How can I avoid "for" loops with an "if" condition inside them with C++?

With almost all code I write, I am often dealing with set reduction problems on collections that ultimately end up with naive "if" conditions inside of them. Here's a simple example:

for(int i=0; i<myCollection.size(); i++)
{
     if (myCollection[i] == SOMETHING)
     {
           DoStuff();
     }
}

With functional languages, I can solve the problem by reducing the collection to another collection (easily) and then perform all operations on my reduced set. In pseudocode:

newCollection <- myCollection where <x=true
map DoStuff newCollection

And in other C variants, like C#, I could reduce with a where clause like

foreach (var x in myCollection.Where(c=> c == SOMETHING)) 
{
   DoStuff();
}

Or better (at least to my eyes)

myCollection.Where(c=>c == Something).ToList().ForEach(d=> DoStuff(d));

Admittedly, I am doing a lot of paradigm mixing and subjective/opinion based style, but I can't help but feel that I am missing something really fundamental that could allow me to use this preferred technique with C++. Could someone enlighten me?

Solution 1:

IMHO it's more straight forward and more readable to use a for loop with an if inside it. However, if this is annoying for you, you could use a for_each_if like the one below:

template<typename Iter, typename Pred, typename Op> 
void for_each_if(Iter first, Iter last, Pred p, Op op) {
  while(first != last) {
    if (p(*first)) op(*first);
    ++first;
  }
}

Usecase:

std::vector<int> v {10, 2, 10, 3};
for_each_if(v.begin(), v.end(), [](int i){ return i > 5; }, [](int &i){ ++i; });

Live Demo

Solution 2:

Boost provides ranges that can be used w/ range-based for. Ranges have the advantage that they don't copy the underlying data structure, they merely provide a 'view' (that is, begin(), end() for the range and operator++(), operator==() for the iterator). This might be of your interest: http://www.boost.org/libs/range/doc/html/range/reference/adaptors/reference/filtered.html

#include <boost/range/adaptor/filtered.hpp>
#include <iostream>
#include <vector>

struct is_even
{
    bool operator()( int x ) const { return x % 2 == 0; }
};

int main(int argc, const char* argv[])
{
    using namespace boost::adaptors;

    std::vector<int> myCollection{1,2,3,4,5,6,7,8,9};

    for( int i: myCollection | filtered( is_even() ) )
    {
        std::cout << i;
    }
}

Solution 3:

Instead of creating a new algorithm, as the accepted answer does, you can use an existing one with a function that applies the condition:

std::for_each(first, last, [](auto&& x){ if (cond(x)) { ... } });

Or if you really want a new algorithm, at least reuse for_each there instead of duplicating the iteration logic:

template<typename Iter, typename Pred, typename Op> 
  void
  for_each_if(Iter first, Iter last, Pred p, Op op) {
    std::for_each(first, last, [&](auto& x) { if (p(x)) op(x); });
  }