Re: [Boost-users] [multi_index] range selection over composit_key problem

From: Joaquín Mª López Muñoz (joaquin_at_[hidden])
Date: 2008-01-08 12:47:50

Hi Oliver,

"Kowalke Oliver (QD IT PA AS)" ha escrito:

> Hello,
> I want to do an range selection over an composit key but I get compiler errors like:

[...]

>                 std::pair<
>                         idx_type::iterator,
>                         idx_type::iterator
>                 > p(
>                         idx.range(
>                                 boost::make_tuple( mi::unbounded, true),
>                                 boost::make_tuple(
>                                         boost::lambda::_1 >= 0.,
>                                         true) ) );

I think there's a misunderstanding here about the usage of composite keys with
range(): note that the composite key-based index sorts the elements of p according to a
lexicographical order on (descending function, descending active); for instance, the values
you insert in your example get sorted as follows:

  (1.1,false),(1,true),(0,true),(-1,true),(-1,false),(-1.1,true).

Now, a fundamental limitation (or chararacteristic, if you wish) of range() is that
it can only retrieve *contiguous* ranges, no matter whether the key used is simple,
composite, custom or whatever . Your (syntactically incorrect) invocation to idx.range()
suggests that you're trying to get the values with function>=0 and active==true, but the
set of matching values is not contiguous (it is so by chance with the particular set of
values you're using in oyur example, but it wouldn't be if for instance you inserted
an additional (1,false) ). In order to do that you'd need to retrieve some larger range
and filter out the "undesirable" elements, for example like this:

  // get all elements with function>=0 and arbitrary active: these do always form
  // a range.
  std::pair<
    idx_type::iterator,
    idx_type::iterator
  > p(
    idx.begin(),
    idx.upper_bound( boost::make_tuple(0.0) ) );

  // filter out the elements with !active
  for ( ; p.first != p.second; ++p.first)
    if(p.first->active)
      std::cout << p.first->function << " " << std::boolalpha << p.first->active << std::endl;

Hope this helps, please come back if something's still not clear.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo