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:
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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;
}
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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
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// 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;
}
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Output:
myvector contains 5 odd values.
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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) |
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