| bytarr type conversion/structures [message #13008] |
Fri, 25 September 1998 00:00 |
Jacobus Koster
Messages: 3
Registered: February 1998
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Junior Member |
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Here's one for the pro's (and DF :-), at least, I think, but I'm no
(big) expert.
Suppose my image files have a standard format with a standard 2048 byte
header.
This header consists of 10 bytes of extraneous information followed by a
descriptor part and a data part (don't worry, I never took a data
structure class so all my terminology is hopelessly inaccurate)
The descriptor part consists of about 100 descriptor fields, each of
which is a record/structure consisting of 3 short ints : key.type,
key.length, and key.offset.
key.type is a code indicating the datafield is string, shortint, char,
etc.
key.length tells you the length of the data field in bytes
key.offset is the offset of the start of the data field, also in bytes.
This is all fairly standard sofar, I would think.
Now :
I would like to read these headers as byte arrays of 2048 bytes, and
then forget forever about the file I got them from. From this byte
array, I want to read the 100 descriptor structures into a 100-element
structure array, with the structure elements described by :
{type:0,length:0,offset:0}. And then, I would like to access the actual
data itself, of course.
Is it possible in IDL to do this kind of type conversion, WITHOUT first
writing the byte array out again into a dummy file and using an - albeit
very beautiful - ASSOC variable or something like that ?
All I know is : it should involve pointers (if only because the type of
the actual data varies). In addition to this, I would like to avoid
FOR-loops, and explicitly calculating integers as 256*byte1 + byte2
strikes me as kinda crude.
Ponder this over your weekend, or answer right away if you like. TIA.
Sjaak Koster
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