need help for STL sort inside a class

[menghiot]

Hi, i tried to sorting an array a, with a condition s[a] <= s[a[j]] (i<j), s is given. Both a and s is contained inside a class. You can take a look at the sample source code.

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
class gg{
public:

bool cmp( int a, int b)
{
return s[a]<s;
}


gg(){
for(int i=0; i<20; i++)
s.push_back(i);

random_shuffle(s.begin(), s.end());
for(int i=0; i<20; i++)
a.push_back(i);
random_shuffle(a.begin(), a.end());

sort(a.begin(), a.end(), cmp );

}
vector< int > s;
vector< int > a;
};

int main()
{
gg tmp;
}




My problem is that it cannot compile. CAN ANYONE HELP ME?

[mamun]

The compare function should be declared outside and before the class gg; and it should be something like this

Code: Select all

bool cmp(int a, int b) {
	return a < b;
}

[Krzysztof Duleba]

If you really want to encapsulate the comparator inside a class, you should use a function object instead of a function (which is a method in this case and thus cannot be used by sort directly). Just create an inner class with a public operator() overloaded for the type you want to compare and then use an instance of that class as the comparator when calling sort.

[menghiot]

Hi ,

I, actually, did all the methods which you mention, before I post the questions . what I tried to do is sorting array a with condition :
s[a] < s[a[j]] (i<j) , not a<a[j] , (i<j) ....

if I declare a inner class like

class gg{
public:

class cmp{
public:
bool operator()(int a, int b)
{
return s[a]<s;
}
};

.....
};

cmp class is not able to see vector<int> s;

[Krzysztof Duleba]

There are plenty of solutions to your problem, but I believe that the best one is to change design - it seems there's something terribly wrong with it. If elements of a and s are so related, you might think of using a separate class for them.

Anyway, if you just want to stick with two vectors, you can try something like this:

Code: Select all

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

using namespace std;

struct foo{
    vector<int> a;
    vector<int> s;

    struct cmp{
        static vector<int> * sref;

        bool operator()(int x, int y)const{
            return ((*sref)[x] < (*sref)[y]);
        }
    };

    void perform(){
        cmp comparer;
        comparer.sref = &s;

        sort(a.begin(), a.end(), comparer);
    }
};

vector<int> * foo::cmp::sref;

int main(){
    foo bar;
    bar.perform();
}

[menghiot]

Many thanks !

[Quantris]

I tend to prefer something like:

Code: Select all

struct sortBy {
  const vector<int> &key;
  sortBy(const vector<int> &S) : key(S) {}
  bool operator()(int a, int b) const {
    return key[a] < key[b];
    }
  };

and then you can use sort(a.begin(), a.end(), sortBy(s));

This is just a bit more modular so you can apply it to vectors that aren't in a class (you can generalise more and pass it a function that assigns a key value to the sorted elements, or template the sortBy structure, etc.)

However, I have to agree with Krzysztof Duleba -- it is probably better to keep a and s as a vector of pairs or something to make it simpler (and also you get to keep the a value and s value at the same location).