用语法糖强制算术优先级

Question

每天早上我起床喝杯咖啡，然后前往 SO 看看 John Skeet 前一天的回答。这是我每天的提醒，我不知道有多少。今天，为此 there was a discussion on the increment operators such as ++ and --. The MSDN doc 表示 ++ 和 -- 的优先级高于 *。考虑这段代码：

         int c = 10;
         c = c++ * --c;

和这个代码

        int c = 10;
        c = (c++) * (--c);

在这两种情况下 c 都是 100。您将如何强制优先级（如果可能的话），以便在乘法之前首先计算括号中的值？

Answer 1

变量后面的 ++ 只会在当前指令之后应用，因为它是一个 post 增量。在变量前使用 ++ 或 -- 来实现你想要的

int c = 10;
c = ++c * --c ;
Console.WriteLine ( c ) ;

此代码将输出 110 因为 10 + 1 = 11 和 11 - 1 = 10 所以 10 * 11 = 110

Answer 2

计算机处理一堆操作。

以下...

int c = 10;
c = c++ * --c;

编译成这些操作...

//OpCode      // Function                                             Stack      Var c
ldc.i4.s   10 // 10 is pushed onto the stack                          {10}
stloc.0       // pop the stack and store the value in location 0      {}         10
ldloc.0       // location 0 is pushed onto the stack                  {10}       10
dup           // stack value is copied and pushed on stack            {10,10}    10
ldc.i4.1      // 1 is added to the stack                              {10,10,1}  10
add           // top 2 values are popped and the sum is pushed        {10, 11}   10
stloc.0       // pop the stack and store the value in location 0      {10}       11
ldloc.0       // location 0 is pushed onto the stack                  {10}       11
ldc.i4.1      // 1 is added to the stack                              {10,11,1}  11
sub           // top 2 values are popped and the difference is pushed {10, 10}   11
dup           // stack value is copied and pushed on stack            {10,10,10} 11
stloc.0       // pop the stack and store the value in location 0      {10,10}    10
mul           // top 2 values are popped and the product is pushed    {100}      10
stloc.0       // pop the stack and store the value in location 0      {}         100
ldloc.0       // location 0 is pushed onto the stack                  {100}      100

...我不确定是否清除了任何内容...但是您可以看到，当您在各处（stloc.0/ldloc.0）使用相同的变量时，实际操作在堆栈上进行。（我在大括号 {} 中的内容）。操作（dup、add、sub 和 mul）不关心变量索引，只关心堆栈。

还有咯咯笑...

int c = 10;
c = ++c * c--;

...编译成这个...

//OpCode      // Function                                             Stack        Var c
ldc.i4.s   10 // 10 is pushed onto the stack                          {10}
stloc.0       // pop the stack and store the value in location 0      {}           10
ldloc.0       // location 0 is pushed onto the stack                  {10}         10
ldc.i4.1      // 1 is added to the stack                              {10,1}       10
add           // top 2 values are popped and the sum is pushed        {11}         10
dup           // stack value is copied and pushed on stack            {11,11}      10
stloc.0       // pop the stack and store the value in location 0      {11}         11
ldloc.0       // location 0 is pushed onto the stack                  {11,11}      11
dup           // stack value is copied and pushed on stack            {11,11,11}   11
ldc.i4.1      // 1 is added to the stack                              {11,11,11,1} 11
sub           // top 2 values are popped and the difference is pushed {11,11,10}   11
stloc.0       // pop the stack and store the value in location 0      {11,11}      10
mul           // top 2 values are popped and the product is pushed    {121}        10
stloc.0       // pop the stack and store the value in location 0      {}           121
ldloc.0       // location 0 is pushed onto the stack                  {121}        121

用语法糖强制算术优先级

Forcing arithmetic precedence with syntactic sugar

c#

operator-precedence

pre-increment

post-increment