C ++ std :: map项按键降序排列 [英] C++ std::map items in descending order of keys

问题描述

我如何使用std :: map容器，其键值按降序排列.

How cal I use std::map container with key value in descending order.

例如，如果插入以下项目:

As an example, if insert the following items:

[2 , 5]
[1 , 34]
[3 , 67]

它们在地图中的排序方式如下:

They will be ordered in the map like:

position 0: [1, 34]
position 1: [2, 5]
position 2: [3, 67]

我可以反向迭代地图，但是假设下次我插入[-1，60]时.它将放置在第一位置吗?

I can iterate through the map reversely, but suppose the next time I am inserting [-1 , 60]. Will it be placed at the first position?

推荐答案

当默认订单不适合您时，请使用自定义比较器.
您将其作为第三个模板参数(通常默认为std::less<KeyType>)传递.
您可以使用std::greater:

Use a custom comparator when the default order doesn't do it for you.
You pass it as the third template parameter ( that's normally defaulted to std::less<KeyType> ).
In your case, you can use std::greater:

std::map<int, int, std::greater<int> > m;

示例代码:

#include <map>
#include <iostream>
#include <functional>

int main() {
  std::map<int, int, std::greater<int>> m { {-1, 77}, {0, 42}, {1, 84} };
  for (const auto& p : m)
    std::cout << '[' << p.first << ',' << p.second << "]\n";
}

结果输出:

[1,84]
[0,77]
[-1,42]

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