R: Setup Collation by ICU

icuSetCollate {base} | R Documentation

## Setup Collation by ICU

### Description

Controls the way collation is done by ICU (an optional part of the R build).

### Usage

    icuSetCollate(...)

    icuGetCollate(type = c("actual", "valid"))

### Arguments

`...` | Named arguments, see ‘Details’.
`type` | character string: can be abbreviated. Either the actual locale in use for collation or the most specific locale which would be valid.

### Details

Optionally, R can be built to collate character strings by ICU (http://site.icu-project.org). For such systems, `icuSetCollate` can be used to tune the way collation is done. On other builds calling this function does nothing, with a warning.

Possible arguments are

`locale`:

  A character string such as `"da_DK"` giving the language and country whose collation rules are to be used. If present, this should be the first argument.

`case_first`:

  `"upper"`, `"lower"` or `"default"`, asking for upper- or lower-case characters to be sorted first. The default is usually lower-case first, but not in all languages (not under the default settings for Danish, for example).

`alternate_handling`:

  Controls the handling of ‘variable’ characters (mainly punctuation and symbols). Possible values are `"non_ignorable"` (primary strength) and `"shifted"` (quaternary strength).

`strength`:

  Which components should be used? Possible values `"primary"`, `"secondary"`, `"tertiary"` (default), `"quaternary"` and `"identical"`.

`french_collation`:

  In a French locale the way accents affect collation is from right to left, whereas in most other locales it is from left to right. Possible values `"on"`, `"off"` and `"default"`.

`normalization`:

  Should strings be normalized? Possible values are `"on"` and `"off"` (default). This affects the collation of composite characters.

`case_level`:

  An additional level between secondary and tertiary, used to distinguish large and small Japanese Kana characters. Possible values `"on"` and `"off"` (default).

`hiragana_quaternary`:

  Possible values `"on"` (sort Hiragana first at quaternary level) and `"off"`.

Only the first three are likely to be of interest except to those with a detailed understanding of collation and specialized requirements.

Some special values are accepted for `locale`:

`"none"`:

  ICU is not used for collation: the OS's collation services are used instead.

`"ASCII"`:

  ICU is not used for collation: the C function `strcmp` is used instead, which should sort byte-by-byte in (unsigned) numerical order.

`"default"`:

  obtains the locale from the OS as is done at the start of the session. If environment variable R_ICU_LOCALE is set to a non-empty value, its value is used rather than consulting the OS.

`""`, `"root"`:

  the ‘root’ collation: see http://www.unicode.org/reports/tr35/tr35-collation.html#Root_Collation.

For the specifications of ‘real’ ICU locales, see http://userguide.icu-project.org/locale. Note that ICU does not report that a locale is not supported, but falls back to its idea of ‘best fit’ (which could be rather different and is reported by `icuGetCollate("actual")`, often `"root"`). Most English locales fall back to `"root"` as although e.g. `"en_GB"` is a valid locale (at least on some platforms), it contains no special rules for collation. Note that `"C"` is not a supported ICU locale.

Some examples are `case_level = "on", strength = "primary"` to ignore accent differences and `alternate_handling = "shifted"` to ignore space and punctuation characters.

Initially ICU will not be used for collation if the OS is set to use the `C` locale for collation. Once this function is called with a value for `locale`, ICU will be used until it is called again with `locale = "none"`.

All customizations are reset to the default for the locale if `locale` is specified: the collation engine is reset if the OS collation locate category is changed by `Sys.setlocale`.

### Value

For `icuGetCollate`, a character string describing the ICU locale in use (which may be reported as `"ICU not in use"`). The ‘actual’ locale may be simpler than the requested locale: for example `"da"` rather than `"da_DK"`: English locales are likely to report `"root"`.

### Note

ICU is used by default wherever it is available: this include macOS, Solaris and many Linux installations. As it works internally in UTF-8, it will be most efficient in UTF-8 locales.

It is optional on Windows: if R has been built against ICU, it will only be used if environment variable R_ICU_LOCALE is set or once `icuSetCollate` is called to select the locale (as ICU and Windows differ in their idea of locale names). Note that `icuSetCollate(locale = "default")` should work reasonably well for R >= 3.2.0 and Windows Vista/Server 2008 and later (but finds the system default ignoring environment variables such as LC_COLLATE).

### See Also

Comparison, `sort`.

`capabilities` for whether ICU is available; `extSoftVersion` for its version.

The ICU user guide chapter on collation (http://userguide.icu-project.org/collation).

### Examples

    ## These examples depend on having ICU available, and on the locale.
    ## As we don't know the current settings, we can only reset to the default.
    if(capabilities("ICU")) {
        print(icuGetCollate())
        print(icuGetCollate("valid"))
        x <- c("Aarhus", "aarhus", "safe", "test", "Zoo")
        print(sort(x))
        icuSetCollate(case_first = "upper"); print(sort(x))
        icuSetCollate(case_first = "lower"); print(sort(x))

        ## Danish collates upper-case-first and with 'aa' as a single letter
        icuSetCollate(locale = "da_DK", case_first = "default"); print(sort(x))
        ## Estonian collates Z between S and T
        icuSetCollate(locale = "et_EE"); print(sort(x))
        icuSetCollate(locale = "default"); print(icuGetCollate("valid"))
    }

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