c++ sorting with custom compare function

Question

I have a vector of following type:

std::vector< std::pair< std::pair< int, int >, std::vector<float> > >  neighbors;

I would like to create sort the following vector as follows

std::sort( neighbors.begin(), neighbors.end(), myFunc(index) );

where,

bool myFunc( const std::pair< std::pair< int, int >, float > &a, const std::pair< std::pair< int, int >, float > &b, const int index ){
     return ( a.second[index] > b.second[index] );
}

I know that the syntax is wrong but I wish to provide an index for comparing only that element of the vector.

I am not sure how to pass this argument to myFunc.

Side Note: You may replace std::pair by more descriptive version — , Jun 20 '16 at 22:06
"comparing only that element" ... with what? you need two elements to compare. — Octopus, Jun 20 '16 at 22:08

score 1 · Accepted Answer · edited May 23 '17 at 10:28

1

Lambdas:

std::sort(
    neighbors.begin(),
    neighbors.end(),
    [index](const std::pair< std::pair< int, int >, std::vector<float> > &a,  
            const std::pair< std::pair< int, int >, std::vector<float> > &b)
            { 
                 return ( a.second[index] > b.second[index] );
            }
);

See What is a lambda expression in C++11? for a detailled introduction.

edited May 23 '17 at 10:28

Community

1
1

answered Jun 20 '16 at 22:06

deviantfan

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Minor nitpick: `std::vector`, not `float` for the second item of the outer `std::pair` – Marco A. Jun 20 '16 at 22:22
@MarcoA. Thanks. (Hm, I was sure I copypasted this part...) – deviantfan Jun 20 '16 at 22:25

score 1 · Answer 2 · answered Jun 20 '16 at 22:22

Pre-C++11 solution: use an object instance as a custom comparator

struct Comparator {
    Comparator(int index) : index_(index) {}
    bool operator () (const std::pair< std::pair< int, int >, std::vector<float> > &a,
                      const std::pair< std::pair< int, int >, std::vector<float> > &b) 
    {
        return ( a.second[index_] > b.second[index_] );
    }

    int index_;
};

sort(neighbors.begin(), neighbors.end(), Comparator(42));

C++11+ solution: use a lambda

std::sort(neighbors.begin(), neighbors.end(), [index]
                         (const std::pair< std::pair< int, int >, std::vector<float> > &a, 
                          const std::pair< std::pair< int, int >, std::vector<float> > &b) 
  { 
    return ( a.second[index] > b.second[index] );
  }
);

My advice: go for a lambda if you're allowed to use C++11 features. It's probably more elegant and readable.

c++ sorting with custom compare function

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