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| <h3 class="section">5.6 Collation Functions</h3> |
| |
| <p><a name="index-collating-strings-533"></a><a name="index-string-collation-functions-534"></a> |
| In some locales, the conventions for lexicographic ordering differ from |
| the strict numeric ordering of character codes. For example, in Spanish |
| most glyphs with diacritical marks such as accents are not considered |
| distinct letters for the purposes of collation. On the other hand, the |
| two-character sequence ‘<samp><span class="samp">ll</span></samp>’ is treated as a single letter that is |
| collated immediately after ‘<samp><span class="samp">l</span></samp>’. |
| |
| <p>You can use the functions <code>strcoll</code> and <code>strxfrm</code> (declared in |
| the headers file <samp><span class="file">string.h</span></samp>) and <code>wcscoll</code> and <code>wcsxfrm</code> |
| (declared in the headers file <samp><span class="file">wchar</span></samp>) to compare strings using a |
| collation ordering appropriate for the current locale. The locale used |
| by these functions in particular can be specified by setting the locale |
| for the <code>LC_COLLATE</code> category; see <a href="Locales.html#Locales">Locales</a>. |
| <a name="index-string_002eh-535"></a><a name="index-wchar_002eh-536"></a> |
| In the standard C locale, the collation sequence for <code>strcoll</code> is |
| the same as that for <code>strcmp</code>. Similarly, <code>wcscoll</code> and |
| <code>wcscmp</code> are the same in this situation. |
| |
| <p>Effectively, the way these functions work is by applying a mapping to |
| transform the characters in a string to a byte sequence that represents |
| the string's position in the collating sequence of the current locale. |
| Comparing two such byte sequences in a simple fashion is equivalent to |
| comparing the strings with the locale's collating sequence. |
| |
| <p>The functions <code>strcoll</code> and <code>wcscoll</code> perform this translation |
| implicitly, in order to do one comparison. By contrast, <code>strxfrm</code> |
| and <code>wcsxfrm</code> perform the mapping explicitly. If you are making |
| multiple comparisons using the same string or set of strings, it is |
| likely to be more efficient to use <code>strxfrm</code> or <code>wcsxfrm</code> to |
| transform all the strings just once, and subsequently compare the |
| transformed strings with <code>strcmp</code> or <code>wcscmp</code>. |
| |
| <!-- string.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>strcoll</b> (<var>const char *s1, const char *s2</var>)<var><a name="index-strcoll-537"></a></var><br> |
| <blockquote><p>The <code>strcoll</code> function is similar to <code>strcmp</code> but uses the |
| collating sequence of the current locale for collation (the |
| <code>LC_COLLATE</code> locale). |
| </p></blockquote></div> |
| |
| <!-- wchar.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>wcscoll</b> (<var>const wchar_t *ws1, const wchar_t *ws2</var>)<var><a name="index-wcscoll-538"></a></var><br> |
| <blockquote><p>The <code>wcscoll</code> function is similar to <code>wcscmp</code> but uses the |
| collating sequence of the current locale for collation (the |
| <code>LC_COLLATE</code> locale). |
| </p></blockquote></div> |
| |
| <p>Here is an example of sorting an array of strings, using <code>strcoll</code> |
| to compare them. The actual sort algorithm is not written here; it |
| comes from <code>qsort</code> (see <a href="Array-Sort-Function.html#Array-Sort-Function">Array Sort Function</a>). The job of the |
| code shown here is to say how to compare the strings while sorting them. |
| (Later on in this section, we will show a way to do this more |
| efficiently using <code>strxfrm</code>.) |
| |
| <pre class="smallexample"> /* <span class="roman">This is the comparison function used with </span><code>qsort</code><span class="roman">.</span> */ |
| |
| int |
| compare_elements (char **p1, char **p2) |
| { |
| return strcoll (*p1, *p2); |
| } |
| |
| /* <span class="roman">This is the entry point---the function to sort</span> |
| <span class="roman">strings using the locale's collating sequence.</span> */ |
| |
| void |
| sort_strings (char **array, int nstrings) |
| { |
