std::map find_if 条件样式混淆

Question

I'd like to use std::find_if to search for the first element in my map that has a certain value in a specific element of its value structure. I'm a little confused though. I think I need to use bind1st or bind2nd, but I'm not positive that's the right way to go.

Here's some pseudo-code:

struct ValueType { int x, int y, int z };

std::map<int, ValueType> myMap;

... {populate map}

std::map<int, ValueType>::iterator pos = std::find_if(myMap.begin(), myMap.end(), <?>); 

So, let's say that I wanted to find the first element of the map where the .x member of the ValueType was equal to a certain integer value (which can change each call).

What would be the best way to write a function or function object to achieve this? I understand that the <?> has to be a unary predicate which makes me think I'll need bind1st or bind2nd to provide the integer value I'm checking for, but I'm not sure how to go about it. It's been way too long since I looked at this stuff! >.<

Answer 1

You can use a lambda function

int val = ...;
auto it = std::find_if(myMap.begin(), myMap.end(), 
   [val](const std::pair<int, ValueType> & t) -> bool { 
      return t.second.x == val;
   }
);

But as Kirill V. Lyadvinsky answer suggests the "first" element may not be what you expect.

Answer 2

Elements in the map are not sorted by value, they are sorted according to the key. So the phrase "the first element" has not much sense.

To find some element (not the first) that has x equal to some value you can write the functor as follows:

struct check_x
{
  check_x( int x ) : x_(x) {}
  bool operator()( const std::pair<int, ValueType>& v ) const 
  { 
    return v.second.x == x_; 
  }
private:
  int x_;
};

Then use it as follows:

// find any element where x equal to 10
std::find_if( myMap.begin(), myMap.end(), check_x(10) );

Answer 3

For the lazy, use a C++17 auto lambda, then you don't need to be verbose with the type.

const auto it = std::find_if(myMap.begin(), myMap.end(), [&val](const auto &it) { 
      return it.second.x == val; // Comparing with the object
   }
);

Answer 4

struct Pred
{
    Pred(int x) : x_(x) { }
    bool operator()(const std::pair<int, ValueType>& p)
    {
        return (x_ == p.second.x);
    }
private:
    int x_;
};

... = std::find_if(myMap.begin(), myMap.end(), Pred(NUMBER));

Answer 5

Building on all the answers above I cheat by using decltype with C++11 semantics.

auto beg_ = myMap.begin();
auto end_ = myMap.end();
auto it = find_if(beg_, end_,
    [&some_val](decltype(*beg_) & vt) {
        return vt.second == some_val;});
if (end_ != it) {
    auto key_found = (*it).first;
} else {
    // throw error not found.
}

Answer 6

This doesn't have anything to do with std::bind1st or std::bind2nd . First of all, you have to keep in mind that the elements of a map are key-value pairs, in your case std::pair<int,ValueType> . Then you just need a predicate that compares the x member of the second member of yuch a pair against a specific value:

struct XEquals : std::unary_function<std::pair<int,ValueType>,bool>
{
    XEquals(int _x)
        : x(_x) {}
    bool operator()(const std::pair<int,ValueType> &v) const
        { return p.second.x == x; }
    int x;
};

Answer 7

如果您还想在值中进行搜索，那么最好使用Boost Bimap以免变慢？

Answer 8

using Boost.Bind and Boost.Lambda :

...
#include <boost/bind.hpp>
#include <boost/lambda/lambda.hpp>
...
typedef std::map<int, ValueType> MapType;
...
MapType::iterator pos = std::find_if(myMap.begin(), myMap.end(), 
    boost::bind(&ValueType::y, boost::bind(&MapType::iterator::value_type::second, _1)) == magic_number);