| Re: Convert a byte array to a structure? [message #28875] |
Thu, 17 January 2002 16:10  |
Craig Markwardt
Messages: 1869
Registered: November 1996
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Senior Member |
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mikef@grsgds6.lpl.arizona.edu (Mike Fitzgibbon) writes:
> Does IDL have a simple and/or elegant way to convert a byte array
> into a structure?
>
> I could write a brute force approach like:
... brute force approach which converts each data type by hand ..
I think you are stuck with the brute force approach. If you have an
array of structures, then you may save some time by slicing out all of
the field "A" tags first, then "B" and so on.
I believe that if you want to go "native" there are C functions to
import C structures into IDL.
Good luck!
Craig
--
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Craig B. Markwardt, Ph.D. EMAIL: craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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| Re: Convert a byte array to a structure? [message #28877 is a reply to message #28875] |
Thu, 17 January 2002 15:43  |
Bhautik Joshi
Messages: 21
Registered: November 2001
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Junior Member |
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> Does IDL have a simple and/or elegant way to convert a byte array
> into a structure?
> ..
> But is there a better, platform independent, way?
> (Windows NT/2K, Linux, and Solaris)
I've had no luck with READS and the FORMAT keword either :(
Problem is this:
Lets say we have an arbitrary array of bytes, of length n. We also have
a field, of n bytes length, that needs to be filled with that data as
the concatenation of those values. As an example:
['ff'x,'01'x] -> 'ff01'x -> 65281
BYTE[2] -> intermediate step (?) -> FIX[1]
Would it be possible to do this with other size fields (like converting
4 bytes to a floating point number etc.)? Simply 'gluing' the elements
of they byte array together, like 255*byte[0]+ byte[1] won't work with
floating point numbers (?). I suspect the solution may involve pointers
somehow, but I'm not sure how it might be done.
Then again, I might be missing something blindingly obvious, so please
let me know :)
cheers,
Bhautik
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| Re: Convert a byte array to a structure? [message #28880 is a reply to message #28877] |
Thu, 17 January 2002 14:57 |
Bhautik Joshi
Messages: 21
Registered: November 2001
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Junior Member |
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> Does IDL have a simple and/or elegant way to convert a byte array
> into a structure?
> ..
> But is there a better, platform independent, way?
> (Windows NT/2K, Linux, and Solaris)
I wouldn't mind finding this out myself (its a nifty trick). However,
from what I can see, a simple method is not possible? Please, correct me
if I'm wrong.
There is a newsgroup post from a while back here (from JD Smith):
http://groups.google.com/groups?q=converting+type+struct+gro up:comp.lang.idl-pvwave++group:comp.lang.idl-pvwave&hl=e n&selm=3BAF6CAD.CFB19104%40astro.cornell.edu&rnum=3
The structure of a structure in memory is something like:
typedef struct { /* Reference to a structure */
IDL_ARRAY *arr; /* ^ to array block containing data */
struct _idl_structure *sdef; /* ^ to structure definition */
} IDL_SREF;
So what you have is data, stored as bytes in an array, that is the data
that fills out the structure fields, all concatenated together. Then
there is the 'context', which defines the data types of the fields and
how many bytes each field takes up. So, if you could extract the data
and context blocks (I haven't tried this before, but using pointers and
stuff, just like good old C) and then play with it from there...
Another option would be to use the READS command and use explicitly
formatted I/O to format the data into fields. I'm having a look at that
now, and I'll get back to you on that one :)
cheers,
Bhautik
--
/--------------------------------------------------(__)----- ----\
| nbj@imag.wsahs.nsw.gov.au | phone: 0404032617 |..|--\ -moo |
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| Re: Convert a byte array to a structure? [message #28976 is a reply to message #28877] |
Sat, 26 January 2002 00:06 |
sven
Messages: 1
Registered: January 2002
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Junior Member |
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> Problem is this:
> Lets say we have an arbitrary array of bytes, of length n. We also have
> a field, of n bytes length, that needs to be filled with that data as
> the concatenation of those values. As an example:
>
> ['ff'x,'01'x] -> 'ff01'x -> 65281
>
> BYTE[2] -> intermediate step (?) -> FIX[1]
>
> Would it be possible to do this with other size fields (like converting
> 4 bytes to a floating point number etc.)? Simply 'gluing' the elements
> of they byte array together, like 255*byte[0]+ byte[1] won't work with
> floating point numbers (?). I suspect the solution may involve pointers
> somehow, but I'm not sure how it might be done.
I am quite possibly missing something here, but why don't you use the
standard typecasting functions like fix() and float() ?
IDL> b = ['66'xb,'66'xb,'8a'xb,'41'xb]
IDL> help,b
B BYTE = Array[4]
IDL> print,b
102 102 138 65
IDL> print,float(b,0)
17.3000
IDL> print,fix(b[0:1],0)
26214
Isn't that what they're there for?
-- Sven Geier
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