On Tue, Jun 24, 2008 at 9:26 PM, Mathias Gaunard <mathias.gaunard@ens-lyon.org> wrote:
Dean Michael Berris wrote:
Almost does it, but not necessarily. Constructing an instance of the transform iterator would require the instantiation of the counting_iterator as well.
template <class ResultType, class InputType> struct generator { typedef transform_iterator< function<ResultType(InputType)>, counting_iterator<InputType>
iterator; typedef iterator const const_iterator;
iterator begin() { return make_transform_iterator( counting_iterator<InputType>(), _function ); }; [...]
By using ranges instead of iterators, you can get that container-like behaviour (grouping of begin and end).
However counting_range doesn't seem to be implemented yet.
Interesting... Would the range provide the overload to operator[] to create an associative pseudo-container that behaves like a container? Perhaps something like (using C++0x auto): auto f_sequence = make_lazy_function_range( begin, end, function() ); f_sequence[1]; // like *(++begin); ? I think the generator works at a different level than ranges and/or iterators. The ideas are complimentary IMO: double function_1(int i) { return (numeric_cast<double>(i)*2.0) + 1.0; }; // 2x + 1 double function_2(int i) { return numeric_cast<double>(i) * numeric_cast<double>(i) + 1.0; }; // 2^x + 1 ... generator<double, int> f_1(function_1); generator<double, int> f_2(function_2); for_each( make_zip_iterator(make_tuple(f_1.begin(), f_2.begin())), make_zip_iterator(make_tuple(f_1.begin() + 5, f_2.begin() + 5))), deal_with_data_points() ); iterator_range< generator<double, int>::const_iterator
first_10_f_1s(f_1.begin(), f_1.begin() + 5);
This way, dereferencing the iterator would cause the function to be invoked and a value to be produced (much like a transform iterator). Also, much like for example some mathematical sequences and/or tables where values can be looked up from, the abstraction provided by a generated sequence container instead of just iterators and ranges can be very useful. If it helps thinking about it like a lazy associative container where values are only computed as necessary, then I think that would be one way you can think about the generator example/proposal. // with phoenix for example generator<int, int> doubles(arg1 * 2); doubles[100]; // 200 HTH -- Dean Michael C. Berris Software Engineer, Friendster, Inc.