如何优化自定义日期结构的比较功能？ [英] How can I optimize compare function for custom date structure?

问题描述

我想对 date s的向量进行排序，并为此编写了compare函数：

I want to sort a vector of dates and I wrote compare function for it:

#include <iostream>

struct date {
    int day;
    int month;
    int year;
};

int compare_dates(date a, date b) {
    if (a.year < b.year) {
        return -1;
    } else if (a.year == b.year) {
        if (a.month < b.month) {
            return -1;
        } else if (a.month == b.month) {
            if (a.day < b.day) {
                return -1;
            } else if (a.day > b.day) {
                return 1;
            }
        } else {
          return 1;
        }
    } else {
        return 1;
    }

    return 0;
}

int main() {
    date a = {};
    date a.day = 19;
    date a.month = 11;
    date a.year = 2016;

    date b = {};
    date b.day = 20;
    date b.month = 11;
    date b.year = 2016;

    compare_dates(a, b) // -1
    compare_dates(b, a) // 1
    compare_dates(b, b) // 0

    return 0;
}

运行良好，但 compare_dates 函数看起来很糟糕。有什么想法可以改进吗？

It is working well, but compare_dates function looks awful. Is there any idea how can I improve it?

推荐答案

我不是C ++专家，其他人则指出 std :: sort（）不需要三向比较，只需要< 。但是要按书面形式清理代码：

I'm not a C++ expert and the others are pointing out that std::sort() doesn't require three-way comparison, only a <. But to clean up your code as written:

您的 compare_dates（）始终对<$进行三向比较c $ c>> /< / == ，并希望返回值 + 1 / -1 / 0 。因此，声明一个三向 cmp（）帮助器函数，就像在Python中一样。现在您的代码简化为：

Your compare_dates() keeps doing three-way comparisons for >/</==, and wants a +1/-1/0 return value. So declare a three-way cmp() helper function which does that, like we do in Python. Now your code reduces to:

int cmp(int x, int y) {
    return (x>y) ? 1 : ((x<y) ? -1 : 0); 
}

int compare_dates(date a, date b) {
    if (cmp(a.year, b.year) != 0)
        return cmp(a.year, b.year);

    if (cmp(a.month, b.month) != 0)
        return cmp(a.month, b.month);

    return cmp(a.day, b.day);
} 

订单比较结果为 ==。这样一来，您就可以避免所有其他的子句，花括号和缩进，这可以使您的缩进级别保持恒定，并且易于观察。它还指出了计算的对称性。

You only fall-through into doing the lower-order comparisons if the higher-order comparison gave '=='. So that allows you to avoid all the else-clauses, braces and indenting, which keeps your indent level constant and is easy on the eyes. It also calls out the symmetry of the computations.

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