Sharing examples of solving inaccurate floating point calculations in PHP

In computer programming, we will always encounter the problem of precision of floating-point operations. This article shares examples of solving inaccurate floating-point operations in PHP. I hope it will be helpful to everyone in solving the problem of precision of floating-point operations.

Comparison of floating point calculation results

An example of floating point calculation is as follows:

$a = 0.2+0.7;
$b = 0.9;
var_dump($a == $b);

The printed result is: bool (false). In other words, the calculation result of 0.2+0.7 here is not equal to 0.9, which is obviously against our common sense.

Regarding this issue, the official PHP manual once stated: Apparently a simple decimal fraction such as 0.2 cannot be converted to an internal binary format without losing a little precision. This has to do with the fact that it is impossible to express certain decimal fractions exactly with a finite number of digits. For example, 1/3 in decimal becomes 0.3333333….

We print out the above variables in double precision format:

$a = 0.2+0.7;
$b = 0.9;
printf("%0.20f", $a);
echo &#39;<br />&#39;;
printf("%0.20f", $b);

The output results are as follows:

0.89999999999999991118
0.90000000000000002220

Obviously here, the actual As floating-point data, part of its accuracy has been lost and cannot be fully accurate. So never trust that a floating-point number result is accurate to the last digit, and never compare two floating-point numbers for equality. It should be noted that this is not a problem with PHP, but a problem with the computer's internal processing of floating point numbers! The same problem will be encountered in languages ​​such as C and JAVA.

So to compare two floating point numbers, we need to control them within the precision range we need before comparing, so use the bcadd() function to add the floating point numbers and convert the precision (to a string) ：

var_dump(bcadd(0.2,0.7,1) == 0.9); // 输出：bool(true)

Floating point number rounding

In the article "PHP rounding function ceil and floor", there was an example:

<?php
echo ceil(2.1/0.7);    // 输出：4
?>

After the above calculation of floating point number After discussion, we know that this is caused by incomplete floating point calculation results:

<?php
printf("%0.20f", (2.1/0.7));    // 输出：3.00000000000000044409
?>

After the above discussion on floating point calculation, Knowing that this is caused by incomplete floating point calculation results, we can use the round() function to process it:

<?php
echo ceil( round((2.1/0.7),1) );
?>

Although the round() function rounds according to the specified precision, it retains one decimal place. , has no impact on our rounding results.

Related recommendations:

#php method of taking remainder from floating point numbers

About php floating point number precision issue

php floating point number rounding function_PHP tutorial

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