count_if function template

template <class InputIterator, class UnaryPredicate>
  typename iterator_traits<InputIterator>::difference_type
    count_if (InputIterator first, InputIterator last, UnaryPredicate pred);

Returns the number of elements within a range satisfying the condition

Number of elements in the range of return values [first,last)for which pred is true.

The behavior of this function template is equivalent to:

template <class InputIterator, class UnaryPredicate>
  typename iterator_traits<InputIterator>::difference_type
    count_if (InputIterator first, InputIterator last, UnaryPredicate pred)
{
  typename iterator_traits<InputIterator>::difference_type ret = 0;
  while (first!=last) {
    if (pred(*first)) ++ret;
    ++first;
  }
  return ret;
}

parameter

First, last

The input iterator to the initial position and the final position of the element of the sequence. Range is used [first,last), it contains all the elements of the first and the last, the last comprising a first member, but not the pointed element pointing.

Expected value

Unary function accepts as a parameter in the range of elements, and can be converted to the return value bool. The value returned indicates whether this function computation elements.
This function may not modify its parameters.
This function may be a function or pointer to the object.

return value

The number of elements in a range [first,last)not return for which the PRED false.
Return type ( iterator_traits :: difference_type) is a signed integer type.

Case

// count_if example
#include <iostream>     // std::cout
#include <algorithm>    // std::count_if
#include <vector>       // std::vector

bool IsOdd (int i) { return ((i%2)==1); }

int main () {
  std::vector<int> myvector;
  for (int i=1; i<10; i++) myvector.push_back(i); // myvector: 1 2 3 4 5 6 7 8 9

  int mycount = count_if (myvector.begin(), myvector.end(), IsOdd);
  std::cout << "myvector contains " << mycount  << " odd values.\n";

  return 0;
}

myvector包含5个奇数值。

complex

Between the first and last from linear: pred called once for each element.

Data contest

[first,last)Access the range of objects (each object only visited once).

exception

Pre throw throw throw or throwing any operations on iterators.
Please note that the invalid parameters can lead to undefined behavior .

count	Count appearances of value in range (function template )
for_each	Apply function to range (function template )
find	Find value in range (function template )