如何在C ++中对map使用排序功能？

Question

Just look at my program , i have declared a vector of map objects .

#include<iostream>
#include<map>
#include<vector>
#include<algorithm>

using std::cout;
using std::cin;
using std::endl;
using std::map;
using std::string;
using std::vector;
using std::make_pair;
using std::multimap;
using std::sort;


int main(void)
{
    vector< multimap<string , string>  > data;

    data.resize(1);
    data[0].insert(make_pair("outlook","sunny"));
    data[0].insert(make_pair("wind","weak"));

    data.resize(data.size() + 1);

    data[1].insert(make_pair("outlook","sunny"));   
    data[1].insert(make_pair("wind","strong"));

    data.resize(data.size() + 1);

    data[2].insert(make_pair("outlook","overcast"));    
    data[2].insert(make_pair("wind","weak"));

    data.resize(data.size() + 1);

    data[3].insert(make_pair("outlook","rain"));    
    data[3].insert(make_pair("wind","weak"));

    data.resize(data.size() + 1);

    data[4].insert(make_pair("outlook","rain"));    
    data[4].insert(make_pair("wind","weak"));

    data.resize(data.size() + 1);

    data[5].insert(make_pair("outlook","rain"));    
    data[5].insert(make_pair("wind","strong"));


    sort(data.begin() , data.end() , []( vector< multimap<string,string> >  a ,  vector< multimap<string,string> >  b)
    {
        return a[0].find("outlook")->second < b[0].find("outlook")->second;
    });


    return 0;
}

I have defined a sort function to sort vector entries with respect to the "outlook" index of map but this function is not working . Can anyone tell me what is wrong with this function and the correct way of sorting ?

Answer 1

The predicate for std::sort is going to be called with arguments of type vector::value_type , and not the vector s themselves. Change the lambda expression to

[](multimap<string,string> const& a, multimap<string,string> const& b)
{
    return a.find("outlook")->second < b.find("outlook")->second;
}

The const& is not necessary, but you probably want to avoid making unnecessary copies of the multimap s each time the predicate is called. You may also want to check the result of multimap::find before deferencing the result.

And instead of calling vector::resize each time to add a new multimap , you can call

data.emplace_back();

which will default construct a new multimap .