Discuss the problem of unequal floating point numbers in PHP

PHP is a popular programming language and a server-side scripting language. In PHP, it has flexible and powerful features, so it is widely used in the field of web development.

In PHP programming, we often encounter calculations of floating point numbers. However, because the numbers after the decimal point of floating point numbers are difficult to accurately represent, this will lead to when comparing whether two floating point numbers are equal. Often get wrong results.

Let’s discuss the issue of unequal floating point numbers in PHP.

Characteristics of floating-point numbers

A floating-point number is a number used to represent decimals. It has two elements: the mantissa and the exponent. In the IEEE 754 standard, floating point numbers are represented in binary.

Because the precision of floating-point numbers will change as the size of the value changes, you need to pay attention to the characteristics of floating-point numbers when performing comparison operations.

For example:

$a = 0.1 + 0.2;
$b = 0.3;
echo ($a == $b) ? "Equal" : "Not Equal";

We expect the output result to be Equal. However, the actual output result is Not Equal.

Why is this so? This is because in computers, floating point numbers are stored in binary form, so 0.1 and 0.2 are an infinitely recurring decimal when converted to binary, and the computer can only use a limited number of digits to store them. Therefore, when the computer converts 0.1 and 0.2 into binary, they are not stored exactly exactly, and the end result is slightly off. This deviation will accumulate during the addition operation, causing the result to be unequal to the expected value.

The correct way to compare floating point numbers

In PHP, there are multiple ways to compare two floating point numbers for equality. Let’s introduce these methods below.

Method 1: Use the round() function to check the error

The round() function can round a floating-point number to a specified number of digits. Therefore, when we want to compare two floating point numbers for equality, we can first round them to the same number of decimal places using the round() function, and then compare them.

For example:

$a = 0.1 + 0.2;
$b = 0.3;
$precision = 14;
if (round($a, $precision) == round($b, $precision)) {
  echo "Equal";
} else {
  echo "Not Equal";
}

In the above example, the $precision variable specifies the number of decimal places to be rounded. In this example, we use the round() function to round two numbers to 14 decimal places and then compare them to get the correct result.

Method 2: Use the precise calculation library

When we need to perform precise floating point number comparisons, we can use the precise calculation library provided by PHP. This library provides some functions that can perform high-precision calculations on floating point numbers, thereby avoiding errors that occur in floating point number operations.

Commonly used accurate calculation libraries include BC Math and GMP libraries. Both libraries provide a series of functions that can perform various high-precision floating point operations, and the calculation results are the same as the actual results.

For example:

$a = "0.1";
$b = "0.2";
$c = "0.3";
$sum = bcadd($a, $b, 2); // 计算a和b的和
if (bccomp($sum, $c, 2) == 0) { // 比较计算结果和期望结果
  echo "Equal";
} else {
  echo "Not Equal";
}

In the above example, we use the bcadd() function to calculate the value of the $sum variable, and then use the bccomp() function to compare $sum and $c Whether the values ​​are equal. The bccomp() function returns 0 for equality, 1 for $sum greater than $c, and -1 for $sum less than $c.

Conclusion

Due to the characteristics of floating point numbers, when we compare floating point numbers for equality, we need to pay attention to the calculation error and cannot simply use "==" to compare. Instead, other comparison methods should be used, such as rounding, precise calculation, etc. Only by mastering the correct comparison method can we correctly handle floating point number operations in PHP.

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