Best way of comparing two containers?

[Dylan]

I'd like to compare two containers. They should be considered
equivalent if both containers have the same number of elements with
the same values, no matter what order the values are in.

For instance the containers

A = [1, 2, 3]
B = [1, 2, 3]

are obviously equal, but so would be

A = [3, 2, 1]
B = [2, 1, 3]

as would

A = [2, 2, 5, 1]
B = [2, 1, 5, 2]

What's the best (quickest) way of comparing containers in this way?

 

[Kai-Uwe Bux]

I'd like to compare two containers. They should be considered
equivalent if both containers have the same number of elements with
the same values, no matter what order the values are in.

For instance the containers

A = [1, 2, 3]
B = [1, 2, 3]

are obviously equal, but so would be

A = [3, 2, 1]
B = [2, 1, 3]

as would

A = [2, 2, 5, 1]
B = [2, 1, 5, 2]

What's the best (quickest) way of comparing containers in this way?

I am not sure if this is optimal, but the obvious way should be worth
trying: copy into two new containers, sort them and check whether they
are equal. That would work in O(n*log(n)) time:

#include <vector>
#include <algorithm>

template < typename T, template <typename> class C >
bool sort_compare ( const C<T> & c_1, const C<T> & c_2 ) {
std::vector<T> v_1 ( c_1.begin(), c_1.end() );
std::vector<T> v_2 ( c_2.begin(), c_2.end() );
std::sort( v_1.begin(), v_1.end() );
std::sort( v_2.begin(), v_2.end() );
return( v_1 == v_2 );
}

#include <iostream>

int main ( void ) {
std::vector< int > c_1;
c_1.push_back( 1 );
c_1.push_back( 1 );
c_1.push_back( 3 );
std::vector< int > c_2;
c_2.push_back( 3 );
c_2.push_back( 1 );
c_2.push_back( 1 );
std::vector< int > c_3;
c_3.push_back( 3 );
c_3.push_back( 0 );
c_3.push_back( 1 );
std::cout << sort_compare( c_1, c_2 )
<< " "
<< sort_compare( c_2, c_3 )
<< "\n";
}


Do you suspect that there is a linear time method?


Best

Kai-Uwe Bux

 

[E. Robert Tisdale]

I'd like to compare two containers. They should be considered
equivalent if both containers have the same number of elements with
the same values, no matter what order the values are in.

For instance the containers

A = [1, 2, 3]
B = [1, 2, 3]

are obviously equal, but so would be

A = [3, 2, 1]
B = [2, 1, 3]

as would

A = [2, 2, 5, 1]
B = [2, 1, 5, 2]

What's the best (quickest) way of comparing containers in this way?

Your *equivalence relationship* is *ill-defined*.

Is [3, 1, 1] equal to [3, 3, 1] for example?

What do you mean by "order"?
1 < 2 < . . . < INT_MAX?

 

[Dylan]

I'd like to compare two containers. They should be considered
equivalent if both containers have the same number of elements with
the same values, no matter what order the values are in.

For instance the containers

A = [1, 2, 3]
B = [1, 2, 3]

are obviously equal, but so would be

A = [3, 2, 1]
B = [2, 1, 3]

as would

A = [2, 2, 5, 1]
B = [2, 1, 5, 2]

What's the best (quickest) way of comparing containers in this way?

I am not sure if this is optimal, but the obvious way should be worth
trying: copy into two new containers, sort them and check whether they
are equal. That would work in O(n*log(n)) time:

#include <vector>
#include <algorithm>

template < typename T, template <typename> class C >
bool sort_compare ( const C<T> & c_1, const C<T> & c_2 ) {
std::vector<T> v_1 ( c_1.begin(), c_1.end() );
std::vector<T> v_2 ( c_2.begin(), c_2.end() );
std::sort( v_1.begin(), v_1.end() );
std::sort( v_2.begin(), v_2.end() );
return( v_1 == v_2 );
}

#include <iostream>

int main ( void ) {
std::vector< int > c_1;
c_1.push_back( 1 );
c_1.push_back( 1 );
c_1.push_back( 3 );
std::vector< int > c_2;
c_2.push_back( 3 );
c_2.push_back( 1 );
c_2.push_back( 1 );
std::vector< int > c_3;
c_3.push_back( 3 );
c_3.push_back( 0 );
c_3.push_back( 1 );
std::cout << sort_compare( c_1, c_2 )
<< " "
<< sort_compare( c_2, c_3 )
<< "\n";
}


Do you suspect that there is a linear time method?


Best

Kai-Uwe Bux

Thanks for your answer, but the reason I stipulated that the elements
can be in any order is that, for the problem I'm working on, it's
unreasonable to assume there is a sorting criteria defined for the
element type (or that one can be defined using the type interface).

 

[Andrey Tarasevich]

...
Thanks for your answer, but the reason I stipulated that the elements
can be in any order is that, for the problem I'm working on, it's
unreasonable to assume there is a sorting criteria defined for the
element type (or that one can be defined using the type interface).
...

In that case you should specify what kind of criteria you _do_ have
defined. Boolean equality criteria only? Something else?

