[R] nmax parameter in factor function

[Bert Gunter]

Well, you won't like this, but it is kind of wimpily (is that a word?)
documented:

If you check the code of factor(), you will see that nmax appears as
an argument in a call to unique(). ?unique says for nmax, "... see
duplicated" . And ?duplicated says:

"If nmax is set too small there is liable to be an error: nmax = 1 is
silently ignored."

So sometimes you get an error when nmax is too small with the hash
table error message; and sometimes you just apparently get the nmax
argument ignored:

> identical(factor(letters,nmax = 25), factor(letters,nmax=26))
[1] TRUE

and that, to paraphrase what Roger Hammerstein said about Kansas City,
is about "as fer as I can go."

(http://lyricsplayground.com/alpha/songs/e/everythingsuptodateinkansascity.shtml)

Cheers,
Bert



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 )


On Sat, Jun 3, 2017 at 6:14 PM, Ramnik Bansal <ramnik.bansal at gmail.com> wrote:
> I have been trying to understand how the argument 'nmax' works in
> 'factor' function. R-Documentation states - "Since factors typically
> have quite a small number of levels, for large vectors x it is helpful
> to supply nmax as an upper bound on the number of unique values."
>
> In the code below what is the reason for error when value of nmax is
> 24. Why did the same error not occur with nmax = 25  and also how come
> there are 26 levels when nmax = 25 ?
>
>> factor(x = letters, nmax = 26)
>  [1] a b c d e f g h i j k l m n o p q r s t u v w x y z
> Levels: a b c d e f g h i j k l m n o p q r s t u v w x y z
>
>> factor(x = letters, nmax = 25)
>  [1] a b c d e f g h i j k l m n o p q r s t u v w x y z
> Levels: a b c d e f g h i j k l m n o p q r s t u v w x y z
>
>> factor(x = letters, nmax = 24)
> Error in unique.default(x, nmax = nmax) : hash table is full
>
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