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count_if
PrototypeCount_if is an overloaded name: there are two count_if functions.template <class InputIterator, class Predicate> iterator_traits<InputIterator>::difference_type count_if(InputIterator first, InputIterator last, Predicate pred); template <class InputIterator, class Predicate, class Size> void count_if(InputIterator first, InputIterator last, Predicate pred, Size& n); DescriptionCount_if finds the number of elements in [first, last) that satisfy the predicate pred. More precisely, the first version of count_if returns the number of iterators i in [first, last) such that pred(*i) is true. The second version of count adds to n the number of iterators i in [first, last) such that pred(*i) is true.The second version of count_if was the one defined in the original STL, and the first version is the one defined in the draft C++ standard; the definition was changed because the older interface was clumsy and errorprone. The older interface required the use of a temporary variable, which had to be initialized to 0 before the call to count_if. Both interfaces are currently supported [1], for reasons of backward compatibility, but eventually the older version will be removed. DefinitionDefined in the standard header algorithm, and in the nonstandard backwardcompatibility header algo.h.Requirements on typesFor the first version, which takes three arguments:
PreconditionsFor the first version:
ComplexityLinear. Exactly last  first applications of pred.Exampleint main() { int A[] = { 2, 0, 4, 6, 0, 3, 1, 7 }; const int N = sizeof(A) / sizeof(int); cout << "Number of even elements: " << count_if(A, A + N, compose1(bind2nd(equal_to<int>(), 0), bind2nd(modulus<int>(), 2))) << endl; } Notes[1] The new count interface uses the iterator_traits class, which relies on a C++ feature known as partial specialization. Many of today's compilers don't implement the complete standard; in particular, many compilers do not support partial specialization. If your compiler does not support partial specialization, then you will not be able to use the newer version of count, or any other STL components that involve iterator_traits. See alsocount, find, find_ifCopyright © 1999 Silicon Graphics, Inc. All Rights Reserved. TrademarkInformation