 

[Dylan]

In that case you should specify what kind of criteria you _do_ have
defined. Boolean equality criteria only? Something else?

Boolean equality criteria only (==)

 

[Dylan]

I'd like to compare two containers. They should be considered
equivalent if both containers have the same number of elements with
the same values, no matter what order the values are in.

For instance the containers

A = [1, 2, 3]
B = [1, 2, 3]

are obviously equal, but so would be

A = [3, 2, 1]
B = [2, 1, 3]

as would

A = [2, 2, 5, 1]
B = [2, 1, 5, 2]

What's the best (quickest) way of comparing containers in this way?

Your *equivalence relationship* is *ill-defined*.

Is it?

Is [3, 1, 1] equal to [3, 3, 1] for example?

no, see above

What do you mean by "order"?
1 < 2 < . . . < INT_MAX?

replace "order" with "position".

 

[E. Robert Tisdale]

I'd like to compare two containers. They should be considered
equivalent if both containers have the same number of elements with
the same values, no matter what order the values are in.

For instance the containers

A = [1, 2, 3]
B = [1, 2, 3]

are obviously equal, but so would be

A = [3, 2, 1]
B = [2, 1, 3]

as would

A = [2, 2, 5, 1]
B = [2, 1, 5, 2]

What's the best (quickest) way of comparing containers in this way?

Your *equivalence relationship* is *ill-defined*.

Is it?
Yes.

Is [3, 1, 1] equal to [3, 3, 1] for example?

no, see above

What above disqualifies this example?

replace "order" with "position".

What *type* of container are you talking about?
Apparently, it's *not* a set.
Can you extract the set of elements from each container
and compare them for equality
to get the equivalence relationship that you want?

 

[Kai-Uwe Bux]

Boolean equality criteria only (==)

Hm,


in this case, I only see a quadratic way of doing it:


template < typename T, template <typename> class C >
bool nosort_compare ( const C<T> & c_1, const C<T> & c_2 ) {
std::vector<T> v_1 ( c_1.begin(), c_1.end() );
std::vector<T> v_2 ( c_2.begin(), c_2.end() );
if ( v_1.size() != v_2.size() ) {
return( false );
}
typename std::vector<T>::size_type i_1 = 0;
typename std::vector<T>::size_type i_2 = 0;
while ( i_1 < v_1.size() ) {
if ( v_1[i_1] == v_2[i_2] ) {
std::swap( v_2[i_1], v_2[i_2] );
++ i_1;
i_2 = i_1;
continue;
} else if ( i_2 == v_2.size() ) {
return( false );
} else {
++ i_2;
}
}
return( true );
}


Beware: as this code is not as transparent as the sorting method, it
may be deeply flawed.


Best

Kai-Uwe Bux

 

[Markus Dehmann]

I'd like to compare two containers. They should be considered
equivalent if both containers have the same number of elements with
the same values, no matter what order the values are in.

For instance the containers

A = [1, 2, 3]
B = [1, 2, 3]

are obviously equal, but so would be

A = [3, 2, 1]
B = [2, 1, 3]

as would

A = [2, 2, 5, 1]
B = [2, 1, 5, 2]

What's the best (quickest) way of comparing containers in this way?

Isn't that a multiset?
http://mathworld.wolfram.com/Multiset.html

If so, use the STL multiset implementation
http://www.sgi.com/tech/stl/multiset.html

Markus

 

[Karl Heinz Buchegger]

Boolean equality criteria only (==)

Hmm. Would it be possible to make up some artificial 'less'
relationship just for the purpose of sorting? It doesn't
matter if that 'less' actually makes some sense in the
assignment space.
(Such a thing is almost always possible to do)

 

[Ioannis Vranos]

I'd like to compare two containers. They should be considered
equivalent if both containers have the same number of elements with
the same values, no matter what order the values are in.

For instance the containers

A = [1, 2, 3]
B = [1, 2, 3]

are obviously equal, but so would be

A = [3, 2, 1]
B = [2, 1, 3]

as would

A = [2, 2, 5, 1]
B = [2, 1, 5, 2]

What's the best (quickest) way of comparing containers in this way?

I think you need to use std::set or std::multiset.






Regards,

Ioannis Vranos

 

[Ioannis Vranos]

Example:


#include <iostream>
#include <set>


int main()
{
using namespace std;

multiset<int>t1, t2;

t1.insert(2);
t1.insert(2);
t1.insert(5);
t1.insert(1);

t2.insert(2);
t2.insert(1);
t2.insert(5);
t2.insert(2);

if(t1==t2)
cout<<"\nEqual!\n";

}






Regards,

Ioannis Vranos

 

[Jeff Flinn]

Example:


#include <iostream>
#include <set>


int main()
{
using namespace std;

multiset<int>t1, t2;

t1.insert(2);
t1.insert(2);
t1.insert(5);
t1.insert(1);

t2.insert(2);
t2.insert(1);
t2.insert(5);
t2.insert(2);

if(t1==t2)
cout<<"\nEqual!\n";

}

Which works for int, but the OP said his T only implements operator==, and
not any of the inequalities.

Jeff F