| /* <span class="roman">Sort </span><code>temp_array</code><span class="roman"> by comparing the strings.</span> */ |
| qsort (array, nstrings, |
| sizeof (char *), compare_elements); |
| } |
| </pre> |
| <p><a name="index-converting-string-to-collation-order-539"></a><!-- string.h --> |
| <!-- ISO --> |
| |
| <div class="defun"> |
| — Function: size_t <b>strxfrm</b> (<var>char *restrict to, const char *restrict from, size_t size</var>)<var><a name="index-strxfrm-540"></a></var><br> |
| <blockquote><p>The function <code>strxfrm</code> transforms the string <var>from</var> using the |
| collation transformation determined by the locale currently selected for |
| collation, and stores the transformed string in the array <var>to</var>. Up |
| to <var>size</var> characters (including a terminating null character) are |
| stored. |
| |
| <p>The behavior is undefined if the strings <var>to</var> and <var>from</var> |
| overlap; see <a href="Copying-and-Concatenation.html#Copying-and-Concatenation">Copying and Concatenation</a>. |
| |
| <p>The return value is the length of the entire transformed string. This |
| value is not affected by the value of <var>size</var>, but if it is greater |
| or equal than <var>size</var>, it means that the transformed string did not |
| entirely fit in the array <var>to</var>. In this case, only as much of the |
| string as actually fits was stored. To get the whole transformed |
| string, call <code>strxfrm</code> again with a bigger output array. |
| |
| <p>The transformed string may be longer than the original string, and it |
| may also be shorter. |
| |
| <p>If <var>size</var> is zero, no characters are stored in <var>to</var>. In this |
| case, <code>strxfrm</code> simply returns the number of characters that would |
| be the length of the transformed string. This is useful for determining |
| what size the allocated array should be. It does not matter what |
| <var>to</var> is if <var>size</var> is zero; <var>to</var> may even be a null pointer. |
| </p></blockquote></div> |
| |
| <!-- wchar.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: size_t <b>wcsxfrm</b> (<var>wchar_t *restrict wto, const wchar_t *wfrom, size_t size</var>)<var><a name="index-wcsxfrm-541"></a></var><br> |
| <blockquote><p>The function <code>wcsxfrm</code> transforms wide character string <var>wfrom</var> |
| using the collation transformation determined by the locale currently |
| selected for collation, and stores the transformed string in the array |
| <var>wto</var>. Up to <var>size</var> wide characters (including a terminating null |
| character) are stored. |
| |
| <p>The behavior is undefined if the strings <var>wto</var> and <var>wfrom</var> |
| overlap; see <a href="Copying-and-Concatenation.html#Copying-and-Concatenation">Copying and Concatenation</a>. |
| |
| <p>The return value is the length of the entire transformed wide character |
| string. This value is not affected by the value of <var>size</var>, but if |
| it is greater or equal than <var>size</var>, it means that the transformed |
| wide character string did not entirely fit in the array <var>wto</var>. In |
| this case, only as much of the wide character string as actually fits |
| was stored. To get the whole transformed wide character string, call |
| <code>wcsxfrm</code> again with a bigger output array. |
| |
| <p>The transformed wide character string may be longer than the original |
| wide character string, and it may also be shorter. |
| |
| <p>If <var>size</var> is zero, no characters are stored in <var>to</var>. In this |
| case, <code>wcsxfrm</code> simply returns the number of wide characters that |
| would be the length of the transformed wide character string. This is |
| useful for determining what size the allocated array should be (remember |
| to multiply with <code>sizeof (wchar_t)</code>). It does not matter what |
| <var>wto</var> is if <var>size</var> is zero; <var>wto</var> may even be a null pointer. |
| </p></blockquote></div> |
| |
| <p>Here is an example of how you can use <code>strxfrm</code> when |
| you plan to do many comparisons. It does the same thing as the previous |
| example, but much faster, because it has to transform each string only |
| once, no matter how many times it is compared with other strings. Even |
