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Re: DOUBLE precision no precise?? [message #29608 is a reply to message #29605]

Tue, 05 March 2002 08:29

James Kuyper
Messages: 425
Registered: March 2000

Senior Member

Vincent Schut wrote:

> David Williams wrote:
...

>> To try and find where the problem is, we tried the following lines...
>>
>> IDL> a = DOUBLE(42766.080001)
>> IDL> print,a,FORMAT='(F24.17)'
>>
>> 42766.07812500000000000
...

> I'm no expert on math precision, but I think that this is a known
> 'problem', caused by the different possible ways to store floating point
> precision data. (For example, different C++ compilers also give
> different values for a double precision float constant of pi, nice eh?

It's not quite the same problem. The C++ problem is due to the fact that
different implementations of C++ are free to implement double precision
math with different degrees of precision. There's a minimum required
precision, but it's not a very strict requirement. And since the C
standard library isn't even required to provide a value of pi,
implementations are free to provide it with whatever precision they choose.

The IDL problem is due to the fact that the default precision in IDL is
'FLOAT', rather than 'double', which is the default precision for C/C++.
Thus,

IDL C/C++
float
42766.080001 42766.080001F
double
42766.080001D 42766.080001
long double N/A 42766.080001L

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[Message index]

		Re: DOUBLE precision no precise?? By: Pete[1] on Tue, 05 March 2002 13:11
		Re: DOUBLE precision no precise?? By: dave_r_williams on Tue, 05 March 2002 09:20
		Re: DOUBLE precision no precise?? By: James Kuyper on Tue, 05 March 2002 08:29
		Re: DOUBLE precision no precise?? By: William Clodius on Tue, 05 March 2002 08:11
		Re: DOUBLE precision no precise?? By: Robert Stockwell on Tue, 05 March 2002 07:32
		Re: DOUBLE precision no precise?? By: Martin Downing on Tue, 05 March 2002 06:30
		Re: DOUBLE precision no precise?? By: Vincent Schut on Tue, 05 March 2002 07:07
		Re: DOUBLE precision no precise?? By: R.Bauer on Tue, 05 March 2002 06:07
		Re: DOUBLE precision no precise?? By: Vincent Schut on Tue, 05 March 2002 05:01
		Re: DOUBLE precision no precise?? By: James Tappin on Tue, 05 March 2002 04:14

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