# sensitivity logical operators in R

6 messages
Open this post in threaded view
|

## sensitivity logical operators in R

 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-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.
Open this post in threaded view
|

## Re: sensitivity logical operators in R

 Marc Jekel 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: +33 (0)4 72 91 34 77 fax: +33 (0)4 72 91 34 61 portable: +33 (0)6 84 10 64 10 http://www.sbri.fr/members/kenneth-knoblauch.html______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.
Open this post in threaded view
|

## Re: sensitivity logical operators in R

 In reply to this post by Marc Jekel-3 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_accuracyfor other examples. Petr Savicky. ______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.
Open this post in threaded view
|

## Re: sensitivity logical operators in R

 In reply to this post by Marc Jekel-3 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-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.