Sorting a vector of structs....

[ropitch]

When I try to compile the following code,
insert

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

using namespace std;
class c
{
public:
struct match
{
  int index;
  int count;

}n;

bool func(const match& a,const match& b)
{
  return a.count<b.count;
}

void compare()
{
  vector<match> chk;
  for(int i=5;i>=0;i++)
    {
      n.index=i;
      n.count=i;
      chk.push_back(n);
    }

  sort(chk.begin(),chk.end(),func);
  for(int i=0;i<chk.size();i++)
    {
      cout<<chk[i].index<<"   "<<chk[i].count<<endl;
    }

  //  return 0;


}
};
int main()
{
  c obj;
  obj.compare();
  return 0;
}

I get the following error....

sample.cpp: In member function ‘void c::compare()’:
sample.cpp:31: error: no matching function for call to ‘sort(__gnu_cxx::__normal_iterator<c::match*, std::vector<c::match, std::allocator<c::match> > >, __gnu_cxx::__normal_iterator<c::match*, std::vector<c::match, std::allocator<c::match> > >, <unresolved overloaded function type>)’
/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_algo.h:2735: note: candidates are: void std::sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<c::match*, std::vector<c::match, std::allocator<c::match> > >, _Compare = bool (c::*)(const c::match&, const c::match&)]


Aren't the iterators for a vector random access by default????Kindly help!!!!!

[anon]

func is a member function of c and as such needs to be called with an instance of c.

C c;
c.func(match_a, match_b);

As such it cannot be used as a predicate.

However, since it doesn't require any knowledge of a c, you might make it a static class function.

[laserlight]

By the way, I suggest that you format your code properly, e.g.,

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

using namespace std;
class c
{
public:
    struct match
    {
        int index;
        int count;
    }n;

    bool func(const match& a,const match& b)
    {
        return a.count<b.count;
    }

    void compare()
    {
        vector<match> chk;
        for (int i=5;i>=0;i++)
        {
            n.index=i;
            n.count=i;
            chk.push_back(n);
        }

        sort(chk.begin(),chk.end(),func);
        for (int i=0;i<chk.size();i++)
        {
            cout<<chk[i].index<<"   "<<chk[i].count<<endl;
        }

        //  return 0;
    }
};

int main()
{
    c obj;
    obj.compare();
    return 0;
}

This way it is easier to even see that func and compare are member functions of c.

[iMalc]

I'd think it would be more useful to put the comparison operator inside match and call it operator less-than. No need for the last parameter to sort unless you want to override the less-than operator of that class.

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

using namespace std;
class c
{
public:
    struct match
    {
        int index;
        int count;
        bool operator < (const match& a, const match& b)
        {
            return a.count < b.count;
        }
    } n;

    void compare()
    {
        vector<match> chk;
        for (int i=5; i>=0; i++)
        {
            n.index = i;
            n.count = i;
            chk.push_back(n);
        }

        sort(chk.begin(), chk.end());
        for (int i=0; i<chk.size(); i++)
        {
            cout << chk[i].index << "   " << chk[i].count << endl;
        }
    }
};

int main()
{
    c obj;
    obj.compare();
    return 0;
}

[ropitch]

Well ... I put the bool function and struct declaration outside class and it worked....

Anyway the above sample code is just a part of my program... in the original code there are three integer variables in struct (percent, totalpercent & score being the three variables). I've sorted them using three bool functions.... I'd like to know if there is a more object oriented way of doin the same(like iMalc's method).

Also thank you for your replies....

[brewbuck]

Nobody noticed this problem?

for(int i=5;i>=0;i++)
    {
      n.index=i;
      n.count=i;
      chk.push_back(n);
    }

[iMalc]

Nobody noticed this problem?

for(int i=5;i>=0;i++)
    {
      n.index=i;
      n.count=i;
      chk.push_back(n);
    }

Hmm... no I didn't.
Oddly enough I haven't written any backwards loops recently.

[ropitch]

Oh sorry ... I forgot to mention .. I corrected the mistake but dint quite bother about it till it successfully got compiled ....