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reverse_fold

Synopsis

template<
      typename Sequence
    , typename State
    , typename BackwardOp
    , typename ForwardOp = _1
    >
struct reverse_fold
{
    typedef unspecified type;
};

Description

Returns the result of the successive application of binary BackwardOp to the result of the previous BackwardOp invocation (State if it's the first call) and every element in the range [begin<Sequence>::type, end<Sequence>::type) in reverse order. If ForwardOp is provided, then it is applied on forward traversal to form the result that is passed to the first BackwardOp call.

Header

#include <boost/mpl/reverse_fold.hpp>

Parameters

Parameters

Parameter Requirement Description
Sequence Forward Sequence A sequence to iterate.
State Any type The initial state for the first BackwardOp / ForwardOp application.
BackwardOp Binary Lambda Expression The operation to be executed on backward traversal.
ForwardOp Binary Lambda Expression The operation to be executed on forward traversal.

Expression semantics

For any Forward Sequence s, binary Lambda Expression backward_op and forward_op, and arbitrary type state:

typedef reverse_fold< s,state,backward_op >::type t; 
Return type:A type
Semantics:

Equivalent to

typedef reverse_iter_fold< 
      s
    , state
    , apply<backward_op,_1,deref<_2> >
    >::type t; 
typedef reverse_fold< s,state,backward_op,forward_op >::type t; 
Return type:A type.
Semantics:

Equivalent to

typedef reverse_fold<
      Sequence
    , fold<s,state,forward_op>::type
    , backward_op
    >::type t;        

Complexity

Linear. Exactly size<s>::value applications of backward_op and forward_op.

Example

Remove negative elements from a sequence [2].

typedef list_c<int,5,-1,0,-7,-2,0,-5,4> numbers;
typedef list_c<int,-1,-7,-2,-5> negatives;
typedef reverse_fold<
      numbers
    , list_c<int>
    , if_< less< _2,int_<0> >, push_front<_1,_2,>, _1 >
    >::type result;

BOOST_MPL_ASSERT(( equal< negatives,result > ));
[2]See remove_if for a more compact way to do this.

See also

Algorithms, fold, reverse_iter_fold, iter_fold