Sequence iterator? Isn't there one in boost? - c++

Question

From time to time I am feeling the need for a certain kind of iterator (for which I can't make up a good name except the one prefixed to the title of this question).
Suppose we have a function (or function object) that maps an integer to type T. That is, we have a definition of a mathematical sequence, but we don't actually have it stored in memory. I want to make an iterator out of it. The iterator class would look something like this:
template <class F, class T>
class sequence_iterator : public std::iterator<...>
{
int i;
F f;
public:
sequence_iterator (F f, int i = 0):f(f), i(i){}
//operators ==, ++, +, -, etc. will compare, increment, etc. the value of i.
T operator*() const
{
return f(i);
}
};
template <class T, class F>
sequence_iterator<F, T> make_sequence_iterator(F f, int i)
{
return sequence_iterator<F, T>(f, i);
}
Maybe I am being naive, but I personally feel that this iterator would be very useful. For example, suppose I have a function that checks whether a number is prime or not. And I want to count the number of primes in the interval [a,b]. I'd do this;
int identity(int i)
{
return i;
}
count_if(make_sequence_iterator<int>(identity, a), make_sequence_iterator<int>(identity, b), isPrime);
Since I have discovered something that would be useful (at least IMHO) I am definitely positive that it exists in boost or the standard library. I just can't find it. So, is there anything like this in boost?. In the very unlikely event that there actually isn't, then I am going to write one - and in this case I'd like to know your opinion whether or not should I make the iterator_category random_access_iterator_tag. My concern is that this isn't a real RAI, because operator* doesn't return a reference.
Thanks in advance for any help.

Answer 1

boost::counting_iterator and boost::transform_iterator should do the trick:
template <typename I, typename F>
boost::transform_iterator<
F,
boost::counting_iterator<I>>
make_sequence_iterator(I i, F f)
{
return boost::make_transform_iterator(
boost::counting_iterator<I>(i), f);
}
Usage:
std::copy(make_sequence_iterator(0, f), make_sequence_iterator(n, f), out);

Answer 2

I would call this an integer mapping iterator, since it maps a function over a subsequence of the integers. And no, I've never encountered this in Boost or in the STL. I'm not sure why that is, since your idea is very similar to the concept of stream iterators, which also generate elements by calling functions.
Whether you want random access iteration is up to you. I'd try building a forward or bidirectional iterator first, since (e.g.) repeated binary searches over a sequence of integers may be faster if they're generated and stored in one go.

Answer 3

Does the boost::transform_iterator fills your needs? there are several useful iterator adaptors in boost, the doc is here.

Answer 4

I think boost::counting_iterator is what you are looking for, or atleast comes the closest. Is there something you are looking for it doesn't provide? One could do, for example:
std::count_if(boost::counting_iterator<int>(0),
boost::counting_iterator<int>(10),
is_prime); // or whatever ...
In short, it is an iterator over a lazy sequence of consecutive values.

Answer 5

Boost.Utility contains a generator iterator adaptor. An example from the documentation:
#include <iostream>
#include <boost/generator_iterator.hpp>
class my_generator
{
public:
typedef int result_type;
my_generator() : state(0) { }
int operator()() { return ++state; }
private:
int state;
};
int main()
{
my_generator gen;
boost::generator_iterator_generator<my_generator>::type it =
boost::make_generator_iterator(gen);
for (int i = 0; i < 10; ++i, ++it)
std::cout << *it << std::endl;
}