Opened 9 years ago
Last modified 8 years ago
#8896 needs_info defect
0.0000000000000000000000000000 is parsed completely differently than 1.0000000000000000000000000000 for no good reason
Reported by: | was | Owned by: | AlexGhitza |
---|---|---|---|
Priority: | minor | Milestone: | sage-pending |
Component: | basic arithmetic | Keywords: | sd32 |
Cc: | jason | Merged in: | |
Authors: | Robert Bradshaw | Reviewers: | Mariah Lenox |
Report Upstream: | N/A | Work issues: | |
Branch: | Commit: | ||
Dependencies: | Stopgaps: |
Description (last modified by )
In Sage 0.0 and 0.00000000000000000000000000000000000000 denote exactly the same thing (a zero with the same precision). However, that 1.0 and 1.00000000000000000000000000000000000000 are different in Sage:
sage: 0.0 0.000000000000000 sage: 0.00000000000000000000000000000000000000 0.000000000000000 sage: parent(0.00000000000000000000000000000000000000) Real Field with 53 bits of precision sage: 1.00000000000000000000000000000000000000 1.0000000000000000000000000000000000000 sage: 1.0 1.00000000000000 sage: parent(1.00000000000000000000000000000000000000) Real Field with 130 bits of precision sage: parent(1.0) Real Field with 53 bits of precision
Some people consider the above inconsistency a bug. See the discussion below and at sage-devel.
Apply 8896-rebased.patch
Attachments (4)
Change History (36)
comment:1 Changed 9 years ago by
comment:2 Changed 9 years ago by
Btw, Sage does not distinguish +0.0 and -0.0 (and 0.0).
Nice how trac interprets long decimal fractions of floats. :D
Changed 9 years ago by
comment:3 Changed 9 years ago by
- Status changed from new to needs_review
comment:4 Changed 9 years ago by
- Cc jason added
comment:5 follow-up: ↓ 6 Changed 9 years ago by
- Status changed from needs_review to needs_work
Looks okay in general, but the last block
else: # Must be 0.00000000000000...0 sigfigs = len(mantissa)
needs to be indented, n'est pas?
In the tests, there should be (brief) words to the effect that the first two have the default minimum precision and that is what is being tested for, while in the new tests we are testing for getting high precision.
comment:6 in reply to: ↑ 5 ; follow-up: ↓ 9 Changed 9 years ago by
Replying to kcrisman:
Looks okay in general, but the last block
else: # Must be 0.00000000000000...0 sigfigs = len(mantissa)needs to be indented, n'est pas?
No, the else
belongs to the for
statement.
But currently, leading zeros contribute to the precision, too: :)
sage: RealNumber(0.000000000000000000).prec() 67 sage: RealNumber(00.000000000000000000).prec() 70 sage: RealNumber(.000000000000000000).prec() 64
I'm not sure if this is intentional, it's at least uncommon.
(And a leading +
also increases the precision.)
It looks as if a decimal point is counted as a significant digit.
And, sorry, the code is extremely ugly (not due to the patch) and inefficient. Examples and parameter description can be improved as well.
I'm not quite sure what kind of strings actually get in (i.e., where/if illegal syntax is catched, it seems all is left to RealNumber._set()
if not already handled by the parser), this should perhaps be documented, too.
comment:7 follow-up: ↓ 10 Changed 9 years ago by
Also, is it intentional that the exponent is ignored when computing the required precision?
E.g., 1.0e-1000000000
as well as 1.0e-10000000000
evaluate to zero, because of only 53 bits precision.
comment:8 Changed 9 years ago by
sage.rings.complex_number.create_ComplexNumber()
also needs work.
(It doesn't "strip" trailing/fractional zeros, but also doesn't treat a given exponent correctly. Bases (other than 10) aren't supported; perhaps it should just call create_RealNumber()
or use the same code to compute the necessary precision once this operates as desired.)
comment:9 in reply to: ↑ 6 ; follow-ups: ↓ 11 ↓ 13 Changed 9 years ago by
- Status changed from needs_work to needs_review
Replying to leif:
No, the
else
belongs to thefor
statement.
Exactly.
But currently, leading zeros contribute to the precision, too: :)
sage: RealNumber(0.000000000000000000).prec() 67 sage: RealNumber(00.000000000000000000).prec() 70 sage: RealNumber(.000000000000000000).prec() 64I'm not sure if this is intentional, it's at least uncommon.
That's the point of this ticket. For 0, all zeros are leading.
(And a leading
+
also increases the precision.)It looks as if a decimal point is counted as a significant digit.
