Thanks Akira, it is good to know that there is a workaround. But I was wondering, is it necessary in the first place for filtered to dereference the underlying range elements twice? Thanks, Nate. ----------------------------------------

Date: Tue, 31 May 2011 15:07:19 +0900 From: faithandbrave@gmail.com To: boost-users@lists.boost.org Subject: Re: [Boost-users] [range] filtered dereferences underlying range elements twice

Hi, Nathan.

Oven Library has solution for this problem. oven::memoized adaptor:

http://p-stade.sourceforge.net/oven/doc/html/oven/range_adaptors.html#oven.r...

#include #include #include #include #include #include int transform(int i) { std::cout << "calling transform() on " << i << "\n"; return i + 1; } bool filter(int i) { return i % 2 == 0; } int main() { int a[] = {1, 2, 3, 4, 5, 6, 7, 8}; int b[8];

boost::copy(a | boost::adaptors::transformed(transform) | pstade::oven::memoized | boost::adaptors::filtered(filter), b);

return 0; }

results: calling transform() on 1 calling transform() on 2 calling transform() on 3 calling transform() on 4 calling transform() on 5 calling transform() on 6 calling transform() on 7 calling transform() on 8

2011/5/31 Nathan Ridge :

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As you can see, the transform function is called twice for elements that pass the filter.

Can this be avoided? In my real use case, the transformation performs a database lookup, and it should not be done twice.

Thanks, Nate.

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======================== Akira Takahashi mailto:faithandbrave@gmail.com blog:http://d.hatena.ne.jp/faith_and_brave/

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High cost sometime if filtered always store dereferenced-value. I think memoized adaptor was better to external store design. or add stored_filtered adaptor. 2011/6/1 Akira Takahashi :

Nate, follow is filtered + transformed adaptor's internal behavior.

[filtered] increment iterator behavior: while (!pred(*it)) ++it;

dereference iterator behavior: *it;

[transformed] increment iterator behavior: ++it;

dereference iterator behavior: f(*it);

[filtered | transformed] increment iterator behavior: while (!pred(*it)) ++it;

dereference iterator behavior: f(*it);

[transformed | filtered] increment iterator behavior: while (!pred(f(*it))) ++it;

dereference iterator behavior: f(*it);

oven::memoized is call once function by stored calculation.

2011/6/1 Nathan Ridge :

...

Thanks Akira, it is good to know that there is a workaround.

But I was wondering, is it necessary in the first place for filtered to dereference the underlying range elements twice?

Thanks, Nate.

...

======================== Akira Takahashi mailto:faithandbrave@gmail.com blog:http://d.hatena.ne.jp/faith_and_brave/

memoized current implementation use std::deque. Return stored value reference by random access iterator position, in memoized iterator dereference. and not been store to store(push_back). 2011/6/1 Nathan Ridge :

How does memoized work? Does it make a copy of the element? Does it use dynamic allocation?