Filtering Values Outside a Given Range

Problem

You want to ignore values from a sequence that fall above or below a given range.

Solution

Use the remove_copy_if function found in the <algorithm>, as shown in Example 11-8.

Example 11-8. Removing elements from a sequence below a value

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

using namespace std;

struct OutOfRange
{
  OutOfRange(int min, int max)
    : min_(min), max_(max)
  { }
  bool operator( )(int x) {
    return (x < min_) || (x > max_);
  }
  int min_;
  int max_;
};

int main( )
{
  vector<int> v;
  v.push_back(6);
  v.push_back(12);
  v.push_back(18);
  v.push_back(24);
  v.push_back(30);
  remove_copy_if(v.begin( ), v.end( ),
    ostream_iterator<int>(cout, "\n"), OutOfRange(10,25));
}

The program in Example 11-8 produces the following output:

12
18
24

Discussion

The remove_copy_if function copies the elements from one container to another container (or output iterator), ignoring any elements that satisfy a predicate that you provide (it probably would have been more accurate if the function was named copy_ignore_if). The function, however, does not change the size of the target container. If, as is often the case, the number of elements copied by remove_copy_if is fewer than the size of the target container, you will have to shrink the target container by calling the erase member function.

The function remove_copy_if requires a unary predicate (a functor that takes one argument and returns a boolean value) that returns true when an element should not be copied. In Example 11-8 the predicate is the function object OutOfRange. The OutOfRange constructor takes a lower and upper range, and overloads operator( ). The operator( ) function takes an integer parameter, and returns true if the passed argument is less than the lower limit, or greater than the upper limit.