Good catch, fixed. That required some adjustment to keep the input/str in sync.
And, sorry, the code is extremely ugly (not due to the patch) and inefficient. Examples and parameter description can be improved as well.
I'm not quite sure what kind of strings actually get in (i.e., where/if illegal syntax is catched, it seems all is left to
RealNumber._set()
if not already handled by the parser), this should perhaps be documented, too.
Updated the docstring a bit. This is mostly for use by the preparser, though of course it gets used directly as well.
comment:10 in reply to: ↑ 7 Changed 9 years ago by
Replying to leif:
Also, is it intentional that the exponent is ignored when computing the required precision? E.g.,
1.0e-1000000000
as well as1.0e-10000000000
evaluate to zero, because of only 53 bits precision.
Yes, of course. The size of the exponent is completely orthogonal to the number of significant figures. I made create_ComplexNumber defer to create_RealNumber as well.
comment:11 in reply to: ↑ 9 Changed 9 years ago by
No, the
else
belongs to thefor
statement.Exactly.
Here is the reference for this - I hadn't seen that before. Thanks for the additional docstring.
comment:12 follow-up: ↓ 18 Changed 9 years ago by
Just took a first glance:
create_ComplexNumber()
now looks nice- in the above function, it's
len<15
instead oflen<=15
min_prec
can be smaller than 53 (if specified)pad
should be (tested to be) non-negative- (
min_prec
perhaps too, ormin_prec+pad>=0
) - compare against
min_prec+pad
rather than individually for the "common" case rnd
description increate_RealNumber()
is slightly messed up- IMHO leading zeros left to the decimal point should be stripped/ignored
More to come... ;-)
(I've only looked at the patch to the patch.)
comment:13 in reply to: ↑ 9 ; follow-ups: ↓ 14 ↓ 17 Changed 9 years ago by
Replying to robertwb:
Replying to leif:
But currently, leading zeros contribute to the precision, too: :)
sage: RealNumber(0.000000000000000000).prec() 67 sage: RealNumber(00.000000000000000000).prec() 70 sage: RealNumber(.000000000000000000).prec() 64I'm not sure if this is intentional, it's at least uncommon.
That's the point of this ticket. For 0, all zeros are leading.
Not really. We were (only I think) considering zeros of the fractional part.
Truncation only makes sense from the right; and you don't express precision by padding zeros to the left. Stating that the earth's diameter is about 013 million meters doesn't give more information than stating it is about 13 million meters.
Updated the docstring a bit. This is mostly for use by the preparser, though of course it gets used directly as well.
I was thinking of scientific notation syntax, too. (Also the examples do not contain that form.)
comment:14 in reply to: ↑ 13 Changed 9 years ago by
Replying to leif:
Not really. We were (only I think) considering zeros of the fractional part.
To be more precise, 0.0 and 00.0 denote the same "thing", i.e. have the same meaning, while 0.0 and 0.00 do not.
comment:15 Changed 9 years ago by
For those interested in printing real numbers in general, I'll just plug #7682, which could use just a bit of polishing work, I believe, and makes printing real/complex numbers much more consistent and easy to adjust.
comment:16 Changed 9 years ago by
Replying to robertwb:
Replying to leif:
Also, is it intentional that the exponent is ignored when computing the required precision? E.g.,
1.0e-1000000000
as well as1.0e-10000000000
evaluate to zero, because of only 53 bits precision.Yes, of course. The size of the exponent is completely orthogonal to the number of significant figures.
I don't agree either. If the "effective" exponent (the one in normalized form) exceeds the maximum, one should either increase prec
accordingly or - more practical - raise an exception.
If not, this could be a pitfall. But I don't mind... ;-)
comment:17 in reply to: ↑ 13 Changed 9 years ago by
Replying to leif:
Replying to robertwb:
That's the point of this ticket. For 0, all zeros are leading.
Not really. We were (only I think) considering zeros of the fractional part.
Truncation only makes sense from the right; and you don't express precision by padding zeros to the left. Stating that the earth's diameter is about 013 million meters doesn't give more information than stating it is about 13 million meters.
Which has the same information as saying the Earth's diameter is about 0.000013 trillion meters. Whether or not a digit is significant is not a function of whether it is on the left or right of the decimal.
