PHP precision calculation problem

php Precision calculation problem

Let’s take a look at an interesting phenomenon in PHP: var_dump( intval(0.58 * 100 ) ); Guess what result this code will output?

The output result is 57, not 58

##Why is this? This is all caused by floating point arithmetic.

Actually, these results are not language bugs, but are related to the implementation principle of the language. All numbers in js are unified as Number, including integers, which are actually all double precision (double) types.

And PHP will distinguish between int and float. No matter what language, as long as floating point operations are involved, there are similar problems, so you must pay attention when using them.

Floating point numbers, taking 64-bit length (double precision) as an example, will be represented by 1 sign bit (E), 11 exponent bits (Q), and 52-bit mantissa (M) (a total of 64 bits) .

Sign bit: The highest bit indicates the sign of the data, 0 indicates a positive number, and 1 indicates a negative number.

Exponent bit: represents the power of the data with base 2, and the exponent is represented by an offset code

Mantissa: represents the valid digits after the decimal point of the data.

The key points here It lies in the representation of decimals in binary. Regarding how to represent decimals in binary, you can search on Baidu. I will not go into details here. The key thing we need to understand is that for binary representation, 0.58 is an infinitely long value (the numbers below Omitting the implicit 1)..

The binary representation of 0.58 is basically (52 bits): 0010100011110101110000101000111101011100001010001111

The binary representation of 0.57 (52 bits) is basically: 0010001111010 11100001010001111010111000010100011110

The binary numbers of the two, if calculated only through these 52 bits, are:

0.58 -> 0.579999999999999960.57 -> 0.56999999999999999

##As for 0.58 * 100 We don’t think about the specific multiplication of floating point numbers in detail. We just look at it vaguely with mental arithmetic... 0.58 * 100 = 57.999999999

Then if you intval it, it will naturally be 57...

It can be seen that, The key point of this problem is: "You seem to have a finite decimal, but it is infinite in the binary representation of the computer."

PHP floating point type has inaccuracy in +-*%/

We can solve the problem of inaccurate floating point operations through the following commonly used high-precision functions:

bcadd — Add two high-precision numbers

bccomp — Compares two high-precision numbers, returns -1, 0, 1　

bcdiv — Divides two high-precision numbers

bcmod — Finds 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 power of a high-precision number and its modulus, very commonly used in number theory

bcscale — Configure the default number of decimal points, which is equivalent to "scale=" in Linux bc

bcsqrt — Find the square root of a high-precision number

bcsub — Concatenate two high-precision numbers reduce

The above is the detailed content of PHP precision calculation problem. For more information, please follow other related articles on the PHP Chinese website!