三元声明中的C逗号 [英] C comma in ternary statement

问题描述

int m = 5, d = 12, y = 1975, val;
    // May 12, 1975

有人可以在下面的代码行中解释逗号运算符的功能/目的:

Can someone please explain the function/purpose of the comma operator in the line of code below:

val = (d+=m<3?y--:y-2,23*m/9+d+4+y/4-y/100+y/400)%7;

以上行由Mike Keith编写，用于计算给定日期的星期几(d =天，m =月，y =年).其中周日= 0，周一= 1，周二= 2，周三= 3，周四= 4，周五= 5，周六= 6. 我了解如果d + = m <3为真，则执行y--，否则执行y-2.我不了解y-2之后的逗号的目的.

The above line was written by Mike Keith to calculate the day of the week given the date (d = day, m = month, y = year). Where Sunday = 0, Monday = 1, Tuesday = 2, Wednesday = 3, Thursday = 4, Friday = 5, Saturday = 6. I understand that the y-- gets executed if d+=m<3 is true, else the y-2 is executed. What I don't understand is the purpose of the comma after y-2.

推荐答案

逗号运算符将要依次执行的表达式分开，就像;一样.但是，使用,时，它们构成了一个完整的表达式，其计算结果为最后一个子表达式的值.例如

The comma operator separates expressions to be executed one after the other, just like ;. But with , they constitute one whole expression that evaluates to the value of the last sub-expression. For example

int i = 1;
int j = (++i, i*2);
printf("%i", j)

打印出4.

例如可以在for表达式中使用，其中标头中需要3个表达式.例如

It can for example be used in for expressions, where 3 expressions need to be in the header. For example

for(i = 0, j = 0; i < n; i++, j++)

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