[R] Weird R behaviour?

[Benilton Carvalho]

FAQ 7.31: Why doesn't R think these numbers are equal?

http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f

b

On Wed, Mar 31, 2010 at 10:10 AM, Gianluca Baio <gianluca at statistica.it> wrote:
> Dear list,
> I have observed a weird behaviour from R --- apologies if I am missing
> something obvious!
>
> df3f826f28
> df3f826f28
> Say you type in R:
>
>>c.preec <- 10074
>>c.gd <- 2200
>>p1 <- .2
>>c.neo <- p1*9451 + (1-p1)*3883
>>n.preec <- 3710
>>n.gd <- 2650
>>n.neo <- 2120
>>n.pcos <- 53000
>>unit.met <- 94
>>cost.met <- 94*n.pcos
>
>>effect <- seq(0,1,.01)
>>gross.save <- ((n.preec*effect*c.preec) + (n.gd*effect*c.gd) +
> (n.neo*effect*c.neo))
>>save.nhs <- gross.save - cost.met
>
>>e.star <- .4
>>s.star <- save.nhs[which(effect==e.star)]/1000000
>
> Now, R should return
>> which(effect==e.star) = 41
>
> and therefore compute the variable s.star.
> All well and trivial; but if you try and set
>>e.star <- .7
>
> R returns
>>which(effect==e.star) = integer(0)
>
> so apparently there is no element in the vector e.star that is equal to .7.
> Of course, there should be, since effect <- seq(0,1,.01), and in fact if you
> ask R to show you the vector effect, you do get
> [1] 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 0.13
> 0.14
> [16] 0.15 0.16 0.17 0.18 0.19 0.20 0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28
> 0.29
> [31] 0.30 0.31 0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39 0.40 0.41 0.42 0.43
> 0.44
> [46] 0.45 0.46 0.47 0.48 0.49 0.50 0.51 0.52 0.53 0.54 0.55 0.56 0.57 0.58
> 0.59
> [61] 0.60 0.61 0.62 0.63 0.64 0.65 0.66 0.67 0.68 0.69 *0.70* 0.71 0.72 0.73
> 0.74
> [76] 0.75 0.76 0.77 0.78 0.79 0.80 0.81 0.82 0.83 0.84 0.85 0.86 0.87 0.88
> 0.89
> [91] 0.90 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1.00
>
> Can anyone think of why this happens?
> Many thanks
> Gianluca
>
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