# Best way of comparing two containers?

Discussion in 'C++' started by Dylan, Jul 9, 2004.

1. ### DylanGuest

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?

Dylan, Jul 9, 2004

2. ### Kai-Uwe BuxGuest

Dylan wrote:

> 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

Kai-Uwe Bux, Jul 9, 2004

3. ### E. Robert TisdaleGuest

Dylan wrote:

> 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?

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

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

E. Robert Tisdale, Jul 9, 2004
4. ### DylanGuest

On Thu, 08 Jul 2004 21:08:11 -0400, Kai-Uwe Bux <>
wrote:

>Dylan wrote:
>
>> 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).

Dylan, Jul 9, 2004
5. ### Andrey TarasevichGuest

Dylan wrote:
> ...
> 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?

--
Best regards,
Andrey Tarasevich

Andrey Tarasevich, Jul 9, 2004
6. ### DylanGuest

On Thu, 08 Jul 2004 18:48:52 -0700, Andrey Tarasevich
<> wrote:

>Dylan wrote:
>> ...
>> 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?

Boolean equality criteria only (==)

Dylan, Jul 9, 2004
7. ### DylanGuest

On Thu, 08 Jul 2004 18:16:33 -0700, "E. Robert Tisdale"
<> wrote:

>Dylan wrote:
>
>> 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?

>

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".

Dylan, Jul 9, 2004
8. ### E. Robert TisdaleGuest

Dylan wrote:

> E. Robert Tisdale wrote:
>
>>Dylan wrote:
>>
>>>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?

>>

>
> Is it?

Yes.

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

>
> no, see above

What above disqualifies this example?

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

>
> 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?

E. Robert Tisdale, Jul 9, 2004
9. ### Kai-Uwe BuxGuest

Dylan wrote:

> On Thu, 08 Jul 2004 18:48:52 -0700, Andrey Tarasevich
> <> wrote:
>
>>Dylan wrote:
>>> ...
>>> 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?

>
>
> 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

Kai-Uwe Bux, Jul 9, 2004
10. ### Markus DehmannGuest

On Fri, 09 Jul 2004 01:07:57 +0100, Dylan <> wrote:

> 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

Markus Dehmann, Jul 9, 2004
11. ### Karl Heinz BucheggerGuest

Dylan wrote:
>
> On Thu, 08 Jul 2004 18:48:52 -0700, Andrey Tarasevich
> <> wrote:
>
> >Dylan wrote:
> >> ...
> >> 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?

>
>
> 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)

--
Karl Heinz Buchegger

Karl Heinz Buchegger, Jul 9, 2004
12. ### Ioannis VranosGuest

Dylan wrote:

> 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, Jul 9, 2004
13. ### Ioannis VranosGuest

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

Ioannis Vranos, Jul 9, 2004
14. ### Jeff FlinnGuest

"Ioannis Vranos" <> wrote in message
news:ccmhat\$l13\$...
> 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

Jeff Flinn, Jul 9, 2004