Math errors in python

[Radioactive Man]

In python 2.3 (IDLE 1.0.3) running under windows 95, I get the
following types of errors whenever I do simple arithmetic:

1st example:

12.10 + 8.30 20.399999999999 999 1.1 - 0.2 0.9000000000000 0013
2nd example(no errors here): bool(130.0 - 129.0 == 1.0) True
3rd example: a = 0.013
b = 0.0129
c = 0.0001
[a, b, c] [0.0129999999999 99999, 0.0129, 0.0001] bool((a - b) == c)

False
This sort of error is no big deal in most cases, but I'm sure it could
become a problem under certain conditions, particularly the 3rd
example, where I'm using truth testing. The same results occur in all
cases whether I define variables a, b, and c, or enter the values
directly into the bool statement. Also, it doesn't make a difference
whether "a = 0.013" or "a = 0.0130".

I haven't checked this under windows 2000 or XP, but I expect the same
thing would happen. Any suggestions for a way to fix this sort of
error?

[DogWalker]

Have a look at the FAQ (before the response to your message builds).

----- Original Message -----
From: Radioactive Man
Newsgroups: comp.lang.pytho n
To: py*********@pyt hon.org
Sent: Saturday, September 18, 2004 9:50 AM
Subject: Math errors in python
In python 2.3 (IDLE 1.0.3) running under windows 95, I get the
following types of errors whenever I do simple arithmetic:

1st example:

12.10 + 8.30 20.399999999999 999 1.1 - 0.2 0.9000000000000 0013
2nd example(no errors here): bool(130.0 - 129.0 == 1.0) True
3rd example: a = 0.013
b = 0.0129
c = 0.0001
[a, b, c] [0.0129999999999 99999, 0.0129, 0.0001] bool((a - b) == c)

False
This sort of error is no big deal in most cases, but I'm sure it could
become a problem under certain conditions, particularly the 3rd
example, where I'm using truth testing. The same results occur in all
cases whether I define variables a, b, and c, or enter the values
directly into the bool statement. Also, it doesn't make a difference
whether "a = 0.013" or "a = 0.0130".

I haven't checked this under windows 2000 or XP, but I expect the same
thing would happen. Any suggestions for a way to fix this sort of
error?
--
http://mail.python.org/mailman/listinfo/python-list

[Tim Peters]

[Radioactive Man]

In python 2.3 (IDLE 1.0.3) running under windows 95, I get the
following types of errors whenever I do simple arithmetic:

1st example:

12.10 + 8.30

20.399999999999 999

....

Please read the Tutorial appendix on floating-point issues:

http://docs.python.org/tut/node15.html

[Gary Herron]

On Saturday 18 September 2004 09:50 am, Radioactive Man wrote:

In python 2.3 (IDLE 1.0.3) running under windows 95, I get the
following types of errors whenever I do simple arithmetic:

1st example:

12.10 + 8.30

20.399999999999 999

It's not a bug, it's a feature of binary arithmetic on ALL coumputers
in ALL languages. (But perhaps Python is the first time it has not
been hidden from you.)

See the Python FAQ entry 1.4.2:

http://www.python.org/doc/faq/genera...-so-inaccurate
Gary Herron

[Jeremy Bowers]

On Sat, 18 Sep 2004 16:50:16 +0000, Radioactive Man wrote:

In python 2.3 (IDLE 1.0.3) running under windows 95, I get the
following types of errors whenever I do simple arithmetic:

Specifically (building on DogWalker's reply),
http://www.python.org/doc/faq/genera...-so-inaccurate

[Peter Otten]

Radioactive Man wrote:

thing would happen. Any suggestions for a way to fix this sort of
error?

Starting with Python 2.4 there will be the 'decimal' module supporting
"arithmetic the way you know it":

from decimal import *
Decimal("12.10" ) + Decimal("8.30") Decimal("20.40" ) Decimal("1.1") - Decimal("0.2") Decimal("0.9") Decimal("130.0" )-Decimal("129.0" ) == Decimal("1.0") True a, b, c = map(Decimal, "0.013 0.0129 0.0001".split() )
a, b, c (Decimal("0.013 "), Decimal("0.0129 "), Decimal("0.0001 ")) (a-b) == c

True
Peter

[Michel Claveau - abstraction méta-galactique non t]

Hi !

Many languages, yes.
All languages, no.

Few languages can use the DCB possibilities of the processors.

@-salutations
--
Michel Claveau

[Dennis Lee Bieber]

On Sat, 18 Sep 2004 20:54:42 +0200, Michel Claveau - abstraction
méta-galactique non triviale en fuite perpétuelle.
<un************ @msupprimerlepo int.claveauPOIN Tcom> declaimed the
following in comp.lang.pytho n:

Few languages can use the DCB possibilities of the processors.
Uhmm... Data Control Block?

Did you mean BCD -- binary coded decimal. Which is a form of
decimal arithmetic, and not native binary floating point.

Considering how often the question about strange floating point
output comes up, I come to the conclusion that CS courses no longer
teach the basics of representation. .. (Try reading the Ada Reference
Manual regarding the differences between float and fixed, neither of
which imply decimal.)

Or find a copy of "Real Computing Made Real"
http://www.amazon.com/exec/obidos/tg...95157?v=glance

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[Michel Claveau - abstraction méta-galactique non t]

Hi !

BCD (english) ==> DCB (french)
And Alcyane-basic run, in 1976, with floating-based-DCB-arithmétique, on the
8080, before than IEEE normalize the floating-arithmétique-pow(256)-based
(and longer before Ada).

And DCB is native in all Intel processors.

And before, when i use Fortran on mini-computer, there was no this problem.
But, then it's very much easy to work with pow(256), And the anscendant
compatibility become... Hélas.
*sorry for my bad english*
Michel Claveau

[Dennis Lee Bieber]

On Sat, 18 Sep 2004 22:58:13 +0200, Michel Claveau - abstraction
méta-galactique non triviale en fuite perpétuelle.
<un************ @msupprimerlepo int.claveauPOIN Tcom> declaimed the
following in comp.lang.pytho n:

And before, when i use Fortran on mini-computer, there was no this problem.
It may have been there, but you never saw it in print <G>

When I took classes in the 70s, we were taught never to rely
upon comparing two floating point numbers for equality. Instead, we were
told to compare for the difference between the two numbers being less
than some epsilon, with the epsilon defined as the smallest difference
that could be considered equal for the numbers being compared (if the
input is only good for 5 significant figures, the epsilon should not be
down into 7 significant)

a = 5.234
b = 5.235
epsilon = 0.0005
if abs(a - b) < epsilon then
equal

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wl*****@ix.netc om.com | Wulfraed Dennis Lee Bieber KD6MOG <
wu******@dm.net | Bestiaria Support Staff <
=============== =============== =============== =============== == <
Home Page: <http://www.dm.net/~wulfraed/> <
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