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

[Albyn Jones]

Quoting John Fox <jfox at mcmaster.ca>:

> 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

It is easy to test.

> pi
[1] 3.141593
> pi %% 2
[1] 1.141593
> trunc(pi) %% 2
[1] 1
> round(pi) %% 2
[1] 1
> is.integer(round(pi) %% 2)
[1] FALSE
> round(pi) %% 2 == 1
[1] TRUE

You are indeed correct; round() yields a double rather than an integer.
However, %% seems to do the right thing anyway, and the equality test 
works....

I expect that for a sufficiently large double, finite precision will 
eventually
force it to be even even :-)  but that's pretty big.

> (2^65-1) %% 2
[1] 0
Warning message:
probable complete loss of accuracy in modulus

it loooks like somebody thought of this problem already!

albyn