9.3 - (2 * 4.5) => 0.3000000000000007 - Haskell-Cafe

9.3 - (2 * 4.5) => 0.3000000000000007

ivan dragolov

16 Jan 2013 16 Jan '13

8:25 a.m.

9.3 - (2 * 4.5) => 0.3000000000000007 I expected 0.3 ? -- Иван Драголов dragolov.net GSM: 0888 63 19 46 GSM за SMS: 0878 82 83 93 facebook.com/ivan.dragolov twitter.com/dragolov

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Patrick Mylund Nielsen

16 Jan 16 Jan

8:30 a.m.

Fun with floating point! # ghci GHCi, version 7.4.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> 9223372036854775807.0 == 9223372036854775808 True http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems On Wed, Jan 16, 2013 at 7:25 AM, ivan dragolov wrote:

...

9.3 - (2 * 4.5) => 0.3000000000000007

I expected 0.3

?

-- Иван Драголов dragolov.net

GSM: 0888 63 19 46 GSM за SMS: 0878 82 83 93 facebook.com/ivan.dragolov twitter.com/dragolov

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Patrick Mylund Nielsen

8:31 a.m.

This may be of interest if you want full-precision decimals: http://www.haskell.org/haskellwiki/Libraries_and_tools/Mathematics#Decimal_n... On Wed, Jan 16, 2013 at 7:30 AM, Patrick Mylund Nielsen < haskell@patrickmylund.com> wrote:

...

Fun with floating point!

# ghci GHCi, version 7.4.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> 9223372036854775807.0 == 9223372036854775808 True

http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems

On Wed, Jan 16, 2013 at 7:25 AM, ivan dragolov wrote:

...
9.3 - (2 * 4.5) => 0.3000000000000007

I expected 0.3

?

-- Иван Драголов dragolov.net

GSM: 0888 63 19 46 GSM за SMS: 0878 82 83 93 facebook.com/ivan.dragolov twitter.com/dragolov

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Austin Seipp

8:31 a.m.

This is a rounding error. It will happen in any language due to the imprecision of floats; for example, using Ruby: $ irb 1.9.3-p286 :001 > 9.3 - (2 * 4.5) => 0.3000000000000007 1.9.3-p286 :001 > ^D $ Read this: http://floating-point-gui.de/ On Wed, Jan 16, 2013 at 7:25 AM, ivan dragolov wrote:

...

9.3 - (2 * 4.5) => 0.3000000000000007

I expected 0.3

?

-- Иван Драголов dragolov.net

GSM: 0888 63 19 46 GSM за SMS: 0878 82 83 93 facebook.com/ivan.dragolov twitter.com/dragolov

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-- Regards, Austin

Tom Davie

8:55 a.m.

Prelude> import Data.Ratio Prelude Data.Ratio> 93 % 10 - (2 * 9 % 2) 3 % 10 Floating point sucks, avoid it if you can. Thanks Tom Davie On 16 Jan 2013, at 13:25, ivan dragolov wrote:

...

9.3 - (2 * 4.5) => 0.3000000000000007

I expected 0.3

?

-- Иван Драголов dragolov.net

GSM: 0888 63 19 46 GSM за SMS: 0878 82 83 93 facebook.com/ivan.dragolov twitter.com/dragolov _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

David Turner

9:56 a.m.

On 16/01/2013 13:55, Tom Davie wrote:

...

Prelude> import Data.Ratio Prelude Data.Ratio> 93 % 10 - (2 * 9 % 2) 3 % 10

Or even just a type annotation: Prelude> (9.3 - (2 * 4.5)) :: Rational 3 % 10

...

On 16 Jan 2013, at 13:25, ivan dragolov mailto:ivan@dragolov.net> wrote:

...
9.3 - (2 * 4.5) => 0.3000000000000007

I expected 0.3

-- David Turner Senior Consultant Tracsis PLC Tracsis PLC is a company registered in England and Wales with number 5019106.

Luis Cabellos

10:04 a.m.

You should read http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.22.6768 If you want to be efficient with floats. Language independent. Luis On Wed, Jan 16, 2013 at 2:25 PM, ivan dragolov wrote:

...

9.3 - (2 * 4.5) => 0.3000000000000007

I expected 0.3

?

-- Иван Драголов dragolov.net

GSM: 0888 63 19 46 GSM за SMS: 0878 82 83 93 facebook.com/ivan.dragolov twitter.com/dragolov

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Albert Y. C. Lai

2:25 p.m.

On 13-01-16 10:04 AM, Luis Cabellos wrote:

...

You should read http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.22.6768

If you want to be efficient with floats. Language independent.

And also http://floating-point-gui.de/

Daniel Fischer

10:32 a.m.

On Wednesday 16 January 2013, 15:25:15, ivan dragolov wrote:

...

9.3 - (2 * 4.5) => 0.3000000000000007

I expected 0.3

?

Prelude Text.FShow.RealFloat> FD 9.3 9.300000000000000710542735760100185871124267578125 The closest Double to 9.3 is somewhat larger than 9.3. Since the first two significant digits are enough to distinguish it from the next smaller number, only those are delivered by the Show instance. If you subtract 9 from that, you get, unsurprisingly, Prelude Text.FShow.RealFloat> it - 9 0.300000000000000710542735760100185871124267578125 which is not the closest Double value to 0.3 (in fact, there are 12 Double values between that and the closest Double to 0.3). To distinguish that number from its neighbours, 16 significant digits are necessary, hence so many are displayed.

Joe Q

4:16 p.m.

Welcome to IEEE floating point math! Wikipedia has a good article on the specification. If you want exact fractional numbers for your own purposes, there are a number of ways around the limits of floats. Rational will represent numbers internally as fractions, and still allow you to use the same syntax as above. The result will be "3 % 10", which you read as the fraction 3 / 10. If you also want irrational numbers, things get tricky. CReal will do it, but has some limitations. On Jan 16, 2013 8:26 AM, "ivan dragolov" wrote:

...

9.3 - (2 * 4.5) => 0.3000000000000007

I expected 0.3

?

-- Иван Драголов dragolov.net

GSM: 0888 63 19 46 GSM за SMS: 0878 82 83 93 facebook.com/ivan.dragolov twitter.com/dragolov

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participants (9)

Albert Y. C. Lai
Austin Seipp
Daniel Fischer
David Turner
ivan dragolov
Joe Q
Luis Cabellos
Patrick Mylund Nielsen
Tom Davie