R help - Feb 2012 - The "less than" (<) operator doesnt seem to perform as expected

The example here puzzles me. It seems like the < operator doesn't work as
expected.
> l <- 0.6
> u <- seq(0.4, 0.7, 0.1)
> u
[1] 0.4 0.5 0.6 0.7
> mygrid <- expand.grid("l" = l, "u" = u)
> mygrid
    l   u
1 0.6 0.4
2 0.6 0.5
3 0.6 0.6
4 0.6 0.7
> mygridcollapsed <- mygrid[mygrid$l < mygrid$u, ]
> mygridcollapsed
    l   u
3 0.6 0.6
4 0.6 0.7

In this little example I expect 'mygridcollapsed' only to return row 4
and for it to return row 3 seems wrong. The strange thing is it seems to work if
I start the u-sequence at 0.5.
> l <- 0.6
> u <- seq(0.5, 0.7, 0.1)
> u
[1] 0.5 0.6 0.7
> mygrid <- expand.grid("l" = l, "u" = u)
> mygrid
    l   u
1 0.6 0.5
2 0.6 0.6
3 0.6 0.7
> mygridcollapsed <- mygrid[mygrid$l < mygrid$u, ]
> mygridcollapsed
    l   u
3 0.6 0.7

Maybe I'm missing something...

Best wishes
Jonas Hal


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	[[alternative HTML version deleted]]

This is R FAQ 7.31, about machine representation of
floating point numbers.
> mygrid$u[3] - mygrid$l[3]
[1] 1.110223e-16

So
mygrid$l[3] < mygrid$u[3]
is true, though the difference is very, very small and due solely
to the limitations of computers.

Sarah

On Thu, Feb 2, 2012 at 5:00 AM, Jonas Hal <JOH at brf.dk>
wrote:
> The example here puzzles me. It seems like the < operator doesn't
work as expected.
>
>> l <- 0.6
>> u <- seq(0.4, 0.7, 0.1)
>> u
> [1] 0.4 0.5 0.6 0.7
>> mygrid <- expand.grid("l" = l, "u" = u)
>> mygrid
> ? ?l ? u
> 1 0.6 0.4
> 2 0.6 0.5
> 3 0.6 0.6
> 4 0.6 0.7
>> mygridcollapsed <- mygrid[mygrid$l < mygrid$u, ]
>> mygridcollapsed
> ? ?l ? u
> 3 0.6 0.6
> 4 0.6 0.7
>
> In this little example I expect 'mygridcollapsed' only to return
row 4 and for it to return row 3 seems wrong. The strange thing is it seems to
work if I start the u-sequence at 0.5.
>
>> l <- 0.6
>> u <- seq(0.5, 0.7, 0.1)
>> u
> [1] 0.5 0.6 0.7
>> mygrid <- expand.grid("l" = l, "u" = u)
>> mygrid
> ? ?l ? u
> 1 0.6 0.5
> 2 0.6 0.6
> 3 0.6 0.7
>> mygridcollapsed <- mygrid[mygrid$l < mygrid$u, ]
>> mygridcollapsed
> ? ?l ? u
> 3 0.6 0.7
>
> Maybe I'm missing something...
>
> Best wishes
> Jonas Hal
>
>
-- 
Sarah Goslee
http://www.functionaldiversity.org

You need to back up a bit to see the root cause of
the problem, which is that seq()'s calculations necessarily
involve some roundoff error (since it works with 52 binary
digits of precision):
  > u <- seq(from=0.4, to=0.7, by=0.1)
  > u - c(0.4, 0.5, 0.6, 0.7)
  [1] 0.000000e+00 0.000000e+00 1.110223e-16 0.000000e+00
  > u - (4:7) * 0.1
  [1]  0.000000e+00  0.000000e+00  0.000000e+00 -1.110223e-16
  > u - (4:7) / 10
  [1] 0.000000e+00 0.000000e+00 1.110223e-16 0.000000e+00
  > u - cumsum(c(0.4, 0.1, 0.1, 0.1))
  [1]  0.000000e+00  0.000000e+00  0.000000e+00 -1.110223e-16
I find the easiest way around this sort of problem is to use
integer sequences (use them as subscripts into your real sequence
and do the tests on the subscripts).

