[R] Unknown anomaly

[Benjamin Robira]

Dear Sir,

I writting to you as I am facing an irregularity in R that I do not know
the origin. When doing a sequence from 0 to 1 by 0.02 and assigning it to
a vector (i.e. code: a <- seq(from=0, to=1, by=0.02)) then, when I try to
use the 36th element (and two others behave the same way) it is not
recognized correctly. For instance a[36]==0.7, what should give TRUE,
gives instead FALSE. However, this works fine for element 35 and 37 and
all other elements but two.
I do not know the reason. I restarted my R session and tried on another
computer. This has been the same. None of my colleagues had an answer. I
hope that you would be able to help me fix that as it must be a pretty
straightforward error that I do not realise.

I would be thankful for any help,
With my very Best Regards,
Benjamin.

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[Boris Steipe]

The concept of equality for numbers that are represented on a computer is frequently misapplied. Consider:

a <- seq(from=0, to=1, by=0.02)

print(a[36])
[1] 0.7

a[36] == 0.7
[1] FALSE

print(a[36], digits=22)
[1] 0.7000000000000000666134

a[36] == 0.7000000000000001
[1] TRUE

All clear?


B.

[Rolf Turner]

On 19/04/17 13:55, Benjamin Robira wrote:
> Dear Sir,
>
> I writting to you as I am facing an irregularity in R that I do not know
> the origin. When doing a sequence from 0 to 1 by 0.02 and assigning it to
> a vector (i.e. code: a <- seq(from=0, to=1, by=0.02)) then, when I try to
> use the 36th element (and two others behave the same way) it is not
> recognized correctly. For instance a[36]==0.7, what should give TRUE,
> gives instead FALSE. However, this works fine for element 35 and 37 and
> all other elements but two.
> I do not know the reason. I restarted my R session and tried on another
> computer. This has been the same. None of my colleagues had an answer. I
> hope that you would be able to help me fix that as it must be a pretty
> straightforward error that I do not realise.
>

> I would be thankful for any help.


See FAQ 7.31.

cheers,

Rolf Turner

--
Technical Editor ANZJS
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276

[Richard M. Heiberger]

This is the standard behavior of floating point arithmetic on a
digital computer. Computers use 53-bit finite precision arithmetic.
They do not use infinite precision real numbers. Please see FAQ 7.31
for details.

The FAQ is in the R documentation on your computer in file
system.file("../../doc/FAQ")
Locate the file from the R prompt and then open in your favorite text editor.

[Bert Gunter]

FAQ 7.31.

-- Bert

(The FAQ's exist for a reason. You should read them!)

Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )