为什么C ++编译器在这个简单的程序中不给予优先级（在赋值下增量运算符）？ [英] Why the C++ compiler does not give precedence (increment operator under assigment) in this simple program?

问题描述

根据C / C ++语言中操作符的优先级表（见维基百科），增量运算符（++）优先于赋值运算符（=）。

有人可以解释为什么编译器首先分配值（1在bill [x]），然后增加索引值（i ++）在这个简单的程序。我认为应该是相反的（首先增加然后分配）：

  #include< iostream> 
 using namespace std; 
 
 int bill [] = {16，17，18，19，20}; 
 
 int main（）
 {
 int i = 3; 
 
 bill [（i ++）] = 1; //我认为应该是bill [4] = 1; 
 
 cout<< bill [0]< endl 
 cout<< bill [1]< endl 
 cout<< bill [2]< endl 
 cout<< bill [3]< endl 
 cout<< bill [4]< endl 
 
 cout<< 索引值：< i<< endl 
 
 return 0; 
} 
  

输出为：

  16 
 17 
 18 
 1 
 20 
索引值：4 
 
解决方案
 
 p>另一种方式你可以看看这：
  bill [（++ i）] = 1; 
  
您可以读取它，先增加'i'然后执行语句。
  bill [（i ++）] = 1; 
  
您可以读取它，先做语句，然后增加'i'。
 
 
 
如果你想知道这是怎么可能的，内部后增量可以像这样实现你所看到的行为：
  int post_increment（int& i）
 {
 int t = i; 
 i = i + 1; 
 return t; 
} 
 
 bill [post_increment（i）] = 1; //访问bill [3]即使i == 4 
  
 ：
  int pre_increment（int& i）
 {
i = i + 1; 
 return i; 
} 
 
 bill [pre_increment（i）] = 1; // access bill [4]其中i == 4 
  
 
According to the table of precedence of operators in C/C++ language (see Wikipedia), the increment operator (++) takes precedence with respect to the assignment operator (=).

Can someone explain why the compiler first assign the value (1 in bill[x]) and then increases the index value (i++) in this simple program. I think it should be the opposite (first increase and then assign):
#include <iostream>
using namespace std;

int bill[] = {16, 17, 18, 19, 20};

int main ()
{
  int i = 3;

  bill[(i++)] = 1; // I think it should be bill[4] = 1;

  cout << bill[0] << endl;
  cout << bill[1] << endl;
  cout << bill[2] << endl;
  cout << bill[3] << endl;
  cout << bill[4] << endl;

  cout << "Index value:" << i << endl;

  return 0;
}
The output is:
16
17
18
1
20
Index value:4
I'm doing something wrong?
解决方案
Another way you can look at this:
bill[(++i)] = 1;
You can read it as, increment 'i' first then do the statement.
bill[(i++)] = 1;
You can read it as, first do the statement then increment 'i'.

If you're wondering how this is possible, internally post-increment can be implemented like this to get the behavior you're seeing:
int post_increment(int &i)
{
  int t = i;
  i = i + 1;
  return t;
}

bill[post_increment(i)] = 1;    // access bill[3] even though i == 4
vs pre-increment which looks like this:
int pre_increment(int &i)
{
  i = i + 1;
  return i;
}

bill[pre_increment(i)] = 1;    // access bill[4] where i == 4


                        
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