[dfdl-wg] How to deal with variable length elements?

[Westhead, Martin (Martin)]

Hi,

 

This looks fine to me (modulo old syntax) except I don't understand the
need to introduce this construct: "@dfdl:index". I guess in today's
terms we might thing of this as a value in the context. However, I claim
that we don't need it. I think we can achieve the same effect with the
XPath function position().

 

position() should tell you where you are in the current sequence. If you
want to know where you are in the parent sequence you can use
../position(). If your index starts from 0 use position()-1. If you
store two elements in your sequence for every index (e.g. flat array of
x-y coordinates) use position()/2.

 

What does @dfdl:index do that you don't get out of position()?

 

Cheers,

 

Martin

 

  _____  

From: owner-dfdl-wg at ggf.org [mailto:owner-dfdl-wg at ggf.org] On Behalf Of
Mike Beckerle
Sent: Friday, March 17, 2006 2:14 PM
To: Robert E. McGrath
Cc: dfdl-wg at ggf.org; owner-dfdl-wg at ggf.org
Subject: Re: [dfdl-wg] How to deal with variable length elements?

 


Hmmm. 

I think layered value calculation formulas which allow for a magic
"myIndex" variable are perhaps an important device to make this class of
layering possible.This makes the iteration over the elements implicit. 

I have one example which is the one where there is a vector of strings
where the lengths of all the strings are stored first, separately from
all the character data. 

A formula involving "myIndex" is used to glue the two pieces together. 

Here's the example as per our prototype from last summer: 




...mikeb





"Robert E. McGrath" <mcgrath at ncsa.uiuc.edu> 
Sent by: owner-dfdl-wg at ggf.org 

03/17/2006 10:54 AM 

To

dfdl-wg at ggf.org 

cc

 

Subject

Re: [dfdl-wg] How to deal with variable length elements?

 

 

 




Following up on my email on ealier this week:

I think there was a major flaw in what I wrote, and it is quite an
"interesting" challenge.

Let me review:

I am thinking about how to describe reading data into a 1D array. Steve
provided a markup for the XML element.  

The challenge I'm looking at is that the data need not be a image of
the memory layout.  To give one example, a very sparse array might be
stored as a series of (index, value) pairs for the non-empty places,
all others implied to be zero or fill or whatever.

The goal is to have the XML array be fully populated from this sparse
form--or whatever layout--on disk.  (Please assume for now that this is
a 
reasonable goal!)

The XML and DFDL will tell us the data type, and presumably we know
the extent of the data on disk.  But we need to decode the storage
to generate all the elements values and fills.

In my earlier email, I offered a description that included an 'Iterator'
conversion.  I now think this is inadequate.  In fact you need two
cooperating 'Iterators'!  Ick!

Here is my revised pipeline.  Data is read from bottom to top. I
sketch what each conversion is tasked to do.  I think the 'Decoder'
needs to know info from both the 'Iterator' (it asks for each element
in the order it wants them) and 'Float' (it tells the size of
the 'value' to get).


==
 <<XML element with multiple occurs: 1D array >>

       ^
       |

 Iterator conversion:  relevant props: minOccurs="0"
maxOccurs="<<setting>>, 
et al
            Get 'maxOccurs' elements of type datatype.

       ^
       |

 Float conversion:     relevant props:  data description of element
            Decode bytes

       ^
       |

 Decoder conversion:  produces the bytes 'nth' _value_ in the array. 
         Input: what position is needed.
                 may need separators and other props: depends on
encoding
         Output: sizeof datatype bytes, the _value_
         Side effect: after whole array is read, consumes all the
              storage.  Difficult to characterize the intermediate
              state.

       ^
       |

 Data:  read as bytes



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