| Re: signed character [message #30412] |
Thu, 25 April 2002 06:24 |
Mark Rivers
Messages: 49
Registered: February 2000
|
Member |
|
|
Carmen <cambicambi@hotmail.com> wrote in message
news:8128b197.0204250239.509a20fb@posting.google.com...
> Hello,
>
> I am trying to access information stored in a matrix, byte per byte.
> If I were using C, I should read every byte as SIGNED CHARACTERS.
> But, IDL has not that definition.
>
> I would appreciate a lot if you can tell me what should I do in order
> to read the data correctly.
If A is the matrix containing an IDL byte array (which IDL assumes is
unsigned) then you could do
; First convert A to a signed 16-bit integer array
a = fix(a)
; Find the indices of the negative elements (MSB of 8-bit value set)
negs = where(a and '80'x)
; Extended the sign to 16 bits
a[negs]=a[negs] or 'ff80'x
The drawbacks of this approach are that your matrix is twice as big (16 bits
vs 8 bits), and that you need to create the temporary array "negs". If your
matrix is only 2-D and not terribly huge then this should be fine.
Mark Rivers
|
|
|
|
| Re: signed character [message #30413 is a reply to message #30412] |
Thu, 25 April 2002 06:19  |
Dick Jackson
Messages: 347
Registered: August 1998
|
Senior Member |
|
|
"Carmen" <cambicambi@hotmail.com> wrote in message
news:8128b197.0204250239.509a20fb@posting.google.com...
> Hello,
>
> I am trying to access information stored in a matrix, byte per byte.
> If I were using C, I should read every byte as SIGNED CHARACTERS.
> But, IDL has not that definition.
>
> I would appreciate a lot if you can tell me what should I do in order
> to read the data correctly.
Signed characters represent values -128 to 127, but IDL Bytes are treated as
0 to 255. I'd convert them to (short) integer, the simplest IDL data type
that can express -128 to 127, as in this example:
IDL> a = [128B, 255B, 0B, 127B]
IDL> b = fix(a) + ([0, -256])[a GT 127B]
(convert to integer, subtract 256 from elements where needed... I think this
will be the fastest method of doing this)
IDL> Print, b
-128 -1 0 127
IDL> Help, b
B INT = Array[4]
If you've read data into a byte array, you may want a function to convert
it:
FUNCTION SignedCharToFix, a
Return, fix(a) + ([0, -256])[a GT 127B]
END
Hope this helps.
Cheers,
--
-Dick
Dick Jackson / dick@d-jackson.com
D-Jackson Software Consulting / http://www.d-jackson.com
Calgary, Alberta, Canada / +1-403-242-7398 / Fax: 241-7392
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
