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
sensitivity logical operators in R
6 messages · Ken Knoblauch, Petr Savicky, Marc Jekel +2 more
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 L?pine 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
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.
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
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: r-help-bounces at r-project.org [mailto:r-help-bounces at r- project.org] On Behalf Of Marc Jekel Sent: Monday, January 24, 2011 11:48 AM Cc: r-help at r-project.org 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
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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
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