count_if


function template
<algorithm>
template <class InputIterator, class Predicate>
  typename iterator_traits<InputIterator>::difference_type
    count_if ( ForwardIterator first, ForwardIterator last, Predicate pred );

Return number of elements in range satisfying condition

Returns the number of elements in the range [first,last) for which condition pred is true.

The behavior of this function template is equivalent to:
1
2
3
4
5
6
7
 
template <class InputIterator, class Predicate>
  ptrdiff_t count_if ( InputIterator first, InputIterator last, Predicate pred )
{
  ptrdiff_t ret=0;
  while (first != last) if (pred(*first++)) ++ret;
  return ret;
}


Parameters

first, last
Forward iterators to the initial and final positions of the sequence of elements. The range used is [first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
pred
Unary predicate taking an element in the range as argument, and returning a value indicating the falsehood (with false, or a zero value) or truth (true, or non-zero) of some condition applied to it. This can either be a pointer to a function or an object whose class overloads operator().


Return value

The number of elements in the range [first,last) for which cond applied to it returns true.
The return type (iterator_traits<InputIterator>::difference_type) is a signed integral type, generally defined as the same type as ptrdiff_t.

Example

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
 
// count_if example
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

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

int main () {
  int mycount;

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

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

  return 0;
}


Output:
myvector contains 5 odd values.


Complexity

Linear: Applies pred as many times as the number of elements between first and last.

See also

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