[R-sig-teaching] checking even/odd in R

[John Fox]

Dear Albyn,

I thought about rounding but was concerned that the arithmetic would still
be performed on real numbers. This is true even if the left argument to %%
is coerced to an integer. (round itself produces a double-precision floating
point number, not an integer.) I'm not sure whether this is a realistic
worry.

Regards,
 John

--------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario
Canada L8S 4M4
905-525-9140x23604
http://socserv.mcmaster.ca/jfox 
-------------------------------- 

> -----Original Message-----
> From: Albyn Jones [mailto:jones at reed.edu] 
> Sent: Sunday, December 03, 2006 9:42 PM
> To: John Fox
> Cc: bob at statland.org; 'Yeasmin Khandakar'; 'R-sig-teaching'
> Subject: Re: [R-sig-teaching] checking even/odd in R
> 
> One solution for the exact equality problem is to start by 
> rounding or truncating to an integer, depending of course on 
> what the origin of the dataset was, and the point of the 
> operation.  The sign doesn't matter;
> 
> > -3 %% 2
> [1] 1
> 
> 
> floor(x) rounds down to an integer
> ceiling(x) rounds up
> round(x) rounds to the nearest integer, rounding to an even integer
>          in case of ties
> 
> albyn
> 
> Quoting John Fox <jfox at mcmaster.ca>:
> 
> > Dear Robert and Yeasmin,
> >
> > You could test that the number mod 2 is 0; e.g.,
> >
> >> (1:10 %% 2) == 0
> > [1] FALSE  TRUE FALSE  TRUE FALSE  TRUE FALSE  TRUE FALSE  TRUE
> >
> > A potential problem is that testing exact equality is 
> risky, so you're 
> > probably better off checking that abs(x) %% 2 is less than 
> some very 
> > small number such as sqrt(.Machine$double.eps).
> >
> > I hope this helps,
> > John
> >
> > --------------------------------
> > John Fox
> > Department of Sociology
> > McMaster University
> > Hamilton, Ontario
> > Canada L8S 4M4
> > 905-525-9140x23604
> > http://socserv.mcmaster.ca/jfox
> > --------------------------------
> >
> >> -----Original Message-----
> >> From: r-sig-teaching-bounces at r-project.org
> >> [mailto:r-sig-teaching-bounces at r-project.org] On Behalf Of 
> Robert W. 
> >> Hayden
> >> Sent: Sunday, December 03, 2006 8:24 PM
> >> To: Yeasmin Khandakar
> >> Cc: R-sig-teaching
> >> Subject: Re: [R-sig-teaching] checking even/odd in R
> >>
> >>
> >> I do not know of anything specific to r but standard programming 
> >> tricks may do it.  For example, if the number is x, compute y=x/2, 
> >> let z be y rounded to the nearest whole number, and check 
> if y=z.  If 
> >> it does, x is even.  There may be rounding issues; I doubt 
> R stores 
> >> data as integers.  You might have to do this in an 
> environment that 
> >> sees the numbers as integers -- or text.  AWK comes to mind.
> >>
> >> You may also want code to check for x values like 23.456.
> >>
> >> >
> >> > Dear all,
> >> >
> >> > I'm in search for a function which can be used to check 
> whether a 
> >> > numeriacl value (say, x) is even/odd.
> >> > Any help is highly appreciated.
> >> >
> >> > Thanks
> >> > Yeasmin
> >> >
> >> > --Boundary_(ID_5eoWS9sJoAbqcSKM1OUsmg)
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> >> >
> >> > --Boundary_(ID_5eoWS9sJoAbqcSKM1OUsmg)--
> >> >
> >> >
> >>
> >>
> >>
> >>  _______
> >> |   ^  |
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