_EOL;echo number_format($a - $b, 2), PHP_EOL;回声 bcsub($a, $b, 2), PHP_EOL;

584.15999999997584.16584.16584.16 <-------- 在 14 变为 16

ini_set('precision', 6);//改为 6回声 $a - $b, PHP_EOL;echo floatval(round($a - $b, 2)), PHP_EOL;echo number_format($a - $b, 2), PHP_EOL;回声 bcsub($a, $b, 2), PHP_EOL;

584.16584.16584.16584.00 <--- 在 6 变为 00

ini_set('precision', 3);//改为 3回声 $a - $b, PHP_EOL;echo floatval(round($a - $b, 2)), PHP_EOL;echo number_format($a - $b, 2), PHP_EOL;回声 bcsub($a, $b, 2), PHP_EOL;

584584584.16 <-------------------------------- 他们只有一致的值0.00 <--- 在 3 .. 一切都消失了

ini_set('precision', 8);$a = 0.19;$b = 0.16;$c = 0.01;$d = 0.01;$e = 0.01;$f = 0.01;$g = 0.01;$h = $a + $b + $c + $d + $e + $f + $g;echo "总计:" , $h , PHP_EOL;$i = $h-$a;$i = $i-$b;$i = $i-$c;$i = $i-$d;$i = $i-$e;$i = $i-$f;$i = $i-$g;回声 $i , PHP_EOL;

总计:0.41.0408341E-17 <--- 我相信你会在这里期望 0.00 ;

echo round($i,2) , PHP_EOL;echo number_format($i,2) , PHP_EOL;

<代码>00.00 <-------- 仍然确认 number_format 最准确地保持 2 位数

<块引用>

问题 2:当精确解决方法失败时如何估计/计算?没有如此疯狂的测试?有没有数学*的直接答案?如何计算会不会失败?

事实仍然是 浮点 有 精度问题，但您可以查看数学解决方案

• 机器精度和后向误差分析
• 最小化准确性问题的影响

我不需要知道浮点计算是否有效，但是如果您知道精度以及 A 和 B 的范围，当变通方法失败时

不知道那句话是什么意思:)

I've found some workaround for floating point problem in PHP:

php.ini setting precision = 14

342349.23 - 341765.07 = 584.15999999992 // floating point problem

php.ini setting, let's say precision = 8

342349.23 - 341765.07 = 584.16 // voila!

Demo: http://codepad.org/r7o086sS

How bad is that?

1. Can I rely on this solution if I need just precise 2 digits calculations (money)?

2. If not can you provide me a clear example when this solutions fails?

Edit: 3. Which php.ini.precision value suits best two digits, money calculations

• Please mind I can't use integer calculations (float*100 = cents), it's far too late for that.
• I am not going to work on numbers higher than 10^6
• I don't need to compare numbers

UPDATE

@Baba answer is good, but he used precision=20, precision=6 in his tests... So still i am not sure is it gonna work or not.

Please consider following:

Let's say precision = 8 and only thing I do is addition + and subtraction -

A + B = C

A - B = C

Question 1: 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.

Simple test would do the job:

// 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)."
";
  }
}

but obviously 99999999 * 2 is too big job so I can't run this test

Question 2: How to estimate/calculate when precision workaround fails? Without such crazy tests? Is there any mathematicial*, straight answer for it? How to calculate is gonna to fail or not?

*i don't need to know floating point calculations works, but when workaround fails if you know precision, and range of A and B

Please mind I really know cents and bcmath are best solution. But still I am not sure is workaround gonna fails or not for substraction and addition

Introduction

Floating-point arithmetic is considered an esoteric subject by many people. This is rather surprising because floating-point is ubiquitous in computer systems. Most fractional numbers don't have an exact representation as a binary fraction, so there is some rounding going on. A good start is What Every Computer Scientist Should Know About Floating-Point Arithmetic

Questions

Question 1

Can I rely on this solution if I need just precise 2 digits calculations (money)?

Answer 1

If you need need precise 2 digits then the answer is NO you can not use the php precision settings to ascertain a 2 digit decimal all the time even if you are not going to work on numbers higher than 10^6.

During calculations there is possibility that the precision length can be increased if the length is less than 8

Question 2

If not can you provide me a clear example when this solutions fails?

Answer 2

ini_set('precision', 8); // your precision
$a =  5.88 ; // cost of 1kg
$q = 2.49 ;// User buys 2.49 kg
$b = $a * 0.01 ; // 10% Discount only on first kg ;
echo ($a * $q) - $b;

Output

14.5824 <---- not precise 2 digits calculations even if precision is 8

Question 3

Which php.ini.precision value suits best two digits, money calculations?

Answer 3

Precision and Money calculation are 2 different things ... it's not a good idea to use PHP precision for as a base for your financial calculations or floating point length

Simple Test

Lest Run some example together using bcmath , number_format and simple minus

Base

$a = 342349.23;
$b = 341765.07;

Example A

ini_set('precision', 20); // set to 20 
echo $a - $b, PHP_EOL;
echo floatval(round($a - $b, 2)), PHP_EOL;
echo number_format($a - $b, 2), PHP_EOL;
echo bcsub($a, $b, 2), PHP_EOL;

Output

584.15999999997438863
584.15999999999996817    <----- Round having a party 
584.16
584.15  <-------- here is 15 because precision value is 20

Example B

ini_set('precision', 14); // change to  14 
echo $a - $b, PHP_EOL;
echo floatval(round($a - $b, 2)), PHP_EOL;
echo number_format($a - $b, 2), PHP_EOL;
echo bcsub($a, $b, 2), PHP_EOL;

Output

584.15999999997
584.16
584.16
584.16  <-------- at 14 it changed to 16

Example C

ini_set('precision', 6); // change to  6 
echo $a - $b, PHP_EOL;
echo floatval(round($a - $b, 2)), PHP_EOL;
echo number_format($a - $b, 2), PHP_EOL;
echo bcsub($a, $b, 2), PHP_EOL;

Output

584.16
584.16
584.16
584.00  <--- at 6 it changed to 00 

Example D

ini_set('precision', 3); // change to 3
echo $a - $b, PHP_EOL;
echo floatval(round($a - $b, 2)), PHP_EOL;
echo number_format($a - $b, 2), PHP_EOL;
echo bcsub($a, $b, 2), PHP_EOL;

Output 

584
584
584.16   <-------------------------------- They only consistent value 
0.00  <--- at 3 .. everything is gone 

Conclusion

Forget about floating point and just calculate in cents then later divided by 100 if that is too late just simply use number_format it looks consistent to me .

Update

Question 1: 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

Form 0 to 999999.99 at increment of of 0.01 is about 99,999,999 the combination possibility of your loop is 9,999,999,800,000,000 I really don't think anyone would want to run such test for you.

Since floating point are binary numbers with finite precision trying to set precision would have limited effect to ensure accuracy Here is a simple test :

ini_set('precision', 8);

$a = 0.19;
$b = 0.16;
$c = 0.01;
$d = 0.01;
$e = 0.01;
$f = 0.01;
$g = 0.01;

$h = $a + $b + $c + $d + $e + $f + $g;

echo "Total: " , $h , PHP_EOL;


$i = $h-$a;
$i = $i-$b;
$i = $i-$c;
$i = $i-$d;
$i = $i-$e;
$i = $i-$f;
$i = $i-$g;

echo $i , PHP_EOL;

Output

Total: 0.4
1.0408341E-17     <--- am sure you would expect 0.00 here ;

Try

echo round($i,2) , PHP_EOL;
echo number_format($i,2) , PHP_EOL;

Output

0
0.00    <------ still confirms number_format is most accurate to maintain 2 digit 

Question 2: How to estimate/calculate when precision workaround fails? Without such crazy tests? Is there any mathematical*, straight answer for it? How to calculate is gonna to fail or not?

The fact sill remains Floating Point have Accuracy Problems but for mathematical solutions you can look at

• Machine precision and backward error analysis
• Minimizing the effect of accuracy problems

i don't need to know floating point calculations works, but when workaround fails if you know precision, and range of A and B

Not sure what that statement means :)

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