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    <p>Sure <code>rolling_count_non_zero</code> sounds like an appropriate name for what you want (although it could have been somewhat more generic, maybe <code>rolling_count_if</code>?).</p> <p><a href="http://www.boost.org/doc/libs/release/doc/html/accumulators/user_s_guide.html" rel="nofollow">Boost.accumulators users guide</a> describes how to write your own accumulators, features, and extractors, simply following that description, the following seems to work:</p> <pre><code>namespace boost { namespace accumulators { namespace impl { template&lt;typename Sample&gt; struct rolling_count_non_zero : accumulator_base { typedef std::size_t result_type; rolling_count_non_zero(dont_care) : cnt_() {} template&lt;typename Args&gt; void operator ()(Args const &amp;args) { if(args[sample] != 0) ++cnt_; if(is_rolling_window_plus1_full(args) &amp;&amp; rolling_window_plus1(args).front() != 0 ) --cnt_; } template&lt;typename Args&gt; result_type result(Args const &amp;args) const { return cnt_; } private: std::size_t cnt_; }; } namespace tag { struct rolling_count_non_zero : depends_on&lt; rolling_window_plus1 &gt; { typedef accumulators::impl::rolling_count_non_zero&lt; mpl::_1 &gt; impl; }; } namespace extract { extractor&lt;tag::rolling_count_non_zero&gt; const rolling_count_non_zero = {}; } using extract::rolling_count_non_zero; }} </code></pre> <p>live demo: <a href="http://coliru.stacked-crooked.com/a/bc4bea090690f26d" rel="nofollow">http://coliru.stacked-crooked.com/a/bc4bea090690f26d</a></p>
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    1. COThanks Cubbi. That would work. Though I was wondering if I could simple use whats existing out there. May be I would just implement simple array and implement rolling count myself. Thats a bit easier than having to incorporate this in the project base. Though it would be great as you said if there was something like rolling_cout_if available in accumulator already. I emailed Eric Niebler and he said I would have to implement own accumulator. Thanks for the code.
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    2. COI think we don't need rolling_window_plus1(args).front() != 0; as we never insert 0. Right ?
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    3. CO@Shanky front() is the element that just left the rolling window
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