c - 对整数变量 nc 的逗号分隔赋值

Question

我无法理解这段代码的工作原理。

#include<stdio.h>
void main(){
  int a,b;
  a=3,1;
  b=(5,4);
  printf("%d",a+b);
}  

输出是7。那是什么任务？

Answer 1

逗号运算符计算其第一个操作数并丢弃结果，然后计算第二个操作数并返回此值。

执行后

a=3,1;  //  (a = 3), 1;

a将有3和之后

b=(5,4);  // Discard 5 and the value of the expression (5,4) will be 4

b会有4。

维基百科上的更多示例：

// Examples:               Descriptions:                                                                   Values after line is evaluated:
int a=1, b=2, c=3, i=0; // commas act as separators in this line, not as an  operator 
                        // ... a=1, b=2, c=3, i=0
i = (a, b);             // stores b into i 
                        // ... a=1, b=2, c=3, i=2
i = a, b;               // stores a into i. Equivalent to (i = a), b;
                        // ... a=1, b=2, c=3, i=1
i = (a += 2, a + b);    // increases a by 2, then stores a+b = 3+2 into i
                        // ... a=3, b=2, c=3, i=5
i = a += 2, a + b;      // increases a by 2, then stores a to i, and discards unused
                        // a + b rvalue. Equivalent to (i = (a += 2)), a + b; 
                        // ... a=5, b=2, c=3, i=5
i = a, b, c;            // stores a into i, discarding the unused b and c rvalues
                        // ... a=5, b=2, c=3, i=5
i = (a, b, c);          // stores c into i, discarding the unused a and b rvalues
                        // ... a=5, b=2, c=3, i=3
return a=4, b=5, c=6;   // returns 6, not 4, since comma operator sequence points
                        // following the keyword 'return' are considered a single
                        // expression evaluating to rvalue of final   subexpression c=6
return 1, 2, 3;         // returns 3, not 1, for same reason as previous example
return(1), 2, 3;        // returns 3, not 1, still for same reason as above.  This
                        // example works as it does because return is a keyword, not 
                        // a function call. Even though most compilers will allow for
                        // the construct return(value), the parentheses are syntactic
                        // sugar that get stripped out without syntactic analysis 

Answer 2

  a=3,1; // (a=3),1 -- value of expression is 1, side effect is changing a to 3
  b=(5,4);
  printf("%d",a+b); // 3 + 4

Answer 3

在这份声明中

a=3,1;

使用了两个运算符：赋值运算符和逗号运算符。赋值运算符的优先级大于逗号运算符的优先级，因此该语句等价于

( a = 3 ), 1;

1被简单地丢弃，因此a被赋值为3。

在这份声明中

b=(5,4);

由于括号，逗号运算符首先被评估。它的值是最后一个表达式的值，4。所以b被赋值4。

结果你得到a + b => 3 + 4which equals 7。