# iter_fold_if

### Synopsis

    template<
          typename Sequence
        , typename State
        , typename BackwardOp
        , typename ForwardOp = _1
        >
    struct iter_fold_backward
    {
        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 the 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.

### Definition

    #include "boost/mpl/iter_fold_backward.hpp"

### Parameters

Parameter | Requirement | Description | Default value
--- | --- | --- | ---
`Sequence` | A model of Sequence | A sequence to iterate. |
`State` | A type | The initial state for the first `BackwardOp`/`ForwardOp` application. |
`BackwardOp` | A model of [Lambda Function] | The operation to be executed on backward traversal. |
`ForwardOp` | A model of [Lambda Function] | The operation to be executed on forward traversal. | `arg<1>`

### Expression semantics

Expression | Expression type | Precondition | Semantics | Postcondition
--- | --- | --- | --- | ---
`typedef iter_fold_backward< Sequence,T,BackwardOp >::type t;` | A type |  | Equivalent to `typedef lambda<BackwardOp>::type bk_op; typedef begin<Sequence>::type i1; typedef i1::next i2; ...; typedef in::next last; typedef apply<bk_op,T,in>::type tn; typedef apply<bk_op,tn,in-1>::type tn-1; ...; typedef apply<bk_op,t2,i1>::type t1; typedef t1 t`, where `n == size<Sequence>::type::value` and `last` is identical to `end<Sequence>::type`; Equivalent to `typedef T t;` if the sequence is empty. |
`typedef iter_fold_backward< Sequence,T,BackwardOp,ForwardOp >::type t;` | A type |  | Equivalent to `typedef iter_fold_backward<Sequence, iter_fold<Sequence,State,ForwardOp>::type, BackwardOp>::type t;`. |

### Complexity

Linear. Exactly `size<Sequence>::type::value` applications of `BackwardOp` and `ForwardOp`.

### Example

Finds an iterator to the last negative element in the sequence.

    typedef list_c<int,5,-1,0,7,2,0,-5,4> numbers;
    typedef iter_fold_backward<
          numbers
        , end<numbers>::type
        , if_< less< deref<_2>, int_c<0> >,_2,_1 >
        >::type last_negative_iter;

    BOOST_STATIC_ASSERT(last_negative_iter::type::value == -5);

### See also

Algorithms, `iter_fold`, `fold_backward`, `fold`, `copy`, `copy_backward`

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Last edited July 19, 2002 1:25 am