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11.2.6 Additional functions for plural forms

The functions of the gettext family described so far (and all the catgets functions as well) have one problem in the real world which have been neglected completely in all existing approaches. What is meant here is the handling of plural forms.

Looking through Unix source code before the time anybody thought about internationalization (and, sadly, even afterwards) one can often find code similar to the following:

   printf ("%d file%s deleted", n, n == 1 ? "" : "s");

After the first complaints from people internationalizing the code people either completely avoided formulations like this or used strings like "file(s)". Both look unnatural and should be avoided. First tries to solve the problem correctly looked like this:

   if (n == 1)
     printf ("%d file deleted", n);
   else
     printf ("%d files deleted", n);

But this does not solve the problem. It helps languages where the plural form of a noun is not simply constructed by adding an ā€˜sā€™ but that is all. Once again people fell into the trap of believing the rules their language is using are universal. But the handling of plural forms differs widely between the language families. For example, Rafal Maszkowski <rzm@mat.uni.torun.pl> reports:

In Polish we use e.g. plik (file) this way:

1 plik
2,3,4 pliki
5-21 pliko'w
22-24 pliki
25-31 pliko'w

and so on (o’ means 8859-2 oacute which should be rather okreska, similar to aogonek).

There are two things which can differ between languages (and even inside language families);

The consequence of this is that application writers should not try to solve the problem in their code. This would be localization since it is only usable for certain, hardcoded language environments. Instead the extended gettext interface should be used.

These extra functions are taking instead of the one key string two strings and a numerical argument. The idea behind this is that using the numerical argument and the first string as a key, the implementation can select using rules specified by the translator the right plural form. The two string arguments then will be used to provide a return value in case no message catalog is found (similar to the normal gettext behavior). In this case the rules for Germanic language is used and it is assumed that the first string argument is the singular form, the second the plural form.

This has the consequence that programs without language catalogs can display the correct strings only if the program itself is written using a Germanic language. This is a limitation but since the GNU C library (as well as the GNU gettext package) are written as part of the GNU package and the coding standards for the GNU project require program being written in English, this solution nevertheless fulfills its purpose.

Function: char * ngettext (const char *msgid1, const char *msgid2, unsigned long int n)

The ngettext function is similar to the gettext function as it finds the message catalogs in the same way. But it takes two extra arguments. The msgid1 parameter must contain the singular form of the string to be converted. It is also used as the key for the search in the catalog. The msgid2 parameter is the plural form. The parameter n is used to determine the plural form. If no message catalog is found msgid1 is returned if n == 1, otherwise msgid2.

An example for the use of this function is:

printf (ngettext ("%d file removed", "%d files removed", n), n);

Please note that the numeric value n has to be passed to the printf function as well. It is not sufficient to pass it only to ngettext.

In the English singular case, the number – always 1 – can be replaced with "one":

printf (ngettext ("One file removed", "%d files removed", n), n);

This works because the ‘printf’ function discards excess arguments that are not consumed by the format string.

If this function is meant to yield a format string that takes two or more arguments, you can not use it like this:

printf (ngettext ("%d file removed from directory %s",
                  "%d files removed from directory %s",
                  n),
        n, dir);

because in many languages the translators want to replace the ‘%d’ with an explicit word in the singular case, just like “one” in English, and C format strings cannot consume the second argument but skip the first argument. Instead, you have to reorder the arguments so that ‘n’ comes last:

printf (ngettext ("%$2d file removed from directory %$1s",
                  "%$2d files removed from directory %$1s",
                  n),
        dir, n);

See c-format for details about this argument reordering syntax.

When you know that the value of n is within a given range, you can specify it as a comment directed to the xgettext tool. This information may help translators to use more adequate translations. Like this:

if (days > 7 && days < 14)
  /* xgettext: range: 1..6 */
  printf (ngettext ("one week and one day", "one week and %d days",
                    days - 7),
          days - 7);

It is also possible to use this function when the strings don’t contain a cardinal number:

puts (ngettext ("Delete the selected file?",
                "Delete the selected files?",
                n));

In this case the number n is only used to choose the plural form.

