了解每月第一天功能的代码

Question

I'm doing a practice exercise. It asks me to create a calendar of the month, year which is the current user time. I have looked up some code on the Internet, it works well, but I can't understand it clearly. Especially the line year -= month < 3 . Can someone explain it, please?

//return the daycode of the first day of month.
int firstDayOfMonth(int month, int year) {
    int dow = 0; 
    int day = 1;

    int t[] = { 0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4 };
    year -= month  < 3; // I cannot understand this.
    cout<<"This is year "<<year<<endl;
    dow = ( year + year/4 - year/100 + year/400 + t[month-1] + day) % 7;

    return dow;
}

int main()
{
    int a;
    cout<<firstDayOfMonth(2,2018)<<endl;
    return 0;
}

Answer 1

In C++, boolean values can be implicitly converted to integers, with false becoming 0 and true becoming 1 . (See bool to int conversion .)

So year -= month < 3; is equivalent to:

if (month < 3) {
    year -= 1; // true -> 1
} else {
    year -= 0; // false -> 0
}

which can be simplified to:

if (month < 3) {
    --year;
}

The motivation is that January and February (months 1 and 2 ) come before any leap day, while the other months come after any leap day, so it's convenient to treat January and February as being at the end of the previous year, and let the leap day be added to the calculation for the entire March-to-February year.

This code is obviously not optimized for readability.

Answer 2

What that means is:

if the condition (month < 3) is true, then decrement by 1. if the condition (month < 3) is false, then decrement by 0 (year stays the same)

The value of 1 & 0 represent false & true of the month & number comparison.