[R] The "less than" (<) operator doesnt seem to perform as expected

[William Dunlap]

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
> 
> 
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