How to round to the nearest significant digit in php

Question

Is there a slippery way to round to the nearest significant digit in php?

So:

0->0
9->9
10->10
17->10
77->70
114->100
745->700
1200->1000

?

Answer 1

$numbers = array(1, 9, 14, 53, 112, 725, 1001, 1200);
foreach($numbers as $number) {
    printf('%d => %d'
            , $number
            , $number - $number % pow(10, floor(log10($number)))
            );
    echo "\n";
}

Unfortunately, this fails when $ number is 0, but it gives the expected result for positive integers. And this is a mathematical solution.

Answer 2

Here is a purely mathematical solution. It is also a more flexible solution if you ever wanted to round up or down, not just down. And it works on 0 :)

if($num === 0) return 0;
$digits = (int)(log10($num));
$num = (pow(10, $digits)) * floor($num/(pow(10, $digits)));

You can replace floorwith roundor ceil. In fact, if you want to round to the nearest, you can simplify the third line even further.

$num = round($num, -$digits);

Answer 3

, :

function floorToFirst($int) {
    if (0 === $int) return 0;

    $nearest = pow(10, floor(log($int, 10)));
    return floor($int / $nearest) * $nearest;
}

Answer 4

- :

$str = (string)$value;
echo (int)($str[0] . str_repeat('0', strlen($str) - 1));

Answer 5

-, , ... , , ,

function significant($number){
    $digits = count($number);
    if($digits >= 2){
        $newNumber = substr($number,0,1);
        $digits--;
        for($i = 0; $i < $digits; $i++){
            $newNumber = $newNumber . "0";
        }
    }
    return $newNumber;
}

Answer 6

:

$mod = pow(10, intval(round(log10($value) - 0.5))); 
$answer = ((int)($value / $mod)) * $mod;

Answer 7

, , , . :

class Math
{
    public static function round($number, $numberOfSigFigs = 1)
    {
        // If the number is 0 return 0
        if ($number == 0) {
            return 0;
        }

        // Deal with negative numbers
        if ($number < 0) {
            $number = -$number;
            return -Math::sigFigRound($number, $numberOfSigFigs);
        }

        return Math::sigFigRound($number, $numberOfSigFigs);
    }

    private static function sigFigRound($number, $numberOfSigFigs)
    {
        // Log the number passed
        $log = log10($number);

        // Round $log down to determine the integer part of the log
        $logIntegerPart = floor($log);

        // Subtract the integer part from the log itself to determine the fractional part of the log
        $logFractionalPart = $log - $logIntegerPart;

        // Calculate the value of 10 raised to the power of $logFractionalPart
        $value = pow(10, $logFractionalPart);

        // Round $value to specified number of significant figures
        $value = round($value, $numberOfSigFigs - 1);

        // Return the correct value
        return $value * pow(10, $logIntegerPart); 
    }
}

Answer 8

, ( ).

N PHP , PHP , .

number_format, , "" ( ) 0 ( , ).

, , , number_format - number_format:

function roundRate($rate, $digits)
{
    $mod = pow(10, intval(round(log10($rate))));
    $mod = $mod / pow(10, $digits);
    $answer = ((int)($rate / $mod)) * $mod;
    $small = strstr($answer,"-");
    if($small)
    {
        $answer = number_format($answer,str_replace("-","",$small));
    }
    return $answer;
}

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