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count_if

Synopsis

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

#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:

typedef count_if<s,pred>::type n; 
Return type:Integral Constant.
Semantics:

Equivalent to

typedef lambda<pred>::type p;
typedef fold< 
      s
    , long_<0>
    , if_< apply_wrap1<p,_2>, next<_1>, _1 >
    >::type n;

Complexity

Linear. Exactly size<s>::value applications of pred.

Example

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