A brief introduction to php floating point exact operation

If you use php +-* to calculate floating point numbers, you may encounter some incorrect computing results. Therefore, most languages provide precision computing class libraries or function libraries, for example, php has the BC high-precision function library. next we will introduce some common BC high-precision functions. Bc is short for Binary Calculator. Bc * function parameters are all operands plus an optional [int scale], such as string bcadd (string $ left_operand, string $ right_operand [, int $ scale]). If scale is not provided, use the default value of bcscale. Here, the large number is represented by a string consisting of 0-9, and the return value is also a string.

Bcadd-add two high-precision numbers
Bccomp-compare two high-precision numbers and return-1, 0, 1
Bcp-division of two high-precision numbers
Bcmod-precise digital remainder
Bcmul-multiply two high-precision numbers
Bcpow-calculate a high-precision digital multiplier
Bcpowmod-modulus of high-precision multiplication of numbers, which is very common in number theory.
Bcscale-the default number of decimal places configured, which is equivalent to "scale =" in Linux bc"
Bcsqrt-calculate the square root of a precise number
Bcsub-subtract two high-precision numbers

First, let's look at a piece of code:

<?php$a = 0.1;$b = 0.7;var_dump(($a + $b) == 0.8);

The printed value is boolean false.

Why? The PHP Manual provides the following warning information for floating point numbers:

Warning
Floating point precision
Obviously, a simple decimal score is like 0.1 or 0.7. it cannot be converted to an internal binary format without losing a little precision. This will lead to chaotic results: for example, floor (0.1 + 0.7) * 10) usually returns 7 instead of 8 in expectation, because the internal representation of the result is similar to 7. 9999999999 ....
This is related to the fact that it is impossible to accurately express certain decimal scores with limited digits. For example, decimal 1/3 is changed to 0.3333333 ....
Therefore, never believe that the result of a floating point number is accurate to the last digit, or compare whether the two floating points are equal. If higher precision is required, use any precision mathematical function or gmp function.

Then we should rewrite the formula above

<?php$a = 0.1;$b = 0.7;var_dump(bcadd($a,$b,2) == 0.8);

This solves the problem of floating point calculation.