/** \page tutorial_12 Filtering ranges with predicates

This examples shows:
- How to use predicates to filter which elements of a mesh you want iterate over

In the previous tutorial we discussed already that the ranges returned by functions like all_vertices(), voh_range() or outgoing_halfedges() provide a few helpful methods such as avg() or to_vector(). Another interesting method is filtered() which requires as argument something that can be called for an element of the range and returns a bool. The resulting range will then only iterate over elements for which the filter returs true.
The filter can be a lambda, a function pointer, a property manager holding a bool property, or a functor object such as the predicates defined in <OpenMesh/Core/Utils/Predicates.hh>. The predefined predicates can check the status of a mesh element and test if they are boundary.
With their help you can for example count all boundary vertices:

\dontinclude 12-predicates/predicates.cc
\skipline Count boundary vertices
\until boundary vertices

Predicates can be composed using the operators ||, &&, and !. This enables you to specify precisely which elements you want process in your loop, e.g. inner vertices that are selected:

\skipline Selected inner vertices
\until std::cout << std::endl

As mentioned above, the filter argument can be anything returning a bool for a given input, e.g. a function pointer:

\skipline Faces whose id is divisible by 3
\until faces whose id is divisible by 3

However, function pointers, lambdas etc are not composable. Fortunately, you can use make_predicate to turn them into composable predicates:


\skipline Faces which are tagged or whose id is not divisible by 3
\until faces which are tagged or whose id is not divisible by 3

Below is the complete source code:

\include 12-predicates/predicates.cc

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*/