CODEWITHSUNDEEP
c++c++11stlstd
Ela782
Ela782

Reputation: 5181

Filter/copy a vector based on elements in another vector

I have a vector of elements of some type (with a name member), and I want to filter (or copy) this vector so that it contains only the elements whose name matches a list of certain names. So basically: "Copy the element if its name is equal to any of the names in vector<string> filter".

I've been trying stuff with std::copy_if and std::any_of, that resulted in code like below, but I couldn't get it to work, for once because any_of can't be applied in this way, and I'm unsure of what the correct tools are in this case.

using namespace std;

template<class T>
struct MyType {
  string name;
  T data;
};

template<class T>
vector<MyType<T>> filter(vector<MyType<T>> items, vector<string> filter)
{
  vector<MyType<T>> filteredItems;
  copy_if(begin(items), end(items), begin(filteredItems), any_of(begin(filter), end(filter), [](const MyType<T>& lhs, const MyType<T>& rhs) {return lhs.name == rhs.name; }));
  return filteredItems;
};

int main() {
  vector<MyType<int>> items { {"a", 1}, {"b", 2}, {"c", 3} };
  vector<string> filter { "a", "c" };
  auto filteredItems = filter(items, filter);
}

How can I accomplish this, preferably with std:: functionality? (C++11/14 welcome)

Above code on ideone if you want to play around.

I guess this might be a good example to use ranges, but my code needs to compile on VS 2013 so I can't use https://github.com/ericniebler/range-v3 and I'm a bit hesitant to use an older ranges library, but I could be convinced otherwise.

Upvotes: 0

Views: 1202

Answers (1)

T.C.
T.C.

Reputation: 137310

copy_if is the right choice, but your predicate is wrong, and you need to use back_inserter.

copy_if(begin(items), end(items), back_inserter(filteredItems), 
       [&](const MyType<T> & item) { return std::find(begin(filter), end(filter), item.name) != end(filter);} );

To find something in a range, use std::find, which performs a simple linear search. If filter is sorted, std::binary_search can be used too.

Upvotes: 2

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