Would you then say that 0.00e9
and 00.0e8
and .000e10
have different precisions?
comment:18 in reply to: ↑ 12 ; follow-up: ↓ 19 Changed 9 years ago by
Replying to leif:
Just took a first glance:
create_ComplexNumber()
now looks nice- in the above function, it's
len<15
instead oflen<=15
Ah, oops.
min_prec
can be smaller than 53 (if specified)
Yes, but I'm not following what you're trying to say here.
pad
should be (tested to be) non-negative- (
min_prec
perhaps too, ormin_prec+pad>=0
)- compare against
min_prec+pad
rather than individually for the "common" case
I don't care if pad is negative (I'll fix the docstring). Same with min_prec, I just pass whatever I get onto the RealField? constructor which will raise a perfectly fine exception if the precision is not valid (as defined by MPFR_MIN_PREC and MPFR_MAX_PREC).
rnd
description increate_RealNumber()
is slightly messed up
Ah, I'll fix that.
E.g.,
1.0e-1000000000
as well as1.0e-10000000000
evaluate to zero, because of only 53 bits precision.Yes, of course. The size of the exponent is completely orthogonal to the number of significant figures.
I don't agree either. If the "effective" exponent (the one in normalized form) exceeds the maximum, one should either increase
prec
accordingly or - more practical - raise an exception.If not, this could be a pitfall. But I don't mind... ;-)
Well, I'd say the above has only two significant digits of precision, no matter what the exponent, which is all this function tries to deduce. The fact that it's subnormal is outside of the scope of this ticket--if an exception should be raise it's not here. (And in this case you can't raise the precision enough, due to memory/library limitations.)
comment:19 in reply to: ↑ 18 ; follow-up: ↓ 20 Changed 9 years ago by
Replying to robertwb:
Replying to leif:
min_prec
can be smaller than 53 (if specified)Yes, but I'm not following what you're trying to say here.
pad
should be (tested to be) non-negative- (
min_prec
perhaps too, ormin_prec+pad>=0
)- compare against
min_prec+pad
rather than individually for the "common" caseI don't care if pad is negative (I'll fix the docstring). Same with min_prec, I just pass whatever I get onto the RealField? constructor which will raise a perfectly fine exception if the precision is not valid (as defined by MPFR_MIN_PREC and MPFR_MAX_PREC).
If you test for min_prec+pad==53 and ...
, more inputs fall into the simple common case (RR
).
(Using RR
, i.e. 53 bit mantissa, if the sum is less than 53 is perhaps not desired.)
comment:20 in reply to: ↑ 19 Changed 9 years ago by
Replying to leif:
(Using
RR
, i.e. 53 bit mantissa, if the sum is less than 53 is perhaps not desired.)
Must have been some spot on the screen that covered min_
... ;-)
Of course it's pretty ok to return RR
in that case, too. (If someone really wants less precision, he can use RealField(prec)("...")
directly.)
Changed 8 years ago by
comment:21 Changed 8 years ago by
Doctest fixes for 4.6.1.
comment:22 Changed 8 years ago by
- Reviewers set to Mariah Lenox
- Status changed from needs_review to positive_review
Patch fixes problem. Ran 'make testlong'. All tests passed. Positive review!
comment:23 Changed 8 years ago by
Thanks!
comment:24 Changed 8 years ago by
- Description modified (diff)
comment:25 follow-ups: ↓ 26 ↓ 27 Changed 8 years ago by
- Status changed from positive_review to needs_info
In my opinion, this ticket is a huge can of worms and a bad idea. I don't see any mathematically consistent reason who 0.0 should be treated differently than 0.0000000000000.
I really think the current behaviour of Sage is what makes the most sense (mathematically) so I am not in favour of this ticket. Of course, if the majority thinks this patch is a good idea then I'm all for it.
One thing about the patch which is very unclear is why zero is treated differently from other numbers. Consider::
sage: (0.0).prec() 53 sage: (0.1).prec() 53 sage: (000000000000000000000.0).prec() 77 sage: (000000000000000000000.1).prec() 53 sage: (0.000000000000000000000).prec() 77 sage: (0.000000000000000000001).prec() 53
comment:26 in reply to: ↑ 25 Changed 8 years ago by
Replying to jdemeyer:
In my opinion, this ticket is a huge can of worms and a bad idea. I don't see any mathematically consistent reason who 0.0 should be treated differently than 0.0000000000000.
It's exactly the same reason that 1.0 is treated differently than 1.0000000000000000000000. The (value-preserving) trailing zeros indicate higher precision.
I really think the current behaviour of Sage is what makes the most sense (mathematically) so I am not in favour of this ticket. Of course, if the majority thinks this patch is a good idea then I'm all for it.
One thing about the patch which is very unclear is why zero is treated differently from other numbers.
