如何首先按值对 std::map 排序，然后按键排序?
我需要先按值对 std::map 进行排序，然后再按键排序.该地图包含如下数据:
I need to sort a std::map by value, then by key. The map contains data like the following:
1 realistically
8 really
4 reason
3 reasonable
1 reasonably
1 reassemble
1 reassembled
2 recognize
92 record
48 records
7 recs
我需要按顺序获取值，但关键是在值按顺序排列后，键需要按字母顺序排列.我该怎么做?
I need to get the values in order, but the kicker is that the keys need to be in alphabetical order after the values are in order. How can I do this?
推荐答案
std::map 将按 keys 对其元素进行排序.排序时它不关心 values.
std::map will sort its elements by keys. It doesn't care about the values when sorting.
您可以使用 std::vector<std::pair<K,V>> 然后使用 std::sort 后跟 std::stable_sort 对其进行排序:
You can use std::vector<std::pair<K,V>> then sort it using std::sort followed by std::stable_sort:
std::vector<std::pair<K,V>> items;
//fill items
//sort by value using std::sort
std::sort(items.begin(), items.end(), value_comparer);
//sort by key using std::stable_sort
std::stable_sort(items.begin(), items.end(), key_comparer);
第一个排序应该使用std::sort，因为它是nlog(n)，然后使用std::stable_sortn(log(n))^2 在最坏的情况下.
The first sort should use std::sort since it is nlog(n), and then use std::stable_sort which is n(log(n))^2 in the worst case.
请注意，虽然选择 std::sort 是出于性能原因，但需要 std::stable_sort 才能正确排序，因为您希望按值排序被保存.
Note that while std::sort is chosen for performance reason, std::stable_sort is needed for correct ordering, as you want the order-by-value to be preserved.
@gsf 在评论中指出，如果您选择首先比较 values 的比较器，则可以使用 only std::sort，并且如果它们相等，则对 keys 进行排序.
@gsf noted in the comment, you could use only std::sort if you choose a comparer which compares values first, and IF they're equal, sort the keys.
auto cmp = [](std::pair<K,V> const & a, std::pair<K,V> const & b)
{
return a.second != b.second? a.second < b.second : a.first < b.first;
};
std::sort(items.begin(), items.end(), cmp);
那应该是有效的.
但是等等，有一个更好的方法:存储 std::pair 而不是 std::pair 然后你根本不需要任何比较器—std::pair 的标准比较器就足够了，因为它首先比较 first(即 V)然后second 即 K:
But wait, there is a better approach: store std::pair<V,K> instead of std::pair<K,V> and then you don't need any comparer at all — the standard comparer for std::pair would be enough, as it compares first (which is V) first then second which is K:
std::vector<std::pair<V,K>> items;
//...
std::sort(items.begin(), items.end());
那应该很好用.
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