| Re: Constructing integer variables from two bytes? [message #78595] |
Wed, 07 December 2011 08:32 |
greg.addr
Messages: 160
Registered: May 2007
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Senior Member |
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Yes - this is useful - I wish I'd known about it a long time ago!
IDL> a=[1b,2b,2b,3b,4b,5b]
IDL> b2=fix(a, 0, 3, type=2)
IDL> print,b2
513 770 1284
Seems to me, though, that offset should be 0 (unless you want to skip bytes), and you have to figure out the final number of integers yourself. However, it would have made more sense if it worked the way David said... specifying to grab 2 bytes at a time.
While on the topic, is there a similarly direct way to convert bytes into an arbitrary structure? To convert the above array into two of these {b:0b,i:0} for example?
cheers,
Greg
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| Re: Constructing integer variables from two bytes? [message #78602 is a reply to message #78595] |
Wed, 07 December 2011 05:19  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Lajos Foldy writes:
> FIX can combine the bytes into integers (in host byte order, use
> SWAP_ENDIAN if the source byte order is different):
>
> IDL> b2=fix(a, 0, 1, type=2)
> IDL> help, b2
> B2 INT = Array[1]
> IDL> print, b2
> 513
> IDL> print, swap_endian(b2)
> 258
You may have missed this in Lejos's explanation, but
all of these "casting" functions (e.g., Fix, Long, Float,
etc.) can extract data from byte arrays in this fashion.
And not just one value at a time, but all the values
at once. The trick is to use the offset parameter to
tell the function how may byte values to "grab" when
it does the conversion.
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.idlcoyote.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")
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| Re: Constructing integer variables from two bytes? [message #78603 is a reply to message #78602] |
Wed, 07 December 2011 04:52 |
Lajos Foldy
Messages: 176
Registered: December 2011
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Senior Member |
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On Dec 7, 6:46 am, Jared Espley <jesp...@gmail.com> wrote:
> I have data in a byte array that I would like to use to construct
> arrays of unsigned integers and integers.
>
> In code:
> A = [1B, 2B]
> B = 0
> B = A[0]*256 + fix(A[1])
>
> Which will give B as an integer type with a value 258.
>
> I realize that I can construct functions to do this (multiple each bit
> by the appropriate value and checking which OS we're in so that we get
> endianness correct) but I was wondering if someone had already done
> this rigorously?
>
> Thanks,
> Jared
FIX can combine the bytes into integers (in host byte order, use
SWAP_ENDIAN if the source byte order is different):
IDL> b2=fix(a, 0, 1, type=2)
IDL> help, b2
B2 INT = Array[1]
IDL> print, b2
513
IDL> print, swap_endian(b2)
258
regards,
Lajos
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