Function: char * dngettext (const char *domain, const char *msgid1, const char *msgid2, unsigned long int n)

The dngettext is similar to the dgettext function in the way the message catalog is selected. The difference is that it takes two extra parameter to provide the correct plural form. These two parameters are handled in the same way ngettext handles them.

Function: char * dcngettext (const char *domain, const char *msgid1, const char *msgid2, unsigned long int n, int category)

The dcngettext is similar to the dcgettext function in the way the message catalog is selected. The difference is that it takes two extra parameter to provide the correct plural form. These two parameters are handled in the same way ngettext handles them.

Now, how do these functions solve the problem of the plural forms? Without the input of linguists (which was not available) it was not possible to determine whether there are only a few different forms in which plural forms are formed or whether the number can increase with every new supported language.

Therefore the solution implemented is to allow the translator to specify the rules of how to select the plural form. Since the formula varies with every language this is the only viable solution except for hardcoding the information in the code (which still would require the possibility of extensions to not prevent the use of new languages).

The information about the plural form selection has to be stored in the header entry of the PO file (the one with the empty msgid string). The plural form information looks like this:

Plural-Forms: nplurals=2; plural=n == 1 ? 0 : 1;

The nplurals value must be a decimal number which specifies how many different plural forms exist for this language. The string following plural is an expression which is using the C language syntax. Exceptions are that no negative numbers are allowed, numbers must be decimal, and the only variable allowed is n. Spaces are allowed in the expression, but backslash-newlines are not; in the examples below the backslash-newlines are present for formatting purposes only. This expression will be evaluated whenever one of the functions ngettext, dngettext, or dcngettext is called. The numeric value passed to these functions is then substituted for all uses of the variable n in the expression. The resulting value then must be greater or equal to zero and smaller than the value given as the value of nplurals.

The following rules are known at this point. The language with families are listed. But this does not necessarily mean the information can be generalized for the whole family (as can be easily seen in the table below).5

Only one form:

Some languages only require one single form. There is no distinction between the singular and plural form. An appropriate header entry would look like this:

Plural-Forms: nplurals=1; plural=0;

Languages with this property include:

Asian family

Japanese, Vietnamese, Korean

Two forms, singular used for one only

This is the form used in most existing programs since it is what English is using. A header entry would look like this:

Plural-Forms: nplurals=2; plural=n != 1;

(Note: this uses the feature of C expressions that boolean expressions have to value zero or one.)

Languages with this property include:

Germanic family

English, German, Dutch, Swedish, Danish, Norwegian, Faroese

Romanic family

Spanish, Portuguese, Italian, Bulgarian

Latin/Greek family

Greek

Finno-Ugric family

Finnish, Estonian

Semitic family

Hebrew

Artificial

Esperanto

Other languages using the same header entry are:

Finno-Ugric family

Hungarian

Turkic/Altaic family

Turkish

Hungarian does not appear to have a plural if you look at sentences involving cardinal numbers. For example, “1 apple” is “1 alma”, and “123 apples” is “123 alma”. But when the number is not explicit, the distinction between singular and plural exists: “the apple” is “az alma”, and “the apples” is “az almák”. Since ngettext has to support both types of sentences, it is classified here, under “two forms”.

The same holds for Turkish: “1 apple” is “1 elma”, and “123 apples” is “123 elma”. But when the number is omitted, the distinction between singular and plural exists: “the apple” is “elma”, and “the apples” is “elmalar”.

