[Rd] Proposal: more accurate seq(from, to, length=n)

[Gabe Becker]

Suharto,

My 2c inline.

On Fri, Sep 7, 2018 at 2:34 PM, Suharto Anggono Suharto Anggono via R-devel
<r-devel using r-project.org> wrote:

> In R,
> seq(0, 1, 0.1)
> gives the same result as
> (0:10)*0.1.
> It is not the same as
> c(0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1) ,
> as 0.1 is not represented exactly. I am fine with it.
>
> In R,
> seq(0, 1, length=11)
> gives the same result as
> seq(0, 1, 0.1).
> However, for
> seq(0, 1, length=11),
> it is more accurate to return
> c(0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1) .

It can be obtained by
> (0:10)/10.
>
> When 'from', 'to', and 'length.out' are specified and length.out > 2, I
> propose for function 'seq.default' in R to use something like
> from + ((0:(length.out - 1))/(length.out - 1)) * (to - from)
> instead of something like
> from + (0:(length.out - 1)) * ((to - from)/(length.out - 1)) .
>

In your example case under 3.50 on my system these two expressions give
results which return TRUE from all.equal, which is the accepted way of
comparing non-integer numerics in R for "sameness".

> from = 0

> to = 1

> length.out = 11

> all.equal(from + ((0:(length.out - 1))/(length.out - 1)) * (to - from),
from + (0:(length.out - 1)) * ((to - from)/(length.out - 1)))

[1] TRUE

Given that I'm wondering what the benefit you're looking for here is that
would outweigh the very large set of existing code whose behavior would
technically change  under this change. Then again, it wouldn't change with
respect to the accepted all.equal test, so I guess you could argue that
either there's "no change" or the change is ok?

I'd still like to know what practical problem you're trying to solve
though. if you're looking for the ability to use == to compare non integer
sequences generated different ways, as far as I understand the answer is
that you shouldn't be expecting to be able to do that.

Best,
~G


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-- 
Gabriel Becker, Ph.D
Scientist
Bioinformatics and Computational Biology
Genentech Research

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