How to combine a function and a predicate in for_each?

Question

How can you call a Function over some part of a container, using for_each() ?

I have created a for_each_if() to do a

for( i in shapes )
    if( i.color == 1 )
        displayShape(i);

and the call looks like

for_each_if( shapes.begin(), shapes.end(),
                       bind2nd( ptr_fun(colorEquals), 0 ),
                       ptr_fun( displayShape ) );

bool colorEquals( Shape& s, int color ) {
    return s.color == color;
}

However, I feel immitating STL-like algorithms is not something that I should be doing.

1. Is there a way to use only existing STL keywords to produce this ?

   I did not want to do a

    for_each( shapes.begin(), shapes.end(),
                      bind2nd( ptr_fun(display_shape_if_color_equals), 0 ) );
   

   because, in a more complicated case, the functor name would be misleading with respect to what the functor

2. *Is there a way to access a struct's member (like colorEquals) for functions like for_each without having to create a function ? *

Solution 1

To use a regular for_each with an if you would need a Functor that emulates an if condition.

#include <algorithm>
#include <vector>
#include <functional>
#include <iostream>
#include <boost/bind.hpp>

using namespace std;

struct incr {
  typedef void result_type;
  void operator()(int& i) { ++i; }
};

struct is_odd {
  typedef bool return_type;
  bool operator() (const int& value) {return (value%2)==1; }
};


template<class Fun, class Cond>
struct if_fun {
  typedef void result_type;
  void operator()(Fun fun, Cond cond, int& i) {
    if(cond(i)) fun(i);
  }
};


int main() {
  vector<int> vec;
  for(int i = 0; i < 10; ++i) vec.push_back(i);

  for_each(vec.begin(), vec.end(), boost::bind(if_fun<incr, is_odd>(), incr(), is_odd(), _1));
  for(vector<int>::const_iterator it = vec.begin(); it != vec.end(); ++it)
    cout << *it << " ";
}

Unfortunately my template hackery isn't good enough to manage this with bind1st and bind2nd as it somehow gets confusing with the binder being returned being a unary_function but it looks pretty good with boost::bind anyhow. My example is no means perfect as it doesn't allow the Func passed into if_fun to return and I guess somebody could point out more flaws. Suggestions are welcome.

Solution 2

Imitating STL-like algorithms is exactly what you should be doing. That's why they're in the STL.

Specifically, you can use a functor instead of creating an actual function and binding it. This is much neater, really.

template<typename Iterator, typename Pred, typename Operation> void 
for_each_if(Iterator begin, Iterator end, Pred p, Operation op) {
    for(; begin != end; begin++) {
        if (p(*begin)) {
            op(*begin);
        }
    }
}
struct colorequals {
    colorequals(int newcol) : color(newcol) {}
    int color;
    bool operator()(Shape& s) { return s.color == color; }
};
struct displayshape {
    void operator()(Shape& s) { // display the shape }
};
for_each_if(shapes.begin(), shapes.end(), colorequals(0), displayshape());

This is usually considered the idiomatic way to go.

Solution 3

Using boost range adaptors is much neater.

using boost::adaptor::filtered;
using boost::bind;

class Shape {
  int color() const;
};

void displayShape(const Shape & c);

bool test_color(const Shape & s, int color ){
    return s.color() == color;
}

boost::for_each
    ( vec | filtered(bind(&test_color, _1, 1)
    , bind(&displayShape, _1)
    )

Note the use of the new range library to abstract away iterators in favor of ranges and the range adaptors library to compose a pipeline of operations.

All the standard stl iterator based algorithms have been ported to range based algorithms.

Imagine this

typedef boost::unordered_map<int, std::string> Map;
Map map;
...
using boost::adaptor::map_keys;
using boost::bind
using boost::ref
using boost::adaptor::filtered; 

bool gt(int a, int b)
{ return a > b };

std::string const & get(const Map & map, int const & a)
{ return map[a] }

// print all items from map whose key > 5
BOOST_FOREACH
    ( std::string const & s
    , map 
        | map_keys 
        | filtered(bind(&gt, _1, 5)) 
        | transformed(bind(&get, ref(map), _1))
    )
    {
        cout << s;
    }

Read Range Adaptors and Range Algorithm.

Solution 4

You can use the C++20 ranges. Here an example where we add one to all even numbers of a std::vector

#include <ranges>
#include <algorithm>
#include <vector> 

namespace ranges = std::ranges;

std::vector<int> vec = {1, 2, 3, 4, 5};
const auto even = [](int i) { return 0 == i % 2; };
ranges::for_each(vec | std::views::filter(even), [](int& i){ i+=1;});

You can find a living example on compiler explorer here

View more solutions

Comments

[Grim Fandango]

I have to work with ~10 c++ compilers, some dating back to late 90's. STL implementation in many of them is so diverse, which implies that S in STL does not stand for 'Standard'. That's why I tend to avoid immitating STL functions, and rather just call them. I thought that if for_each_if() is the way to solve the problem, then it should have already been in STL, shouldn't it? (I mean remove_if, find_if, count_if is there already).

[Puppy]

@Grim: People normally just stick the if in their functor for for_each. There's no need for such a for_each_if construct.

[Grim Fandango]

Cool. I suppose I can now add a helper function to instantiate an if_fun object so that I can reduce the template parameters when calling for_each()...

[bradgonesurfing]

boost::bind works equally well with normal functions so the extra noise of class based Functors is not really necessary

[just somebody]

the automatic url->a href conversion skipped the algorithms one, presumably because of length.

[newhouse]

This solution is hell of an unituitive mess. Boost adaptors and range is what you want to use here.

[Felix Dombek]

I would love to see this answer updated to modern C++.