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Front Page / Algorithms / Iteration Algorithms / reverse_iter_fold

reverse_iter_fold

Synopsis

template<
      typename Sequence
    , typename State
    , typename BackwardOp
    , typename ForwardOp = _1
    >
struct reverse_iter_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 each iterator in the range [begin<Sequence>::type, end<Sequence>::type) in reverse order. If ForwardOp is provided, then it's applied on forward traversal to form the result which is passed to the first BackwardOp call.

Header

#include <boost/mpl/reverse_iter_fold.hpp>

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_iter_fold< s,state,backward_op >::type t;
Return type:

A type.

Semantics:

Equivalent to

typedef begin<s>::type i1;
typedef next<i1>::type i2;
...
typedef next<in>::type last;
typedef apply<backward_op,state,in>::type staten;
typedef apply<backward_op,staten,in-1>::type staten-1;
...
typedef apply<backward_op,state2,i1>::type state1;
typedef state1 t;

where n == size<s>::value and last is identical to end<s>::type; equivalent to typedef state t; if empty<s>::value == true.

typedef reverse_iter_fold< s,state,backward_op,forward_op >::type t;
Return type:

A type.

Semantics:

Equivalent to

typedef reverse_iter_fold<
      Sequence
    , iter_fold<s,state,forward_op>::type
    , backward_op
    >::type t;

Complexity

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

Example

Build a list of iterators to the negative elements in a sequence.

typedef vector_c<int,5,-1,0,-7,-2,0,-5,4> numbers;
typedef list_c<int,-1,-7,-2,-5> negatives;
typedef reverse_iter_fold<
      numbers
    , list<>
    , if_< less< deref<_2>,int_<0> >, push_front<_1,_2>, _1 >
    >::type iters;

BOOST_MPL_ASSERT(( equal<
      negatives
    , transform_view< iters,deref<_1> >
    > ));

See also

Algorithms, iter_fold, reverse_fold, fold