Iterators#

The definitions of the iterators are available through the <oneapi/dpl/iterator> header. All iterators are implemented in the oneapi::dpl namespace.

counting_iterator: a random-access iterator-like type whose dereferenced value is an integer counter. Instances of a counting_iterator provide read-only dereference operations. The counter of an counting_iterator instance changes according to the arithmetic of the random-access iterator type:

dpl::counting_iterator<int> count_a(0);
dpl::counting_iterator<int> count_b = count_a + 10;
int init = count_a[0]; // OK: init == 0
*count_b = 7; // ERROR: counting_iterator does not provide write operations
auto sum = dpl::reduce(dpl::execution::dpcpp_default,
                       count_a, count_b, init); // sum is (0 + 0 + 1 + ... + 9) = 45

zip_iterator: an iterator constructed with one or more iterators as input. The result of zip_iterator dereferencing is a tuple-like object of an unspecified type that holds the values returned by dereferencing the member iterators, which the zip_iterator wraps. Arithmetic operations performed on a zip_iterator instance are also applied to each of the member iterators.

The make_zip_iterator function is provided to simplify the construction of zip_iterator instances. The function returns zip_iterator instances with all the arguments held as member iterators.

The zip_iterator is useful in defining by key algorithms where input iterators representing keys and values are processed as key-value pairs. The example below demonstrates a stable sort by key, where only the keys are compared but both keys and values are swapped:
```
auto zipped_begin = dpl::make_zip_iterator(keys_begin, vals_begin);
dpl::stable_sort(dpl::execution::dpcpp_default, zipped_begin, zipped_begin + n,
    [](auto lhs, auto rhs) { return get<0>(lhs) < get<0>(rhs); });
```
The dereferenced object of zip_iterator supports the structured binding feature (C++17 and above) for easier access to wrapped iterators values:
```
auto zipped_begin = dpl::make_zip_iterator(sequence1.begin(), sequence2.begin(), sequence3.begin());
auto found = dpl::find(dpl::execution::dpcpp_default, zipped_begin, zipped_begin + n,
    [](auto tuple_like_obj) {
      auto [e1, e2, e3] = tuple_like_obj;
      return e1 == e2 && e1 == e3;
    }
);
```
Since dereferencing zip_iterator is semantically a tuple of references, the copying of such an object is supposed to be cheap. In the example above e1, e2 and e3 are references.

For more examples with zip_iterator, see the code snippet provided for discard_iterator below.
discard_iterator: a random-access iterator-like type that provides write-only dereference operations that discard values passed.

The discard_iterator is useful in the implementation of stencil algorithms where the stencil is not part of the desired output. An example of this would be a copy_if algorithm that receives an input iterator range, a stencil iterator range, and copies the elements of the input whose corresponding stencil value is 1. Use discard_iterator so you do not declare a temporary allocation to store the copy of the stencil:
```
auto zipped_first = dpl::make_zip_iterator(first, stencil);
dpl::copy_if(dpl::execution::dpcpp_default,
             zipped_first, zipped_first + (last - first),
             dpl::make_zip_iterator(result, dpl::discard_iterator()),
             [](auto t){return get<1>(t) == 1;}
```
transform_iterator: an iterator defined over another iterator whose dereferenced value is the result of a function applied to the corresponding element of the base iterator. Both the type of the base iterator and the unary function applied during dereference operations are required template parameters of the transform_iterator class.

The unary functor provided to a transform_iterator should have a const-qualified call operator which accepts the reference type of the base iterator as argument. The functor’s call operator should not have any side effects and should not modify the state of the functor object.

The transform_iterator class provides the following constructors:
- transform_iterator(): instantiates the iterator using a default constructed base iterator and unary functor. This constructor participates in overload resolution only if the base iterator and unary functor are both default constructible.
- transform_iterator(iter): instantiates the iterator using the base iterator provided and a default constructed unary functor. This constructor participates in overload resolution only if the unary functor is default constructible.
- transform_iterator(iter, func): instantiates the iterator using the base iterator and unary functor provided.
To simplify the construction of the iterator, oneapi::dpl::make_transform_iterator is provided. The function receives the base iterator and transform operation instance as arguments, and constructs the transform_iterator instance:
```
dpl::counting_iterator<int> first(0);
dpl::counting_iterator<int> last(10);
auto transform_first = dpl::make_transform_iterator(first, std::negate<int>());
auto transform_last = transform_first + (last - first);
auto sum = dpl::reduce(dpl::execution::dpcpp_default,
                       transform_first, transform_last); // sum is (0 + -1 + ... + -9) = -45
```
permutation_iterator: an iterator whose dereferenced value set is defined by the source iterator provided, and whose iteration order over the dereferenced value set is defined by either another iterator or a functor whose index operator defines the mapping from the permutation_iterator index to the index of the source iterator. The permutation_iterator is useful in implementing applications where noncontiguous elements of data represented by an iterator need to be processed by an algorithm as though they were contiguous. An example is copying every other element to an output iterator. The source iterator cannot be a host-side iterator in cases where algorithms are executed with device policies.

The make_permutation_iterator is provided to simplify construction of iterator instances. The function receives the source iterator and the iterator or function object representing the index map:
```
struct multiply_index_by_two {
    template <typename Index>
    Index operator()(const Index& i) const
    { return i*2; }
};

// first and last are iterators that define a contiguous range of input elements
// compute the number of elements in the range between the first and last that are accessed
// by the permutation iterator
size_t num_elements = std::distance(first, last) / 2 + std::distance(first, last) % 2;
auto permutation_first = dpl::make_permutation_iterator(first, multiply_index_by_two());
auto permutation_last = permutation_first + num_elements;
dpl::copy(dpl::execution::dpcpp_default, permutation_first, permutation_last, result);
```