Finding 2 lists if there is equal element?

[Allerdyce.John]

Hi,

I have 2 list of class A, is there a STL algorithm which find out if
there is equal element between them (using == operator of A)?

Thank you.

 

[Neil Cerutti]

Hi,

I have 2 list of class A, is there a STL algorithm which find out if
there is equal element between them (using == operator of A)?

Thank you.

std::sort the lists and use std::set_intersection to create a
list of common elements.

 

[Andrew Koenig]

I have 2 list of class A, is there a STL algorithm which find out if
there is equal element between them (using == operator of A)?

If the only comparison operator available is ==, then anything you do is
going to be slow because you have to compare all pairs of elements from the
two lists. You can use the find_first_of algorithm to do those comparisons.

If you have an order relation such as < available, there are other
algorithms, typically involving sorting, that will answer your question more
quickly on average. If you tell us what operations are available, we may be
able to suggest a strategy.

 

[Michiel.Salters]

std::sort the lists and use std::set_intersection to create a
list of common elements.

That assumes an operator<. You could also assume a function int
hash(A);
that could reduce the complexity to O(1).

HTH,
Michiel Salters