# [R] diff question

(Ted Harding) Ted.Harding at wlandres.net
Sun Jan 11 09:48:06 CET 2015

```Troels, this is due to the usual tiny difference between numbers
as computed by R and the numbers that you think they are!

tt  <- seq(0,20,by=0.02)
dtt <- diff(tt)
length(dtt)
# [1] 1000
r02 <- rep(0.02,1000)
unique(r02 - dtt)
# [1]  0.000000e+00  3.469447e-18 -3.469447e-18  1.040834e-17
# [5] -1.734723e-17  3.816392e-17  9.367507e-17  2.046974e-16
# [9]  4.267420e-16 -4.614364e-16 -1.349615e-15 -3.125972e-15

Hoping this helps!
Ted.

On 11-Jan-2015 08:29:26 Troels Ring wrote:
> R version 3.1.1 (2014-07-10) -- "Sock it to Me"
> Copyright (C) 2014 The R Foundation for Statistical Computing
> Platform: x86_64-w64-mingw32/x64 (64-bit)
>
> Dear friends - I have a small problem with diff (I guess)
> I made a sequence with fixed interval between consecutive elements - and
> hence thought the  diff would be as specified
> but had a vector with apparently identical 12 elements returned from diff
> tt <- seq(0,20,by=0.02)
> unique(diff(tt)) #[1] 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02
> 0.02 0.02
> Trying to see if these elements in diff were duplicated
> duplicated(diff(tt))
>#[1] FALSE  TRUE FALSE FALSE  TRUE FALSE FALSE  TRUE  TRUE and from
> sum(duplicated(diff(tt)))
> [1] 988
> saw that 12 of the elements in duplicated(diff(tt)) were FALSE. Would it
> be expected that the first was FALSE and the rest TRUE?
>|duplicated()|determines which elements of a vector or data frame are
> duplicates of elements with smaller subscripts, and returns a logical
> vector indicating which elements (rows) are duplicates.
>
> All best wishes
> Troels
> Aalborg, Denmark
>
>       [[alternative HTML version deleted]]
>
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-------------------------------------------------
E-Mail: (Ted Harding) <Ted.Harding at wlandres.net>
Date: 11-Jan-2015  Time: 08:48:03
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