sensitivity logical operators in R

[kognDisso]

Hello R Fans,

Another question for the community that really frightened me today. The
following logical comparison produces a "false" as output:

t = sum((c(.7,.69,.68,.67,.66)-.5)*c(1,1,-1,-1,1))
tt = sum((c(.7,.69,.68,.67,.66)-.5)*c(1,-1,1,1,-1))

t == tt

This is really strange behavior. Most likely this has something to do
how R represents numbers internally and the possible sensitivity of a
computer? Does anyone know when this strange behavior occurs and how to
fix it?

Thank you all! This list is pleasure!!!

Marc

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[Ken Knoblauch]

Marc Jekel <feuerwald <at> gmx.de> writes:

> Another question for the community that really frightened me today. The
> following logical comparison produces a "false" as output:
>
> t = sum((c(.7,.69,.68,.67,.66)-.5)*c(1,1,-1,-1,1))
> tt = sum((c(.7,.69,.68,.67,.66)-.5)*c(1,-1,1,1,-1))
>
> t == tt
>
> This is really strange behavior. Most likely this has something to do
> how R represents numbers internally and the possible sensitivity of a
> computer? Does anyone know when this strange behavior occurs and how to
> fix it?
>
> Thank you all! This list is pleasure!!!
>
> Marc
>

But, try

all.equal(tt, t)
[1] TRUE

and see the R FAQ 7.31

--
Ken Knoblauch
Inserm U846
Stem-cell and Brain Research Institute
Department of Integrative Neurosciences
18 avenue du Doyen Lépine
69500 Bron
France
tel: [phone]
fax: [phone]
portable: [phone]
http://www.sbri.fr/members/kenneth-knoblauch.html

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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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[Petr Savicky-2]

On Sun, Jan 23, 2011 at 11:13:11PM +0100, Marc Jekel wrote:

> Hello R Fans,
>
> Another question for the community that really frightened me today. The
> following logical comparison produces a "false" as output:
>
> t = sum((c(.7,.69,.68,.67,.66)-.5)*c(1,1,-1,-1,1))
> tt = sum((c(.7,.69,.68,.67,.66)-.5)*c(1,-1,1,1,-1))
>
> t == tt
>
> This is really strange behavior. Most likely this has something to do
> how R represents numbers internally and the possible sensitivity of a
> computer? Does anyone know when this strange behavior occurs and how to
> fix it?

The number 0.7 has infinite expansion in binary
  0.1011001100110011001100110011...
so is rounded in the standard numeric data type, which is used for
speed needed in complex computations. If you know in advance that
the result has at most 2 decimal positions, then round(, digits=2)
yields the correct comparison

  round(t, 2) == round(tt, 2)
  # [1] TRUE

athough 0.2 is also not exactly representable. Both sides are rounded
to the same representable number.

See also
  http://rwiki.sciviews.org/doku.php?id=misc:r_accuracy
for other examples.

Petr Savicky.

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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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[kognDisso]

Hi again,

I have checked the same code (see below) using MATLAB. It produces the
same error (i.e., equal numbers are evaluated as unequal). Do I miss
something?

Thanks for help!

Marc

Marc Jekel schrieb:

> Hello R Fans,
>
> Another question for the community that really frightened me today.
> The following logical comparison produces a "false" as output:
>
> t = sum((c(.7,.69,.68,.67,.66)-.5)*c(1,1,-1,-1,1))
> tt = sum((c(.7,.69,.68,.67,.66)-.5)*c(1,-1,1,1,-1))
>
> t == tt
>
> This is really strange behavior. Most likely this has something to do
> how R represents numbers internally and the possible sensitivity of a
> computer? Does anyone know when this strange behavior occurs and how
> to fix it?
>
> Thank you all! This list is pleasure!!!
>
> Marc
>

______________________________________________
[hidden email] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

[Nordlund, Dan (DSHS/RDA)]

Marc

You have been given the answer already and a solution.  See the R FAQ 7.31.  As you have discovered this issue is not specific to R.  In order to eliminate this "problem" entirely, you will need a computer system with infinite precision.

Dan

Daniel J. Nordlund
Washington State Department of Social and Health Services
Planning, Performance, and Accountability
Research and Data Analysis Division
Olympia, WA 98504-5204

> -----Original Message-----
> From: [hidden email] [mailto:r-help-bounces@r-
> project.org] On Behalf Of Marc Jekel
> Sent: Monday, January 24, 2011 11:48 AM
> Cc: [hidden email]
> Subject: Re: [R] sensitivity logical operators in R
>
> Hi again,
>
> I have checked the same code (see below) using MATLAB. It produces the
> same error (i.e., equal numbers are evaluated as unequal). Do I miss
> something?
>
> Thanks for help!
>
> Marc
>
> Marc Jekel schrieb:
> > Hello R Fans,
> >
> > Another question for the community that really frightened me today.
> > The following logical comparison produces a "false" as output:
> >
> > t = sum((c(.7,.69,.68,.67,.66)-.5)*c(1,1,-1,-1,1))
> > tt = sum((c(.7,.69,.68,.67,.66)-.5)*c(1,-1,1,1,-1))
> >
> > t == tt
> >
> > This is really strange behavior. Most likely this has something to do
> > how R represents numbers internally and the possible sensitivity of a
> > computer? Does anyone know when this strange behavior occurs and how
> > to fix it?
> >
> > Thank you all! This list is pleasure!!!
> >
> > Marc
> >
>
> ______________________________________________
> [hidden email] mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-
> guide.html
> and provide commented, minimal, self-contained, reproducible code.

______________________________________________
[hidden email] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

[Berend Hasselman]

kognDisso wrote

Hi again,

I have checked the same code (see below) using MATLAB. It produces the
same error (i.e., equal numbers are evaluated as unequal). Do I miss
something?

1. It is NOT an error
2. The numbers are NOT equal
3. Please read FAQ 7.31.
3. Do

t - tt

and you will see something like

[1] -2.220446e-16

as answer.

Berend