Detailed graphic explanation of PHP floating point number comparison method

This article mainly introduces the PHP floating point number comparison method, which has a good reference value. Let’s take a look at it with the editor

The problem of precision of floating point operations

First look at an example:

<?php
$a = 0.1;
$b = 0.9;
$c = 1;
var_dump(($a+$b)==$c);
var_dump(($c-$b)==$a);
?>

$a $b==$c returns true, correct
$c-$b==$a returns false, error

Why is this so?

After the operation, the actual returned content when the precision is 20 digits is as follows:

<?php
$a = 0.1;
$b = 0.9;
$c = 1;
printf("%.20f", $a+$b); // 1.00000000000000000000
printf("%.20f", $c-$b); // 0.09999999999999997780
?>

$c-$b is 0.09999999999999997780, so comparing with 0.1 returns false

This problem occurs because floating point number calculation involves precision. When floating point numbers are converted to binary, precision may be caused lost.

Floating point number conversion method

The integer part is divided by 2 to take the remainderMethod The decimal part adopts

multiplied by 2 to round method

For example: convert the number 8.5 to binary

The integer part is 8 8/2=4 8%2=0

4/2=2 4%2=0

2/2=1 2%2=0

1 is smaller than 2, so there is no need to calculate it. The binary representation of the integer 8 is

1000

The decimal part is 0.5 0.5x2 = 1.0

Because the decimal part is 0 after rounding, there is no need to calculate anymore

Decimal The binary representation of 0.5 is 0.1

The binary representation of 8.5

is1000.1

Calculate the binary representation of the number 0.90.9x2=1.8

0.8x2=1.6

0.6x2=1.2
0.2x2=0.4
0.4x2=0.8
0.8x2=1.6

…. After that, the loop continues. When the interception accuracy is N, the number after N will be rounded off, resulting in loss of accuracy.

In the above example, the precision of 0.9 is lost when converted to binary, resulting in errors during comparison.

So never believe that a floating point number is accurate to the last digit, and never compare two floating point numbers for equality.

How to correctly compare floating point numbers

1. Use the round method to process and then compareExample:

<?php
$a = 0.1;
$b = 0.9;
$c = 1;
var_dump(($c-$b)==$a);          // false
var_dump(round(($c-$b),1)==round($a,1)); // true
?>

##2. Use high-precision operation methods

When performing calculations first, use high-precision calculation methods to ensure that accuracy is not lost.

The method of high-precision operation is as follows:

bcadd

Combine two high-precision numbers Addition

bccomp

Compares two high-precision numbers and returns -1,0,1

bcp

Divide two high-precision numbers

bcmod

Find the remainder of a high-precision number

bcmul

Multiply two high-precision numbers

bcpow

Find the power of a high-precision number

bcpowmod

Find the modulus of the power of high-precision numbers

bcscale

Configure the default number of decimal points, which is equivalent to "scale="## in Linux bc #bcsqrt

Find the square root of a high-precision numberbcsub

Subtract two high-precision numbersExample:

<?php
$a = 0.1;
$b = 0.9;
$c = 1;
var_dump(($c-$b)==$a);     // false
var_dump(bcsub($c, $b, 1)==$a); // true
?>

##Related recommendations:

Detailed explanation of JS

Floating point number

Arithmetic processing (graphic tutorial)

##javascript implementation to find the square root of Floating point numberDetailed explanation

Sharing examples of solving floating point number inaccurate calculations in PHP

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