functor or extractor for a vector question

I'm new to functors (still trying to grasp the concept) and I wanted to know if I can do the same as the following code with a functor? It basically converts a vector of a two item struct into two vectors.

Code:

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typedef struct
{
short A;
short B;
} MyData;

..........

std::vector<MyData> history;
........

const unsigned int listSize = history.size();
std::vector<short> myA, myB;
 
for (int i = 0; i < listSize; i++)
{
        //extract A data from history
        myA.push_back(history[i].A);
        //extract p-p interval data from history
        myB.push_back(history[i].B);
}

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Yeah, you could. If it were me I'd do:
1) Write a functor which takes pointers or references to myA and myB on construction, and takes a vector<MyData>::value_type as the argument to its operator().
2) The operator() does the two push_backs.
3) Use the functor with std::for_each.

Yes, you can. Create a functor that takes references to your two vectors as arguments to its constructor. The operator() should take one argument of type MyData.

Code:

---

#include <vector>
#include <algorithm>
#include <iostream>

struct MyData
{
  short A;
  short B;
  MyData(short a, short b) : A(a), B(b) {}
};

struct Functor {
  std::vector<short> &ra, &rb;
  Functor(std::vector<short>& a , std::vector<short>& b) : ra(a), rb(b) {}
  void operator()(MyData arg1) {
    ra.push_back(arg1.A);
    rb.push_back(arg1.B);
  }
};

int main() {
  std::vector<MyData> history;
  history.push_back(MyData(1,1));
  history.push_back(MyData(2,4));
  history.push_back(MyData(3,9));

  std::vector<short> myA, myB;

  // Create a functor with references to myA and myB
  Functor f(myA, myB);
  std::for_each(history.begin(), history.end(), f);

  std::cout << "Content of vector myA\n";
  for (std::vector<short>::const_iterator it = myA.begin(); it != myA.end(); ++it)
    std::cout << *it << "\n";
  std::cout << "Content of vector myB\n";
  for (std::vector<short>::const_iterator it = myB.begin(); it != myB.end(); ++it)
    std::cout << *it << "\n";
}

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Edit: Just saw that Lindley already replied while I was trying out an example

Quote:

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Originally Posted by Lindley

Yeah, you could. If it were me I'd do:
1) Write a functor which takes pointers or references to myA and myB on construction, and takes a vector<MyData>::value_type as the argument to its operator().
2) The operator() does the two push_backs.
3) Use the functor with std::for_each.

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thanks I think that is what I was looking for. The key being using pointers and value_type which I had never used. Let me see if I can code this now.

Quote:

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Originally Posted by treuss

Yes, you can. Create a functor that takes references to your two vectors as arguments to its constructor. The operator() should take one argument of type MyData.

Code:

---

#include <vector>
#include <algorithm>
#include <iostream>

struct MyData
{
  short A;
  short B;
  MyData(short a, short b) : A(a), B(b) {}
};

struct Functor {
  std::vector<short> &ra, &rb;
  Functor(std::vector<short>& a , std::vector<short>& b) : ra(a), rb(b) {}
  void operator()(MyData arg1) {
    ra.push_back(arg1.A);
    rb.push_back(arg1.B);
  }
};

int main() {
  std::vector<MyData> history;
  history.push_back(MyData(1,1));
  history.push_back(MyData(2,4));
  history.push_back(MyData(3,9));

  std::vector<short> myA, myB;

  // Create a functor with references to myA and myB
  Functor f(myA, myB);
  std::for_each(history.begin(), history.end(), f);

  std::cout << "Content of vector myA\n";
  for (std::vector<short>::const_iterator it = myA.begin(); it != myA.end(); ++it)
    std::cout << *it << "\n";
  std::cout << "Content of vector myB\n";
  for (std::vector<short>::const_iterator it = myB.begin(); it != myB.end(); ++it)
    std::cout << *it << "\n";
}

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Edit: Just saw that Lindley already replied while I was trying out an example

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Thanks. I got stuck and came back to see your post. Unfortunately I had done things in reverse. For some reason this stuff is counter intuitive for me right now.

If speed is a concern, which it is for me, would this functor method be slower than my primitive loop method?

In an optimized build there isn't likely to be a notable difference.

However, what *could* speed you up is calling .reserve() on myA and myB with the size of the history vector beforehand.

Quote:

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value_type which I had never used

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For vectors it probably doesn't matter---it's just going to be another name for MyData. However, it's useful for sets and maps since the type to use there isn't always obvious.

Quote:

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Originally Posted by sdcode

Thanks. I got stuck and came back to see your post. Unfortunately I had done things in reverse. For some reason this stuff is counter intuitive for me right now.

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Which part is counter-intuitive? The functor itself or the way it is used?

Maybe it helps that the following code

Code:

---

  Functor f(myA, myB);
  std::for_each(history.begin(), history.end(), f);

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is basically equivalent to

Code:

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  Functor f(myA, myB);
  for (std::vector<MyData>::iterator it = history.begin(); it != history.end(); ++it)
      f.operator()(*it);  // or in short f(*it);

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or, using indexes instead of iterators

Code:

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  Functor f(myA, myB);
  for (unsigned int i = 0; i < history.size(); ++i)
      f.operator()(history[i]);  // or in short f(history[i]);

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Now consider operator() to be a normal member function of Functor and I hope you will understand.