Re: Calculations while sorting?

[Ben Pfaff]

John Darrington <address@hidden> writes:

>      On Sun, Jul 27, 2008 at 09:21:56PM -0700, Ben Pfaff wrote:
>           
>           It would be easy to create a "moments_reader" that does calculations
>           like this as side effects and otherwise passes cases through
>           without modification.
>           
>           There would be no need to make this part of sorting.  Just make a
>           "moments_reader" out of your data and then pass the
>           moments_reader to sort_execute.
>      
> Having thought about this, I have some more ideas:
>
> 1. Ben's suggestion is good idea, but I'd like to generalize upon it.
>
>   In my local dir, I have a number of new modules in src/math with a
> similar interface to moments.c   How about we abstract this interface
> into a virtual base class (let's call it "struct statistics" for
> now).  Then instead of a "moments_reader" we can have  a polymorphic
> "statistics_reader" is possible, which takes an array of heterogenous
> "struct statistics" object.

This is perfectly feasible.

However, now I remember why I didn't do something like this
earlier: because there is an easier way!  And this way, you don't
need a whole menagerie of processing classes, or even just one.

Instead of using sort_execute(), write a loop:

    sorter = sort_create_writer (ordering, casereader_get_value_cnt (input));
    moments = moments_create (...);
    foo = foo_create (...);
    bar = bar_create (...);
    while (casereader_read (input, &c)) 
      {
        moments_pass_one (moments, ..., ...);
        foo_process (foo, &c);
        bar_process (bar, &c);
        casewriter_write (sorter, &c);
      }
    output = casewriter_make_reader (sorter);

Note that this is all sort_execute() does, without the extra
moments and whatever else thrown in, as you can see from its
source:

    struct casereader *
    sort_execute (struct casereader *input, struct case_ordering *ordering)
    {
      struct casewriter *output =
        sort_create_writer (ordering, casereader_get_value_cnt (input));
      casereader_transfer (input, output);
      return casewriter_make_reader (output);
    }
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