Peter - 9 months ago 42

PHP Question

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?

- 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

@Baba answer is good, but he used

`precision=20`

`precision=6`

Please consider following:

Let's say

`precision = 8`

`+`

`-`

`A + B = C`

`A - B = C`

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)."\n";

}

}

but obviously

`99999999 * 2`

*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

Answer

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

Can I rely on this solution if I need just

precise 2 digits calculations(money)?

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

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

```
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
```

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

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

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
```

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 .

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

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 :)