| Re: Differences between IDL's floats and Java's floats - a problem [message #36951 is a reply to message #36950] |
Thu, 13 November 2003 05:05  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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necr@pml.ac.uk writes:
> I am currently porting some IDL code across to java, and I've run into
> a couple of snags with the different ways IDL and Java deal with
> numbers. Most of these I have sorted, with trips to the APIs and a
> fair bit of googling. However, there is one I haven't managed to sort
> out yet.
>
> IDL and Java appear to load floating point numbers from a file in very
> different ways.
>
> In IDL, I can read seperate bytes into memory like so:
>
> myvar=0b & readu, 10, myvar
>
> (where 10 is the filehandle of the open file which is being read).
> This will result in an unsigned byte being retrieved.
>
> I could read the byte in with java using a DataInputStream like so:
>
> byte a = input.readUnsignedByte();
>
>
> (where input is my open DataInputStream).
>
> Both these pieces of code would give the same result if run on the
> same byte in a file
>
> To read in a float in idl, I would use:
>
> readu, 10, floatvar
>
>
> (the floatvar would not previously have been set to anything).
>
> And in java:
>
> float f = input.readFloat();
>
> However, running these two pieces of code will not result in the same
> float being given out.
>
> My Example
>
> I read in four bytes from a file, both in IDL and Java. Both systems
> give me the results 0, 64, 206, 67.
>
> If I run the float code on these same four bytes though, IDL will give
> me 412.50 (the value I want), while Java will give 5.951465E-39 -
> clearly not the number I'm looking for!
>
> I have tested this on both a Sun and a Windows machine, and have
> received the same results.
>
> So, has anyone got any ideas as to why this is happening? And more
> importantly, does anyone know what I can do to get the same float
> value being loaded in Java?
>
> Thankyou in advance for any help you can give me.
This is a byte order problem. I don't know Java, but
consider this IDL experiment:
IDL> a=[0b, 64b, 206b, 67b]
IDL> print, float(a,0)
412.500
IDL> b=[67b,206b,64b,0b]
IDL> print, float(b,0)
5.95146e-039
Java is apparently opposite-ended from whatever
it is you are running on. :-)
Cheers,
David
--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http:/www.dfanning.com/
Phone: 970-221-0438, IDL Book Orders: 1-888-461-0155
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