count significant digits in PHP

Question

i am writing a class that does simple bcmath operations on numbers. tho i need to set the scale automatically throughout this so i need a way to determine how many significant digits a number has.

for small and simple numbers converting it to string and a simple explode and strlen would do the job but bigger numbers are converted into scientific form therefor i cant think of anything.

$test = 100000000000000.00000000000000001;

var_dump($test);
var_dump($test.'');

"bigger numbers are converted into scientific form" - I cannot reproduce that. Can you please post your code? (Please post it as text, rather than pictures, so we don't need to re-type it again). — Álvaro González, Nov 28 '22 at 17:05
@WOUNDEDStevenJones defining the value as numeric initially would cause the conversion to scientific form. Defining it as a string initially as in the question does not https://3v4l.org/J0SqS — SubjectCurio, Nov 28 '22 at 17:19
Correct, it happens when switching types: https://3v4l.org/muRBP — WOUNDEDStevenJones, Nov 28 '22 at 17:23

score 3 · Answer 1 · answered Nov 28 '22 at 17:23

3

You're defining your input data as float, which basically defeats the purpose of having an arbitrary precision library because you've already lost precision before data has the chance of reaching BC Math:

$test = 100000000000000.00000000000000001;

You need to handle input as string:

$input = '100000000000000.00000000000000001';
[$integerPart, $decimalPart] = explode('.', $input);
$result = bcadd($input, '9', strlen($decimalPart));

var_dump($input, $result);

answered Nov 28 '22 at 17:23

Álvaro González

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well yes but the problem is that i *have* a float, from json, file or db. i think best way would be to parse the scientific form back to string. – محمدرضا خدامی Nov 29 '22 at 12:07
That makes the problem slightly different from what the question states. If you are using native JSON decoding, there's an opt-in feature for big integers, but I'm not aware of anything similar for floats (please see [demo](https://3v4l.org/YRFhc#v8.1.13)). You'll have to write your own JSON parser or find a third-party one that suits your needs. – Álvaro González Nov 29 '22 at 12:14

count significant digits in PHP

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