| Re: fix(4.70*100) is... 469 [message #53628 is a reply to message #53547] |
Fri, 20 April 2007 00:33   |
mmeron
Messages: 44
Registered: October 2003
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Member |
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In article <1177051423.028405.260680@y80g2000hsf.googlegroups.com>, Qing <csis@bigpond.net.au> writes:
> On Apr 20, 3:44 am, b_efrem...@yahoo.com wrote:
>> I'll try one more time , see if it works.
>>
>> Here is my initial statement:
>> When I handle a floating point number, 470.000
>> it better stay the same number with the precision I use it.
>>
>> It may well be 469.99999999999999999999999 and this is
>> perfectly fine with me if when used with a floating point precision
>> it is rounded to 470.000
>>
>> OK, so if I do something, like
>> converting 470.00 to double, I have no right to complain when
>> the number I get is 469.99999999999999435462346 with any
>> arbitrary numbers in the digits exceeding the precision I had
>> provided.
>>
>> On the other side, I would expect when I use this number with the
>> provided precision or lower to matter if I had given the number
>> 470.000 or 469.999.
>>
>> In short, when converting my number to something of lower precision
>> like integer
>> I would expect the number to be rounded.
>>
>> but the integer of 469.9999 is 469. which is not true.
>>
>> I also had the wrong assumption that FIX will FIRST round the number
>> to the precision I'm
>> working with and THEN truncate.
>> Now when I think of this, it is not very intellicgent assumption.
>> Well, I did it.
>> On the other hand I stilll think that
>>
>> print,469.9999,format='(i3)'
>> should be
>> 470
>>
>> but it is 469 insted.
>> What is wrong about this assumption of mine?
>> Cheers
>> Boryana
>
> Definately "sky is falling" introduces great fun and interests to us
> scientists! To continue ...
>
> (1) It is not a problem about just the FIX function:
>
> IDL> print, floor(4.7*100)
> 469
>
> (2) Can we always use ROUND instead of FIX or FLOOR? Then why the hell
> to have FIX and FLOOR to get confused?
>
Because they provide answers to different questions. Given a floating
point number X, you may be (depending on the problem you're dealing
with) interested in:
1) The closest integer approximation to X
2) The upper bound of all integers <= X
3) The lower bound of all integers >= X
ROUND provides the answer to the first question, FLOOR to the second,
CEIL to the third. So they're all useful. FIX is the worst choice in
general since it behaves like FLOOR for positives integers but like
CEIL for negative ones.
> (3) A precision issue? definitely need to read again at
> http://www.dfanning.com/math_tips/sky_is_falling.html .
> "There's nothing worse that trying to debug code and discovering weird
> results are related to the precision of the represetation" - it can
> also be fun!
> But what about:
>
> IDL> print, fix(100D*4.7) ============= it can still be argued
> as a precision issue as long as you use 4.7 as a example !!!
> 469
>
> (4) "... maybe apart from an insidious compiler bug, but that would
> never happen with IDL!"
> does the problem happen in just IDL (on Windows, Lenux, MaxOS, ...)?
> Do we have the same problem in C/C++, FORTRAN or even BASIC?
>
> Lets continue the hunt... it may not be just fun. Is is possible that
> the difference between 469 and 470 could end up sending a satellite
> off its track :-((
>
> Really, I am serious! :-)))
> Qing
>
>
>
>
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"
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