copy and copy_if Function in C++

std::copy() and std::copy_if() function in C++ copies the elements in the range, to another range. However std::copy_if() function copies the element based on result of unary function passed to it.

Syntax

Syntax of copy() and copy_if() function is

template <class InputIterator, class OutputIterator>
OutputIterator copy (InputIterator first, InputIterator last, OutputIterator result);

template <class InputIterator, class OutputIterator, class UnaryPredicate>
OutputIterator copy_if (InputIterator first, InputIterator last, OutputIterator result, UnaryPredicate pred);

where

• first, last : Iterators to the initial and final positions in a sequence. Range copied is [first,last), which contains all the elements between first and last, including the element pointed by first.
• result : Iterator to the initial position of the range where the resulting sequence is stored. The range includes as many elements as [first,last).
• pred : It accepts an element in the range as argument, and returns a value convertible to bool. If returned value is true, it will be copied to result.

copy() copies the elements in the range [first,last) into the range beginning at result. copy_if() copies the elements in the range [first,last) for which pred returns true to the range beginning at result.

Behavior of copy_if() function is equivalent to:

OutputIterator copy_if (InputIterator first, InputIterator last, OutputIterator result, UnaryPredicate pred)
{
  while (first!=last) {
    if (pred(*first)) {
      *result = *first;
      ++result;
    }
    ++first;
  }
  return result;
}

Example

The following code uses copy() and copy_if() to copy the contents of one vector to another and to display the resulting vector:

#include <iostream>     // std::cout
#include <algorithm>    // std::copy
#include <vector>       // std::vector

int main () {

  int myints[]={10,20,30,40,50,60,70};
  std::vector<int> myvector (7);

  // Copy using copy()
  std::copy (myints, myints+7, myvector.begin());

  for (std::vector<int>::iterator it = myvector.begin(); it!=myvector.end(); ++it){
    std::cout << ' ' << *it;
  }

  // Output
  //  10 20 30 40 50 60 70  
  
  std::cout <<"\n";
    
  std::vector<int> foo = {25,15,5,-5,-15};
  std::vector<int> bar (foo.size());

  // copy only positive numbers:
  auto it = std::copy_if(foo.begin(), foo.end(), bar.begin(), [](int i){return !(i<0);} );

  // Shrink to output size
  bar.resize(std::distance(bar.begin(),it));  

  for (int& x: bar) {
    std::cout << ' ' << x;
  }

  // Output
  //  25 15 5 

  return 0;
}