-----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users- bounces@lists.boost.org] On Behalf Of Nathan Ridge Sent: 08 July 2012 06:18 To: Boost Mailing List Subject: Re: [Boost-users] chaining ouput iterator to input iterator

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I have a function that transforms a serie of numbers into another serie:

template void mytransform( InputIterator begin, InputIterator end, OutputIterator out ) { }

and a 2nd function that calculates the standard deviation of a sample

template <typename InputIterator > double stdev( InputIterator begin, InputIterator end) { }

Is there a way to chain these existing functions in order to get the std dev of the transformed serie, without storing the intermediate numbers? Maybe with the help of some lazy iterator, or some lazy function?

In this particular case, you can use boost::transform_iterator rather than std::transform, as Jeff pointed out.

However, this does not resolve the problem in general, which is how to treat the sequence of values a function writes to an output iterator as a range (or pair of iterators) input to another function (without accumulating the elements in a container, of course).

I'm not too sure how to call the transform_iterator in this case. I have these 2 functions that I need to provide separately because they are used separately. And I also need a 3rd function, called double stddev_of_transformed(InputIterator begin, InputIterator end) { return mytransform(begin, end, boost::transform_iterator( ... ) ); // ???? }

This seems like a textbook use case for coroutines. I wonder whether a solution using Boost.Context would impose sufficiently little overhead to provide a general solution to this problem.

I'll take a look at this too, thanks, MM

-----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users- bounces@lists.boost.org] On Behalf Of Nathan Ridge Sent: 08 July 2012 15:33 To: Boost Mailing List Subject: Re: [Boost-users] chaining ouput iterator to input iterator

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I have a function that transforms a serie of numbers into another serie:

template void mytransform( InputIterator begin, InputIterator end, OutputIterator out ) { }

and a 2nd function that calculates the standard deviation of a sample

template <typename InputIterator > double stdev( InputIterator begin, InputIterator end) { }

Is there a way to chain these existing functions in order to get the std dev of the transformed serie, without storing the intermediate numbers? Maybe with the help of some lazy iterator, or some lazy function?

In this particular case, you can use boost::transform_iterator rather than std::transform, as Jeff pointed out.

However, this does not resolve the problem in general, which is how to treat the sequence of values a function writes to an output iterator as a range (or pair of iterators) input to another function (without accumulating the elements in a container, of course).

I'm not too sure how to call the transform_iterator in this case.

I have these 2 functions that I need to provide separately because they are used separately. And I also need a 3rd function, called

double stddev_of_transformed(InputIterator begin, InputIterator end) { return mytransform(begin, end, boost::transform_iterator( ... ) ); // ???? }

Sorry, I thought by transform() you meant std::transform(). If you have your own mytransform() function (and you can't replace it with a call to std::transform()), then boost::transform_iterator may not be applicable. In that case, your options are 1) accumulate the outputs in a container in between the two calls, or 2) use a Boost.Context-based solution as I mentioned before.

Regards, Nate

Boost.Coroutine? Is that in boost vault? MM

-----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users- bounces@lists.boost.org] On Behalf Of Nat Linden Sent: 09 July 2012 13:24 To: boost-users@lists.boost.org Subject: Re: [Boost-users] chaining ouput iterator to input iterator

On Sun, Jul 8, 2012 at 5:33 PM, MM wrote:

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-----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users- bounces@lists.boost.org] On Behalf Of Nathan Ridge Sent: 08 July 2012 15:33 To: Boost Mailing List Subject: Re: [Boost-users] chaining ouput iterator to input iterator

your options are ... 2) use a Boost.Context-based solution as I mentioned before.

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Boost.Coroutine? Is that in boost vault?

Heh. There is a Boost.Coroutine in the Vault, but you're better advised to use the new Boost.Coroutine just coming up for review this week (I believe).

IMO, either would be better than directly using Boost.Context. Context is a low-level library intended for use by application-level libraries such as Coroutine. It's just that any application-level library based on user-space context switching necessarily deals with the same platform-dependent issues; Oliver Kowalke has very usefully abstracted those out as Context.

Now Oliver has presented us with a reimplementation of the original Boost.Coroutine prototype (in the Vault), based on his Context library. If you're willing, it would be very timely to add your opinion to the review about to be announced.

Presumably the doc is in the boost vault. I'll take a look Frankly, the coroutine seems very unnatural as I am accustomed to the separation of concerns, and the mechanism of of function call, and so on... I was also wondering if some lazy function from phoenix could help with this, or really a boost::transform_iterator could do it somehow MM

-----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users- bounces@lists.boost.org] On Behalf Of Nathan Ridge Sent: 09 July 2012 19:20 To: Boost Mailing List Subject: Re: [Boost-users] chaining ouput iterator to input iterator

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your options are ... 2) use a Boost.Context-based solution as I mentioned before.

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Boost.Coroutine? Is that in boost vault?

[snip]

Frankly, the coroutine seems very unnatural as I am accustomed to the separation of concerns, and the mechanism of of function call, and so on...

I was also wondering if some lazy function from phoenix could help with this, or really a boost::transform_iterator could do it somehow

Is it possible for you to change the implementation of my_transform? If so, could you post its implementation (or an outline of it)? We may be able to suggest a way to express the same thing with boost::transform_iterator instead.

Regards, Nate

I wouldn't be able to change the signature... template void ratios( InputIterator begin, InputIterator end, OutputIterator out ) { auto i=begin; ++i; auto prev=begin; while (i!=end) { *out = (*i)/(*prev); ++out; ++i; ++prev; } } MM