| .. Algorithms/Querying Algorithms//count_if |50 |
| |
| count_if |
| ======== |
| |
| Synopsis |
| -------- |
| |
| .. parsed-literal:: |
| |
| template< |
| typename Sequence |
| , typename Pred |
| > |
| struct count_if |
| { |
| typedef |unspecified| type; |
| }; |
| |
| |
| |
| Description |
| ----------- |
| |
| Returns the number of elements in ``Sequence`` that satisfy the predicate ``Pred``. |
| |
| |
| Header |
| ------ |
| |
| .. parsed-literal:: |
| |
| #include <boost/mpl/count_if.hpp> |
| |
| |
| |
| Parameters |
| ---------- |
| |
| +---------------+-------------------------------+-----------------------------------+ |
| | Parameter | Requirement | Description | |
| +===============+===============================+===================================+ |
| | ``Sequence`` | |Forward Sequence| | A sequence to be examined. | |
| +---------------+-------------------------------+-----------------------------------+ |
| | ``Pred`` | Unary |Lambda Expression| | A count condition. | |
| +---------------+-------------------------------+-----------------------------------+ |
| |
| |
| Expression semantics |
| -------------------- |
| |
| |
| For any |Forward Sequence| ``s`` and unary |Lambda Expression| ``pred``: |
| |
| .. parsed-literal:: |
| |
| typedef count_if<s,pred>::type n; |
| |
| :Return type: |
| |Integral Constant|. |
| |
| :Semantics: |
| Equivalent to |
| |
| .. parsed-literal:: |
| |
| typedef lambda<pred>::type p; |
| typedef fold< |
| s |
| , long_<0> |
| , if_< apply_wrap\ ``1``\<p,_2>, next<_1>, _1 > |
| >::type n; |
| |
| |
| Complexity |
| ---------- |
| |
| Linear. Exactly ``size<s>::value`` applications of ``pred``. |
| |
| |
| Example |
| ------- |
| |
| .. parsed-literal:: |
| |
| typedef vector<int,char,long,short,char,long,double,long> types; |
| |
| BOOST_MPL_ASSERT_RELATION( (count_if< types, is_float<_> >::value), ==, 1 ); |
| BOOST_MPL_ASSERT_RELATION( (count_if< types, is_same<_,char> >::value), ==, 2 ); |
| BOOST_MPL_ASSERT_RELATION( (count_if< types, is_same<_,void> >::value), ==, 0 ); |
| |
| |
| See also |
| -------- |
| |
| |Querying Algorithms|, |count|, |find|, |find_if|, |contains| |
| |
| |
| .. copyright:: Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) |
