The problem that double type cannot be accurately calculated

Preface

Recently, I was maintaining a C# project , Party a proposed a bug, as follows ：

in other words , The user entered a number with two decimal places , But there was an error in our verification .

problem

The verification method of the original code in the project is as follows ：

In the code, the user will enter decimal 100 Rounding times , And with this decimal 100 Times for comparison , If less than, it means that the user has entered more than two decimal places .

The idea is very simple , If the user enters less than two decimal places , So his 100 Times round and his 100 Times should be equal , conversely , It should be less than .

For example, user input 12.345, be money by 12.345, As expected ,centFloor by 1234, and cent by 1234.5,centFloor<cent, The user did enter more than decimals .

Good idea , But the reality is cruel , The problem lies in the use of double Calculate ,double As a double precision floating-point number , It is originally expressed in the form of base and index of several bits , It is not accurate , The result calculated with it is also more inaccurate . therefore It can only be used for storage , It cannot be used for calculation

Based on the input proposed by Party A 525.70 For example , The calculation results here are as follows ：

We can see cent It's not what we expected 52570, But an exact decimal , such , Our judgment is in trouble .

In fact, as long as the number entered by the user is decimal , So his 100 Times and his 100 Times and then rounding cannot be equal ！

I learned before Java When , Have you noticed this problem .

Java To solve double The method of calculating the problem is wrapper class BigDecimal, The code is as follows ：

BigDecimal a=new BigDecimal(0.1);
BigDecimal b=new BigDecimal(0.2);
System.out.println(a.add(b));
BigDecimal c=new BigDecimal("0.1");
BigDecimal d=new BigDecimal("0.2");
System.out.println(c.add(d));

The results are as follows ：

It should be noted that ,BigDecimal Initialization parameters cannot be used double Variable of type （Java The middle decimal is... By default double）, To use String Type .

stay C# in , I found the use of decimal Type can support financial calculation with decimal .

The above double Switch to decimal after , Solved the problem ：


But I still don't recommend this kind of writing , Regular expressions don't smell good ？