Discuss the reasons why PHP float is inaccurate and how to avoid it

PHP (Hypertext Preprocessor) is a popular open source server-side programming language commonly used for dynamic web development.

In PHP, float is a data type used to represent numbers with decimal parts, such as 3.14. However, some PHP developers have discovered that some inaccuracies occur when using floats.

This article will discuss the reasons why PHP float is inaccurate and how to use float correctly.

1. Why float is not allowed

In computers, floating point numbers are limited because computers can only represent numbers of limited length. Therefore, some inaccuracies can occur when using floating point numbers for calculations.

For example, in PHP, the following code:

$a = 0.1;
$b = 0.2;
$c = $a + $b;
echo $c;

The expected output is 0.3, but the actual output is 0.30000000000000004.

This is because 0.1 and 0.2 are infinite in the computer and cannot be represented accurately. When calculating 0.1 0.2, the computer adds two infinite numbers, and the result needs to be expressed using some approximations, resulting in inaccurate results.

2. How to avoid inaccurate float

Although float is inaccurate, it still needs to be used in some scenarios. At this time, developers need to pay attention to the following points to avoid inaccuracies as much as possible:

1. Avoid direct comparison of float

Since float may have different representations, directly compare float May cause unexpected results.

For example:

$a = 0.1;
$b = 0.2;
$c = 0.3;
if ($a + $b == $c) {
    echo "equals";
} else {
    echo "not equals";
}

The output result of this code is "not equals" because the result of $a $b is not exactly equal to the result of $c.

If you need to compare, you can use the following method:

$epsilon = 0.00001;
if (abs($a+$b-$c) < $epsilon) {
    echo "equals";
} else {
    echo "not equals";
}

In this way, you can use a "fault tolerance value" to determine whether two floats are equal.

2. Use the round function

The round function can round a float to a specified number of decimal places. This can correct the float inaccuracy problem to a certain extent.

For example:

$a = 0.1;
$b = 0.2;
$c = $a + $b;
echo round($c, 1);  //输出0.3

3. Use integer instead of float

When precise calculations are required, integer can be used instead of float. For example, you can convert a currency value to cents, perform the calculation using an integer, and then convert the result to dollars.

$price = 2.68;  //2.68元
$price_in_cents = intval($price * 100);  //268分
$discount = 20;  //20%
$final_price_in_cents = $price_in_cents * (100 - $discount) / 100;
$final_price = number_format($final_price_in_cents / 100, 2);  //最终价格，2位小数

This can avoid inaccuracies when using float for calculations.

3. Summary

In PHP, float is inaccurate because the computer cannot accurately represent infinite length numbers. In order to avoid the impact of float inaccuracy, the following measures can be taken:

1. Avoid direct comparison of floats and use a fault tolerance value to determine whether two floats are equal;
2. Use the round function to round the float to Specified number of decimal places;
3. Use integer instead of float for precise calculation.

Of course, when using float, you need to consider multiple situations and handle them appropriately to ensure the accuracy of the program.

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