使用lambda的c ++ 11排序列表 [英] c++11 sorting list using lambda

问题描述

在练习使用lambda时，我编写了该程序，该程序应该按pair的list的第二个元素(int)进行排序.

While practicing the use of lambdas, I wrote this program which is supposed to sort a list of pairs by their second element (an int).

#include <iostream>
#include <algorithm>
#include <list>

using namespace std;
int main()
{
    list<pair <string, int>> s = {{"two", 2}, {"one", 1}, {"three", 3}};

    sort(s.begin(), s.end(), [](pair<string,int> a, pair<string, int> b) -> bool {
        return (a.second) > (b.second);
    });

    for_each(s.begin(), s.end(), [](pair<string, int> a) {
        cout << a.first << " " << a.second << endl;
    });
}

不过，我得到了那些错误:

I get those errors, though:

c:\qt\qt5.2.0\tools\mingw48_32\lib\gcc\i686-w64-mingw32\4.8.0\include\c++\bits\stl_algo.h:5513: error: no match for 'operator-' (operand types are 'std::_List_iterator<std::pair<std::basic_string<char>, int> >' and 'std::_List_iterator<std::pair<std::basic_string<char>, int> >')
     std::__lg(__last - __first) * 2, __comp);
                  ^

c:\qt\qt5.2.0\tools\mingw48_32\lib\gcc\i686-w64-mingw32\4.8.0\include\c++\bits\stl_algo.h:2245: ошибка: 'void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = std::_List_iterator<std::pair<std::basic_string<char>, int> >; _Compare = main()::__lambda0]', declared using local type 'main()::__lambda0', is used but never defined [-fpermissive]
     __final_insertion_sort(_RandomAccessIterator __first,
     ^

我的代码有什么问题?

推荐答案

您不能将std::sort与顺序容器(例如std::list或std::forward_list)一起使用，因为它们没有标准算法所需的随机访问迭代器std::sort.因此，两个容器都有自己的成员函数sort.

You may not use std::sort with sequential containers such as std::list or std::forward_list because they have no random access iterator that is required by the standard algorithm std::sort. By this reason the both containers have their own member functions sort.

在这种情况下，代码将采用以下方式:

In you case the code will look the following way:

#include <iostream>
#include <list>
#include <string>

using namespace std;

int main()
{
    list<pair <string, int>> s = {{"two", 2}, {"one", 1}, {"three", 3}};
    s.sort( []( const pair<string,int> &a, const pair<string,int> &b ) { return a.second > b.second; } );

    for ( const auto &p : s )
    {
        cout << p.first << " " << p.second << endl;
    }
}

请注意，您需要包含标头<string>，否则您的程序将无法与其他编译器一起编译.

Take into account that you need to include header <string> otherwise your program will not be compiled with other compilers.

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