What are the R sorting rules of character vectors?

Question

R sorts character vectors in a sequence which I describe as alphabetic, not ASCII.

For example:

sort(c("dog", "Cat", "Dog", "cat"))
[1] "cat" "Cat" "dog" "Dog"

Three questions:

1. What is the technically correct terminology to describe this sort order?
2. I can not find any reference to this in the manuals on CRAN. Where can I find a description of the sorting rules in R?
3. is this any different from this sort of behaviour in other languages like C, Java, Perl or PHP?

Coiffeur

Related to Do not ignore case in sorting character strings. – Vagus

Answer 1

Details: for sort() states:

 The sort order for character vectors will depend on the collating
 sequence of the locale in use: see ‘Comparison’.  The sort order
 for factors is the order of their levels (which is particularly
 appropriate for ordered factors).

and help(Comparison) then shows:

 Comparison of strings in character vectors is lexicographicwithin
 the strings using the collating sequence of the locale in use:see
 ‘locales’.  The collating sequence of locales such as ‘en_US’ is
 normally different from ‘C’ (which should use ASCII) and can be
 surprising.  Beware of making _any_ assumptions about the 
 collation order: e.g. in Estonian ‘Z’ comes between ‘S’ and ‘T’,
 and collation is not necessarily character-by-character - in
 Danish ‘aa’ sorts as a single letter, after ‘z’.  In Welsh ‘ng’
 may or may not be a single sorting unit: if it is it follows ‘g’.
 Some platforms may not respect the locale and always sort in
 numerical order of the bytes in an 8-bit locale, or in Unicode
 point order for a UTF-8 locale (and may not sort in the same order
 for the same language in different character sets).  Collation of
 non-letters (spaces, punctuation signs, hyphens, fractions and so
 on) is even more problematic.

so it depends on your locale setting.

Demimondaine

D'oh. I was trying to find this in cran.r-project.org/doc/manuals/R-ints.html. Thank you. – Coiffeur

I won't attempt to improve upon Dirk & the help's description, but outside of R, one might find it described as lexicographic sorting, albeit case-invariant. The collation rule is a serious consideration, as naive text processing is usually done with regard to English order, which is bad for some other languages. A good example is it makes name sorting look really weird to either native speakers or people who think only of 26 letters in strict A-Z order. – Labial

and I've just spent a long time discovering that space characters may or may not be ignored, and that this changed depending on whether I was running tests locally, or doing R CMD check – Sounding

A better method is to use stringr::str_sort and you can assign locale so it will have consistent result. – Anatola

Only if you are willing or required to accept the heavier burden of stringr dependencies. – Demimondaine

Answer 2

Sorting depends on locale. My solution for that is the following...

I create ~/.Renviron file

cat ~/.Renviron 
#LC_ALL=C

then in R sorting is in C locale

x=c("A", "B", "d", "F", "g", "H")
sort(x)
#[1] "A" "B" "F" "H" "d" "g"

Smyrna