How to compare floating point numbers in php?

Floating point numbers will have errors during the conversion process, so floating point numbers cannot be directly compared in size. Generally, when comparing two floating point numbers, the difference between them is compared. If the difference between the two numbers If it is within an acceptable range, then we consider the two floating point numbers to be equal. 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, incorrect

Why is this?

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 it is the same as 0.1 The comparison returns false

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

Floating point number to binary conversion method

The integer part adopts divided by 2 to take the remaindermethod

The decimal part is 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 system of the integer 8 is

1000

The decimal part is 0.5

0.5x2 = 1.0

Since the decimal part is 0 after rounding, there is no need to calculate further

The binary system of decimal 0.5 is 0.1

8.5The binary system of 0.5 is 1000.1

Calculate the number 0.9 Binary

0.9x2=1.8

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

…. Afterwards, 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 compare

Example:

<?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 calculation method

When performing calculations first, use high-precision calculation method operation method 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 Linux in bc ”scale=”

bcsqrt Find the square root of a high-precision number

##bcsub Combine two High-precision number subtractionExample:

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

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