| the time needed to allocate and free storage is much less than the time |
| we save, when there are many strings. |
| |
| <pre class="smallexample"> struct sorter { char *input; char *transformed; }; |
| |
| /* <span class="roman">This is the comparison function used with </span><code>qsort</code> |
| <span class="roman">to sort an array of </span><code>struct sorter</code><span class="roman">.</span> */ |
| |
| int |
| compare_elements (struct sorter *p1, struct sorter *p2) |
| { |
| return strcmp (p1->transformed, p2->transformed); |
| } |
| |
| /* <span class="roman">This is the entry point---the function to sort</span> |
| <span class="roman">strings using the locale's collating sequence.</span> */ |
| |
| void |
| sort_strings_fast (char **array, int nstrings) |
| { |
| struct sorter temp_array[nstrings]; |
| int i; |
| |
| /* <span class="roman">Set up </span><code>temp_array</code><span class="roman">. Each element contains</span> |
| <span class="roman">one input string and its transformed string.</span> */ |
| for (i = 0; i < nstrings; i++) |
| { |
| size_t length = strlen (array[i]) * 2; |
| char *transformed; |
| size_t transformed_length; |
| |
| temp_array[i].input = array[i]; |
| |
| /* <span class="roman">First try a buffer perhaps big enough.</span> */ |
| transformed = (char *) xmalloc (length); |
| |
| /* <span class="roman">Transform </span><code>array[i]</code><span class="roman">.</span> */ |
| transformed_length = strxfrm (transformed, array[i], length); |
| |
| /* <span class="roman">If the buffer was not large enough, resize it</span> |
| <span class="roman">and try again.</span> */ |
| if (transformed_length >= length) |
| { |
| /* <span class="roman">Allocate the needed space. +1 for terminating</span> |
| <code>NUL</code><span class="roman"> character.</span> */ |
| transformed = (char *) xrealloc (transformed, |
| transformed_length + 1); |
| |
| /* <span class="roman">The return value is not interesting because we know</span> |
| <span class="roman">how long the transformed string is.</span> */ |
| (void) strxfrm (transformed, array[i], |
| transformed_length + 1); |
| } |
| |
| temp_array[i].transformed = transformed; |
| } |
| |
| /* <span class="roman">Sort </span><code>temp_array</code><span class="roman"> by comparing transformed strings.</span> */ |
| qsort (temp_array, sizeof (struct sorter), |
| nstrings, compare_elements); |
| |
| /* <span class="roman">Put the elements back in the permanent array</span> |
| <span class="roman">in their sorted order.</span> */ |
| for (i = 0; i < nstrings; i++) |
| array[i] = temp_array[i].input; |
| |
| /* <span class="roman">Free the strings we allocated.</span> */ |
| for (i = 0; i < nstrings; i++) |
| free (temp_array[i].transformed); |
| } |
| </pre> |
| <p>The interesting part of this code for the wide character version would |
| look like this: |
| |
| <pre class="smallexample"> void |
| sort_strings_fast (wchar_t **array, int nstrings) |
| { |
| ... |
| /* <span class="roman">Transform </span><code>array[i]</code><span class="roman">.</span> */ |
| transformed_length = wcsxfrm (transformed, array[i], length); |
| |
| /* <span class="roman">If the buffer was not large enough, resize it</span> |
| <span class="roman">and try again.</span> */ |
| if (transformed_length >= length) |
| { |
| /* <span class="roman">Allocate the needed space. +1 for terminating</span> |
| <code>NUL</code><span class="roman"> character.</span> */ |
| transformed = (wchar_t *) xrealloc (transformed, |
| (transformed_length + 1) |
| * sizeof (wchar_t)); |
| |
| /* <span class="roman">The return value is not interesting because we know</span> |
| <span class="roman">how long the transformed string is.</span> */ |
| (void) wcsxfrm (transformed, array[i], |
| transformed_length + 1); |
| } |
| ... |
| </pre> |
| <p class="noindent">Note the additional multiplication with <code>sizeof (wchar_t)</code> in the |
| <code>realloc</code> call. |
| |
| <p><strong>Compatibility Note:</strong> The string collation functions are a new |
| feature of ISO C90<!-- /@w -->. Older C dialects have no equivalent feature. |
| The wide character versions were introduced in Amendment 1<!-- /@w --> to ISO C90<!-- /@w -->. |
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