Because otherwise there's now way to write a high-precision zero (in fact I can't think of any other interpretation of a large number of trailing zeros).
Consider::
sage: (0.0).prec() 53 sage: (0.1).prec() 53 sage: (000000000000000000000.0).prec() 77 sage: (000000000000000000000.1).prec() 53 sage: (0.000000000000000000000).prec() 77 sage: (0.000000000000000000001).prec() 53
comment:27 in reply to: ↑ 25 Changed 8 years ago by
Another argument is that the BDFL suggested this change fairly strongly when opening this ticket :)
More seriously, I do have a question here. Robert's reasoning seems sound in the following example - we want a high-precision zero, and this is the best way to acquire it.
> sage: (0.000000000000000000000).prec() > 77 > sage: (0.000000000000000000001).prec() > 53
But I agree with Jeroen that the following seems problematic. Didn't you say yourself (robertwb) that leading zeros before the decimal point shouldn't make a difference? So in the case here maybe we should have the default precision...
> sage: (000000000000000000000.0).prec() > 77 > sage: (000000000000000000000.1).prec() > 53
Keeping in mind that I have very little invested in either outcome for now, so take these only as observations.
comment:28 follow-up: ↓ 29 Changed 8 years ago by
Replying to robertwb:
Replying to jdemeyer:
In my opinion, this ticket is a huge can of worms and a bad idea. I don't see any mathematically consistent reason who 0.0 should be treated differently than 0.0000000000000.
It's exactly the same reason that 1.0 is treated differently than 1.0000000000000000000000. The (value-preserving) trailing zeros indicate higher precision.
It is absolutely not the same because with non-zero numbers only digits after a non-zero digit contribute to the precision. Indeed, 0.437
and 0.00000000000437
have exactly the same precision.
Because otherwise there's now way to write a high-precision zero (in fact I can't think of any other interpretation of a large number of trailing zeros).
Well, you can always do:
sage: zero = RealField(1000)(0) sage: zero.prec() 1000
An other way to demonstrate the problem with this patch is that truncating decimal numbers (i.e. removing digits from the right) can increase the precision, which is very unlogical::
sage: (0.000000000000000000001234567890123456789).prec() 67 sage: (0.00000000000000000000123456789012345678).prec() 64 sage: (0.000000000000000000001).prec() 53 sage: (0.00000000000000000000).prec() 74
comment:29 in reply to: ↑ 28 ; follow-up: ↓ 30 Changed 8 years ago by
Replying to jdemeyer:
Replying to robertwb:
Replying to jdemeyer:
In my opinion, this ticket is a huge can of worms and a bad idea. I don't see any mathematically consistent reason who 0.0 should be treated differently than 0.0000000000000.
It's exactly the same reason that 1.0 is treated differently than 1.0000000000000000000000. The (value-preserving) trailing zeros indicate higher precision.
It is absolutely not the same because with non-zero numbers only digits after a non-zero digit contribute to the precision. Indeed,
0.437
and0.00000000000437
have exactly the same precision.
This hinges on the ambiguity of the term "trialing zero." Sage interprets trailing zeros after the decimal point as additional precision. To me, trailing zeros are those that are not followed by a non-zero digit, to you trailing zeros must also follow a non-zero digit.
Because otherwise there's now way to write a high-precision zero (in fact I can't think of any other interpretation of a large number of trailing zeros).
Well, you can always do:
sage: zero = RealField(1000)(0) sage: zero.prec() 1000
Yeah, but that's a pain, but to me the fact that 0.00000000000000000000000000000000 is not high precision is more surprising, as it has many significant digits.
An other way to demonstrate the problem with this patch is that truncating decimal numbers (i.e. removing digits from the right) can increase the precision, which is very unlogical::
sage: (0.000000000000000000001234567890123456789).prec() 67 sage: (0.00000000000000000000123456789012345678).prec() 64 sage: (0.000000000000000000001).prec() 53 sage: (0.00000000000000000000).prec() 74
It also changes the relative value by a huge amount :)
Is it worth taking this discussion to sage-devel?
comment:30 in reply to: ↑ 29 Changed 8 years ago by
Replying to robertwb:
to me the fact that 0.00000000000000000000000000000000 is not high precision is more surprising, as it has many significant digits.
I would say that it does have a lot of digits but that none of them is a significant digit.
comment:31 Changed 8 years ago by
- Description modified (diff)
- Milestone changed from sage-4.7.1 to sage-pending
comment:32 Changed 8 years ago by
- Keywords sd32 added
This is because 0.00000000000000000000000000000000005 is not considered to have "high precision." We should special-case 0.