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 
> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at
r-project.org] On Behalf Of Jonas Hal
> Sent: Thursday, February 02, 2012 2:01 AM
> To: r-help at r-project.org
> Subject: [R] The "less than" (<) operator doesnt seem to
perform as expected
> 
> The example here puzzles me. It seems like the < operator doesn't
work as expected.
> 
> > l <- 0.6
> > u <- seq(0.4, 0.7, 0.1)
> > u
> [1] 0.4 0.5 0.6 0.7
> > mygrid <- expand.grid("l" = l, "u" = u)
> > mygrid
>     l   u
> 1 0.6 0.4
> 2 0.6 0.5
> 3 0.6 0.6
> 4 0.6 0.7
> > mygridcollapsed <- mygrid[mygrid$l < mygrid$u, ]
> > mygridcollapsed
>     l   u
> 3 0.6 0.6
> 4 0.6 0.7
> 
> In this little example I expect 'mygridcollapsed' only to return
row 4 and for it to return row 3
> seems wrong. The strange thing is it seems to work if I start the
u-sequence at 0.5.
> 
> > l <- 0.6
> > u <- seq(0.5, 0.7, 0.1)
> > u
> [1] 0.5 0.6 0.7
> > mygrid <- expand.grid("l" = l, "u" = u)
> > mygrid
>     l   u
> 1 0.6 0.5
> 2 0.6 0.6
> 3 0.6 0.7
> > mygridcollapsed <- mygrid[mygrid$l < mygrid$u, ]
> > mygridcollapsed
>     l   u
> 3 0.6 0.7
> 
> Maybe I'm missing something...
> 
> Best wishes
> Jonas Hal
> 
> 
>
______________________________________________________________________________________________________
> ___________________________________
> BRFkredit sender e-mails og vedhaeftede dokumenter i ikke-krypteret form.
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> modtage oplysninger fra BRFkredit pr. e-mail, beder vi dig meddele os det
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> e-mail kan indeholde fortrolig information. Hvis du modtager e-mailen ved
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> informere os om det hurtigst muligt. Samtidig beder vi dig slette e-mailen
uden at videresende eller
> kopiere indholdet.
>
______________________________________________________________________________________________________
> ___________________________________
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at r-project.org mailing list
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> PLEASE do read the posting guide
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> and provide commented, minimal, self-contained, reproducible code.

It's likely an infelicity of floating point representations (R FAQ
7.31) but admittedly, not a case I would have expected to present
itself.

If you want it to work out as expected, try this:

l <- 0.6
u <- seq(0.4, 0.7, 0.1)

l.int <- (6L) / 10
u.int <- seq(4, 7) / 10

l < u
l.int < u.int

Michael

On Thu, Feb 2, 2012 at 5:00 AM, Jonas Hal <JOH at brf.dk>
wrote:
> The example here puzzles me. It seems like the < operator doesn't
work as expected.
>
>> l <- 0.6
>> u <- seq(0.4, 0.7, 0.1)
>> u
> [1] 0.4 0.5 0.6 0.7
>> mygrid <- expand.grid("l" = l, "u" = u)
>> mygrid
> ? ?l ? u
> 1 0.6 0.4
> 2 0.6 0.5
> 3 0.6 0.6
> 4 0.6 0.7
>> mygridcollapsed <- mygrid[mygrid$l < mygrid$u, ]
>> mygridcollapsed
> ? ?l ? u
> 3 0.6 0.6
> 4 0.6 0.7
>
> In this little example I expect 'mygridcollapsed' only to return
row 4 and for it to return row 3 seems wrong. The strange thing is it seems to
work if I start the u-sequence at 0.5.
>
>> l <- 0.6
>> u <- seq(0.5, 0.7, 0.1)
>> u
> [1] 0.5 0.6 0.7
>> mygrid <- expand.grid("l" = l, "u" = u)
>> mygrid
> ? ?l ? u
> 1 0.6 0.5
> 2 0.6 0.6
> 3 0.6 0.7
>> mygridcollapsed <- mygrid[mygrid$l < mygrid$u, ]
>> mygridcollapsed
> ? ?l ? u
> 3 0.6 0.7
>
> Maybe I'm missing something...
>
> Best wishes
> Jonas Hal
>
>
>
_________________________________________________________________________________________________________________________________________
> BRFkredit sender e-mails og vedhaeftede dokumenter i ikke-krypteret form.
Hvis du ikke ?nsker at modtage oplysninger fra BRFkredit pr. e-mail, beder vi
dig meddele os det via brev eller e-mail. Denne e-mail kan indeholde fortrolig
information. Hvis du modtager e-mailen ved en fejl, beder vi dig informere os om
det hurtigst muligt. Samtidig beder vi dig slette e-mailen uden at videresende
eller kopiere indholdet.
>
_________________________________________________________________________________________________________________________________________
>
> ? ? ? ?[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org 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.

On Thu, Feb 02, 2012 at 10:00:58AM +0000, Jonas Hal
wrote:
> The example here puzzles me. It seems like the < operator doesn't
work as expected.
> 
> > l <- 0.6
> > u <- seq(0.4, 0.7, 0.1)
> > u
> [1] 0.4 0.5 0.6 0.7
> > mygrid <- expand.grid("l" = l, "u" = u)
> > mygrid
>     l   u
> 1 0.6 0.4
> 2 0.6 0.5
> 3 0.6 0.6
> 4 0.6 0.7
> > mygridcollapsed <- mygrid[mygrid$l < mygrid$u, ]
> > mygridcollapsed
>     l   u
> 3 0.6 0.6
> 4 0.6 0.7
> 
> In this little example I expect 'mygridcollapsed' only to return
row 4 and for it to return row 3 seems wrong. The strange thing is it seems to
work if I start the u-sequence at 0.5.
Hi.

As others pointed out, the problem is in different
rounding error of 0.6 and seq(0.4, 0.7, 0.1)[3]. Try

  print(0.6, digits=20)

  [1] 0.5999999999999999778

  print(seq(0.4, 0.7, 0.1)[3], digits=20)

  [1] 0.60000000000000008882

Use round(, digits=1) to force the same rounding in
seq(0.4, 0.7, 0.1) and in c(0.4, 0.5, 0.6, 0.7)

  round(seq(0.4, 0.7, 0.1), digits=1) == c(0.4, 0.5, 0.6, 0.7)

Hope this helps.

Petr Savicky.