Two forms, singular used for zero and one

Exceptional case in the language family. The header entry would be:

Plural-Forms: nplurals=2; plural=n>1;

Languages with this property include:

Romanic family

Brazilian Portuguese, French

Three forms, special case for zero

The header entry would be:

Plural-Forms: nplurals=3; plural=n%10==1 && n%100!=11 ? 0 : n != 0 ? 1 : 2;

Languages with this property include:

Baltic family

Latvian

Three forms, special cases for one and two

The header entry would be:

Plural-Forms: nplurals=3; plural=n==1 ? 0 : n==2 ? 1 : 2;

Languages with this property include:

Celtic

Gaeilge (Irish)

Three forms, special case for numbers ending in 00 or [2-9][0-9]

The header entry would be:

Plural-Forms: nplurals=3; \
    plural=n==1 ? 0 : (n==0 || (n%100 > 0 && n%100 < 20)) ? 1 : 2;

Languages with this property include:

Romanic family

Romanian

Three forms, special case for numbers ending in 1[2-9]

The header entry would look like this:

Plural-Forms: nplurals=3; \
    plural=n%10==1 && n%100!=11 ? 0 : \
           n%10>=2 && (n%100<10 || n%100>=20) ? 1 : 2;

Languages with this property include:

Baltic family

Lithuanian

Three forms, special cases for numbers ending in 1 and 2, 3, 4, except those ending in 1[1-4]

The header entry would look like this:

Plural-Forms: nplurals=3; \
    plural=n%10==1 && n%100!=11 ? 0 : \
           n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2;

Languages with this property include:

Slavic family

Russian, Ukrainian, Belarusian, Serbian, Croatian

Three forms, special cases for 1 and 2, 3, 4

The header entry would look like this:

Plural-Forms: nplurals=3; \
    plural=(n==1) ? 0 : (n>=2 && n<=4) ? 1 : 2;

Languages with this property include:

Slavic family

Czech, Slovak

Three forms, special case for one and some numbers ending in 2, 3, or 4

The header entry would look like this:

Plural-Forms: nplurals=3; \
    plural=n==1 ? 0 : \
           n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2;

Languages with this property include:

Slavic family

Polish

Four forms, special case for one and all numbers ending in 02, 03, or 04

The header entry would look like this:

Plural-Forms: nplurals=4; \
    plural=n%100==1 ? 0 : n%100==2 ? 1 : n%100==3 || n%100==4 ? 2 : 3;

Languages with this property include:

Slavic family

Slovenian

You might now ask, ngettext handles only numbers n of type ‘unsigned long’. What about larger integer types? What about negative numbers? What about floating-point numbers?

About larger integer types, such as ‘uintmax_t’ or ‘unsigned long long’: they can be handled by reducing the value to a range that fits in an ‘unsigned long’. Simply casting the value to ‘unsigned long’ would not do the right thing, since it would treat ULONG_MAX + 1 like zero, ULONG_MAX + 2 like singular, and the like. Here you can exploit the fact that all mentioned plural form formulas eventually become periodic, with a period that is a divisor of 100 (or 1000 or 1000000). So, when you reduce a large value to another one in the range [1000000, 1999999] that ends in the same 6 decimal digits, you can assume that it will lead to the same plural form selection. This code does this:

#include <inttypes.h>
uintmax_t nbytes = ...;
printf (ngettext ("The file has %"PRIuMAX" byte.",
                  "The file has %"PRIuMAX" bytes.",
                  (nbytes > ULONG_MAX
                   ? (nbytes % 1000000) + 1000000
                   : nbytes)),
        nbytes);

Negative and floating-point values usually represent physical entities for which singular and plural don’t clearly apply. In such cases, there is no need to use ngettext; a simple gettext call with a form suitable for all values will do. For example:

printf (gettext ("Time elapsed: %.3f seconds"),
        num_milliseconds * 0.001);

Even if num_milliseconds happens to be a multiple of 1000, the output

Time elapsed: 1.000 seconds

is acceptable in English, and similarly for other languages.

The translators’ perspective regarding plural forms is explained in Translating plural forms.


Footnotes

(5)

Additions are welcome. Send appropriate information to bug-gnu-gettext@gnu.org and bug-glibc-manual@gnu.org.


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