Question

为什么第一个代码中if()的正文执行虽然第二个代码中if()的正文不执行

工作

double num1 = 0.2;
double num2 = 0.2;
double num3 = num1 + num2;
if (num3 == 0.4)
{
    MessageBox.Show("1st");
}

不工作

double num1 = 0.1;
double num2 = 0.2;
double num3 = num1 + num2;
if (num3 == 0.3)
{
    MessageBox.Show("2nd");
}

Answer 1

原因

机器使用二进制语言当它将数字转换为二进制时，转换回十进制数值得到改变，由于某些数字无法完全转换为二进制

当你转换0.1 into base 2 (binary)时，你会得到小数点后的重复模式，

喜欢 1/3 in base 10 ; 1/3=0.333333333333333333333333333333333..........＆amp;从来没有得到确切的价值

因此，您无法使用常规浮点方法获取每个数字的精确值。

示例（转换模式）

0.1二进制模式......

Binary Pattern of 0.1

Source: Why 0.1 Does Not Exist In Floating-Point

0.1转换 0.1 is one-tenth, or 1/10. To show it in binary您可以看到100正在重复间隔作为输出为1001如图所示

Conversion Of 0.1 to binary

Source: Why 0.1 Does Not Exist In Floating-Point

第二个代码片段{0.1 + 0.2的答案与实际值0.3不相同}

--------------------------------------------------------------
Actual {Answer of 0.1+0.2 Is not Same as actual value of 0.3}
--------------------------------------------------------------
0.1=0.1000000000000000055511151231257827021181583404541015625
0.2=0.200000000000000011102230246251565404236316680908203125
0.3=0.299999999999999988897769753748434595763683319091796875

==============================================================
 Calculation 0.1 + 0.2
______________________________________________________________ 
 0.1=0.1000000000000000055511151231257827021181583404541015625
+0.2=0.200000000000000011102230246251565404236316680908203125
--------------------------------------------------------------
 0.3=0.3000000000000000444089209850062616169452667236328125

==============================================================
Answer
--------------------------------------------------------------
  0.3000000000000000444089209850062616169452667236328125
!=0.299999999999999988897769753748434595763683319091796875
---------------------------------------------------------------

第一段代码片段{答案0.2 + 0.2与实际值0.4相同}

---------------------------------------------------------------
Actual {Answer of 0.2+0.2 Is Same as actual value of 0.4}
---------------------------------------------------------------
 0.2=0.200000000000000011102230246251565404236316680908203125
 0.2=0.200000000000000011102230246251565404236316680908203125
 0.4=0.40000000000000002220446049250313080847263336181640625  
==============================================================
 Calculation 0.2 + 0.2
---------------------------------------------------------------
 0.2=0.200000000000000011102230246251565404236316680908203125
+0.2=0.200000000000000011102230246251565404236316680908203125
---------------------------------------------------------------
 0.4=0.40000000000000002220446049250313080847263336181640625

==============================================================
Answer
--------------------------------------------------------------
  0.4=0.40000000000000002220446049250313080847263336181640625
==0.4=0.40000000000000002220446049250313080847263336181640625
---------------------------------------------------------------

添加浮点，为什么第一个代码工作

1 个答案: