[c++] Sorting a vector of custom objects



Answers

In the interest of coverage. I put forward an implementation using lambda expressions.

C++11

#include <vector>
#include <algorithm>

using namespace std;

vector< MyStruct > values;

sort( values.begin( ), values.end( ), [ ]( const MyStruct& lhs, const MyStruct& rhs )
{
   return lhs.key < rhs.key;
});

C++14

#include <vector>
#include <algorithm>

using namespace std;

vector< MyStruct > values;

sort( values.begin( ), values.end( ), [ ]( const auto& lhs, const auto& rhs )
{
   return lhs.key < rhs.key;
});
Question

How does one go about sorting a vector containing custom (i.e. user defined) objects.
Probably, standard STL algorithm sort along with a predicate (a function or a function object) which would operate on one of the fields (as a key for sorting) in the custom object should be used.
Am I on the right track?




Below is the code using lambdas

#include "stdafx.h"
#include <vector>
#include <algorithm>

using namespace std;

struct MyStruct
{
    int key;
    std::string stringValue;

    MyStruct(int k, const std::string& s) : key(k), stringValue(s) {}
};

int main()
{
    std::vector < MyStruct > vec;

    vec.push_back(MyStruct(4, "test"));
    vec.push_back(MyStruct(3, "a"));
    vec.push_back(MyStruct(2, "is"));
    vec.push_back(MyStruct(1, "this"));

    std::sort(vec.begin(), vec.end(), 
        [] (const MyStruct& struct1, const MyStruct& struct2)
        {
            return (struct1.key < struct2.key);
        }
    );
    return 0;
}






You can use user defined comparator class.

class comparator
{
    int x;
    bool operator()( const comparator &m,  const comparator &n )
    { 
       return m.x<n.x;
    }
 }



typedef struct Freqamp{
    double freq;
    double amp;
}FREQAMP;

bool struct_cmp_by_freq(FREQAMP a, FREQAMP b)
{
    return a.freq < b.freq;
}

main()
{
    vector <FREQAMP> temp;
    FREQAMP freqAMP;

    freqAMP.freq = 330;
    freqAMP.amp = 117.56;
    temp.push_back(freqAMP);

    freqAMP.freq = 450;
    freqAMP.amp = 99.56;
    temp.push_back(freqAMP);

    freqAMP.freq = 110;
    freqAMP.amp = 106.56;
    temp.push_back(freqAMP);

    sort(temp.begin(),temp.end(), struct_cmp_by_freq);
}

if compare is false, it will do "swap".




You are on the right track. std::sort will use operator< as comparison function by default. So in order to sort your objects, you will either have to overload bool operator<( const T&, const T& ) or provide a functor that does the comparison, much like this:

 struct C {
    int i;
    static bool before( const C& c1, const C& c2 ) { return c1.i < c2.i; }
 };

 bool operator<( const C& c1, const C& c2 ) { return c1.i > c2.i; }

 std::vector<C> values;

 std::sort( values.begin(), values.end() ); // uses operator<
 std::sort( values.begin(), values.end(), C::before );

The advantage of the usage of a functor is that you can use a function with access to the class' private members.




Links