The C API for Collator performs locale-sensitive String comparison. You use this class to build searching and sorting routines for natural language text.

Like other locale-sensitive classes, you can use the function ucol_open(), to obtain the appropriate pointer to UCollator object for a given locale. If you need to understand the details of a particular collation strategy or if you need to modify that strategy.

The following example shows how to compare two strings using the UCollator for the default locale.

// Compare two strings in the default locale
UErrorCode success = U_ZERO_ERROR;
UCollator* myCollator = ucol_open(NULL, &success);
UChar source[4], target[4];
u_uastrcpy(source, "abc");
u_uastrcpy(target, "ABC");
if( u_strcoll(myCollator, source, u_strlen(source), target, u_strlen(target)) == UCOL_LESS) {
printf("abc is less than ABC\n");
}else{
printf("abc is greater than or equal to ABC\n");
}

You can set a Collator's strength property to determine the level of difference considered significant in comparisons. Four strengths are provided: UCOL_PRIMARY, UCOL_SECONDARY, UCOL_TERTIARY, and UCOL_IDENTICAL. The exact assignment of strengths to language features is locale dependant. For example, in Czech, "e" and "f" are considered primary differences, while "e" and "\u00EA" are secondary differences, "e" and "E" are tertiary differences and "e" and "e" are identical. The following shows how both case and accents could be ignored for US English.

//Get the Collator for US English and set its strength to UCOL_PRIMARY
UErrorCode success = U_ZERO_ERROR;
UCollator* usCollator = ucol_open("en_US", &success);
ucol_setStrength(usCollator, UCOL_PRIMARY);
UChar source[4], target[4];
u_uastrcpy(source, "abc");
u_uastrcpy(target, "ABC");
if( u_strcoll(myCollator, source, u_strlen(source), target, u_strlen(target)) == UCOL_EQUAL) {
printf("'abc' and 'ABC' strings are equivalent with strength UCOL_PRIMARY\n");
}

For comparing Strings exactly once, the u_strcoll method provides the best performance. When sorting a list of Strings however, it is generally necessary to compare each String multiple times. In this case, sortKeys provide better performance. The ucol_getsortKey method converts a String to a series of bits that can be compared bitwise against other sortKeys using memcmp()

Note: UCollators with different Locale, Collation Strength and Decomposition Mode settings will return different sort orders for the same set of strings. Locales have specific collation rules, and the way in which secondary and tertiary differences are taken into account, for example, will result in a different sorting order for same strings.