使用sort()对unordered_map进行排序 [英] sort an unordered_map using sort()

问题描述

我正在尝试使用 sort()函数对 unordered_map 进行排序，但是我一直遇到编译器错误.有人可以帮忙吗?

I am trying to sort an unordered_map using sort() function but I keep getting a compiler error. Can anyone help?

bool comp(pair<char,int> a, pair<char,int> b) {
    return a.second < b.second;
}

void rearrangeKDist(char str[], int d) {
    int n = strlen(str);
    unordered_map<char, int> table;
    for (int i=0; i<n; i++) {
        unordered_map<char, int>::iterator it = table.find(str[i]);   
        if (it == table.end()) {
            table.insert(make_pair(str[i], 1));
        } else {
            it->second = it->second+1;
        }
    }
    for (unordered_map<char, int>::iterator it=table.begin(); it!=table.end(); it++)
        cout<<it->first<<" "<<it->second<<endl;
    sort(table.begin(), table.end(), comp);
    for (unordered_map<char, int>::iterator it=table.begin(); it!=table.end(); it++)
        cout<<it->first<<" "<<it->second<<endl;

}

推荐答案

从编译和逻辑角度来看，这都是不可能的.从类型的角度来看， std :: sort 要求:

This is impossible from both a compilation and logical standpoint. From a type standpoint, std::sort requires:

-RandomIt必须满足ValueSwappable和RandomAccessIterator的要求.
-取消引用的RandomIt的类型必须满足MoveAssignable和MoveConstructible的要求.

-RandomIt must meet the requirements of ValueSwappable and RandomAccessIterator.
-The type of dereferenced RandomIt must meet the requirements of MoveAssignable and MoveConstructible.

std :: unordered_map 上的迭代器类型是ForwardIterator，而不是RandomAccessIterator，因此不满足第一个要求.取消引用的迭代器的类型为 pair< const Key，T> ，它不是MoveAssignable(无法分配给 const )，因此第二个要求也不满足.

The iterator type on std::unordered_map is a ForwardIterator, not a RandomAccessIterator, so the first requirement is unsatisfied. The type of the dereferenced iterator is pair<const Key, T>, which is not MoveAssignable (can't assign to const), so the second requirement is also unsatisfied.

从逻辑上讲，对无序容器进行排序是没有意义的.它是无序的.而且，复杂性保证了 unordered_map 能够实现的要求要求非常具体的顺序，您不应该也不允许这样做.

From a logical standpoint, sorting an unordered container makes no sense. It's unordered. And the complexity guarantees that unordered_map is able to achieve require a very specific ordering that you shouldn't be, and aren't, allowed to mess with.

如果要对 unordered_map 进行排序"，请将它们放入 vector :

If you want to "sort" your unordered_map, put them in a vector:

std::vector<std::pair<char, int>> elems(table.begin(), table.end());
std::sort(elems.begin(), elems.end(), comp);

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