using STL to find all elements in a vector

Question

I have a collection of elements that I need to operate over, calling member functions on the collection:

std::vector v;
... // vector is popula         


        

Answer 1

For what its worth for_each_if is being considered as an eventual addition to boost. It isn't hard to implement your own.

Answer 2

Boost Lambda makes this easy.

#include <boost/lambda/lambda.hpp>
#include <boost/lambda/bind.hpp>
#include <boost/lambda/if.hpp>

std::for_each( v.begin(), v.end(), 
               if_( MyPred() )[ std::mem_fun(&MyType::myfunc) ] 
             );

You could even do away with defining MyPred(), if it is simple. This is where lambda really shines. E.g., if MyPred meant "is divisible by 2":

std::for_each( v.begin(), v.end(), 
               if_( _1 % 2 == 0 )[ std::mem_fun( &MyType::myfunc ) ]
             );

---

Update: Doing this with the C++0x lambda syntax is also very nice (continuing with the predicate as modulo 2):

std::for_each( v.begin(), v.end(),
               [](MyType& mt ) mutable
               {
                 if( mt % 2 == 0)
                 { 
                   mt.myfunc(); 
                 }
               } );

At first glance this looks like a step backwards from boost::lambda syntax, however, it is better because more complex functor logic is trivial to implement with c++0x syntax... where anything very complicated in boost::lambda gets tricky quickly. Microsoft Visual Studio 2010 beta 2 currently implements this functionality.

Answer 3

You can use Boost.Foreach:

BOOST_FOREACH (vector<...>& x, v)
{
    if (Check(x)
        DoStuff(x);
}

Answer 4

Lamda functions - the idea is to do something like this

for_each(v.begin(), v.end(), [](MyType& x){ if (Check(x) DoSuff(x); })  

Origial post here.

Answer 5

std::vector<int> v, matches;
std::vector<int>::iterator i = v.begin();
MyPred my_pred;
while(true) {
    i = std::find_if(i, v.end(), my_pred);
    if (i == v.end())
        break;
    matches.push_back(*i);
}

For the record, while I have seen an implementation where calling end() on a list was O(n), I haven't seen any STL implementations where calling end() on a vector was anything other than O(1) -- mainly because vectors are guaranteed to have random-access iterators.

Even so, if you are worried about an inefficient end(), you can use this code:

std::vector<int> v, matches;
std::vector<int>::iterator i = v.begin(), end = v.end();
MyPred my_pred;
while(true) {
    i = std::find_if(i, v.end(), my_pred);
    if (i == end)
        break;
    matches.push_back(*i);
}

Answer 6

Is it ok to change the vector? You may want to look at the partition algorithm.
Partition algorithm

Another option would be to change your MyType::myfunc to either check the element, or to take a predicate as a parameter and use it to test the element it's operating on.