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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=04/2=2 4 %2=0
2/2=1 2%2=0
1000
The decimal part is 0.5
0.5x2 = 1.0Since the decimal part is 0 after rounding, there is no need to calculate furtherThe binary system of decimal 0.5 is 0.18.5The binary system of 0.5 is 1000.1
Calculate the number 0.9 Binary
0.9x2=1.80.8x2=1.6
0.6x2=1.2
0.2x2=0.4
0.4x2=0.8
0.8x2 =1.6
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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