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| <h3 class="section">5.5 String/Array Comparison</h3> |
| |
| <p><a name="index-comparing-strings-and-arrays-515"></a><a name="index-string-comparison-functions-516"></a><a name="index-array-comparison-functions-517"></a><a name="index-predicates-on-strings-518"></a><a name="index-predicates-on-arrays-519"></a> |
| You can use the functions in this section to perform comparisons on the |
| contents of strings and arrays. As well as checking for equality, these |
| functions can also be used as the ordering functions for sorting |
| operations. See <a href="Searching-and-Sorting.html#Searching-and-Sorting">Searching and Sorting</a>, for an example of this. |
| |
| <p>Unlike most comparison operations in C, the string comparison functions |
| return a nonzero value if the strings are <em>not</em> equivalent rather |
| than if they are. The sign of the value indicates the relative ordering |
| of the first characters in the strings that are not equivalent: a |
| negative value indicates that the first string is “less” than the |
| second, while a positive value indicates that the first string is |
| “greater”. |
| |
| <p>The most common use of these functions is to check only for equality. |
| This is canonically done with an expression like ‘<samp><span class="samp">! strcmp (s1, s2)</span></samp>’<!-- /@w -->. |
| |
| <p>All of these functions are declared in the header file <samp><span class="file">string.h</span></samp>. |
| <a name="index-string_002eh-520"></a> |
| <!-- string.h --> |
| <!-- ISO --> |
| |
| <div class="defun"> |
| — Function: int <b>memcmp</b> (<var>const void *a1, const void *a2, size_t size</var>)<var><a name="index-memcmp-521"></a></var><br> |
| <blockquote><p>The function <code>memcmp</code> compares the <var>size</var> bytes of memory |
| beginning at <var>a1</var> against the <var>size</var> bytes of memory beginning |
| at <var>a2</var>. The value returned has the same sign as the difference |
| between the first differing pair of bytes (interpreted as <code>unsigned |
| char</code> objects, then promoted to <code>int</code>). |
| |
| <p>If the contents of the two blocks are equal, <code>memcmp</code> returns |
| <code>0</code>. |
| </p></blockquote></div> |
| |
| <!-- wcjar.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>wmemcmp</b> (<var>const wchar_t *a1, const wchar_t *a2, size_t size</var>)<var><a name="index-wmemcmp-522"></a></var><br> |
| <blockquote><p>The function <code>wmemcmp</code> compares the <var>size</var> wide characters |
| beginning at <var>a1</var> against the <var>size</var> wide characters beginning |
| at <var>a2</var>. The value returned is smaller than or larger than zero |
| depending on whether the first differing wide character is <var>a1</var> is |
| smaller or larger than the corresponding character in <var>a2</var>. |
| |
| <p>If the contents of the two blocks are equal, <code>wmemcmp</code> returns |
| <code>0</code>. |
| </p></blockquote></div> |
| |
| <p>On arbitrary arrays, the <code>memcmp</code> function is mostly useful for |
| testing equality. It usually isn't meaningful to do byte-wise ordering |
| comparisons on arrays of things other than bytes. For example, a |
| byte-wise comparison on the bytes that make up floating-point numbers |
| isn't likely to tell you anything about the relationship between the |
| values of the floating-point numbers. |
| |
| <p><code>wmemcmp</code> is really only useful to compare arrays of type |
| <code>wchar_t</code> since the function looks at <code>sizeof (wchar_t)</code> bytes |
| at a time and this number of bytes is system dependent. |
| |
| <p>You should also be careful about using <code>memcmp</code> to compare objects |
| that can contain “holes”, such as the padding inserted into structure |
| objects to enforce alignment requirements, extra space at the end of |
| unions, and extra characters at the ends of strings whose length is less |
| than their allocated size. The contents of these “holes” are |
| indeterminate and may cause strange behavior when performing byte-wise |
| comparisons. For more predictable results, perform an explicit |
| component-wise comparison. |
| |
| <p>For example, given a structure type definition like: |
| |
| <pre class="smallexample"> struct foo |
| { |
| unsigned char tag; |
| union |
| { |
| double f; |
| long i; |
| char *p; |
| } value; |
| }; |
| </pre> |
| <p class="noindent">you are better off writing a specialized comparison function to compare |
| <code>struct foo</code> objects instead of comparing them with <code>memcmp</code>. |
| |
| <!-- string.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>strcmp</b> (<var>const char *s1, const char *s2</var>)<var><a name="index-strcmp-523"></a></var><br> |
| <blockquote><p>The <code>strcmp</code> function compares the string <var>s1</var> against |
| <var>s2</var>, returning a value that has the same sign as the difference |
| between the first differing pair of characters (interpreted as |
| <code>unsigned char</code> objects, then promoted to <code>int</code>). |
| |
| <p>If the two strings are equal, <code>strcmp</code> returns <code>0</code>. |
| |
| <p>A consequence of the ordering used by <code>strcmp</code> is that if <var>s1</var> |
| is an initial substring of <var>s2</var>, then <var>s1</var> is considered to be |
| “less than” <var>s2</var>. |
| |
| <p><code>strcmp</code> does not take sorting conventions of the language the |
| strings are written in into account. To get that one has to use |
| <code>strcoll</code>. |
| </p></blockquote></div> |
| |
| <!-- wchar.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>wcscmp</b> (<var>const wchar_t *ws1, const wchar_t *ws2</var>)<var><a name="index-wcscmp-524"></a></var><br> |
| <blockquote> |
| <p>The <code>wcscmp</code> function compares the wide character string <var>ws1</var> |
| against <var>ws2</var>. The value returned is smaller than or larger than zero |
| depending on whether the first differing wide character is <var>ws1</var> is |
| smaller or larger than the corresponding character in <var>ws2</var>. |
| |
| <p>If the two strings are equal, <code>wcscmp</code> returns <code>0</code>. |
| |
| <p>A consequence of the ordering used by <code>wcscmp</code> is that if <var>ws1</var> |
| is an initial substring of <var>ws2</var>, then <var>ws1</var> is considered to be |
| “less than” <var>ws2</var>. |
| |
| <p><code>wcscmp</code> does not take sorting conventions of the language the |
| strings are written in into account. To get that one has to use |
| <code>wcscoll</code>. |
| </p></blockquote></div> |
| |
| <!-- string.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>strcasecmp</b> (<var>const char *s1, const char *s2</var>)<var><a name="index-strcasecmp-525"></a></var><br> |
| <blockquote><p>This function is like <code>strcmp</code>, except that differences in case are |
| ignored. How uppercase and lowercase characters are related is |
| determined by the currently selected locale. In the standard <code>"C"</code> |
| locale the characters Ä and ä do not match but in a locale which |
| regards these characters as parts of the alphabet they do match. |
| |
| <p class="noindent"><code>strcasecmp</code> is derived from BSD. |
| </p></blockquote></div> |
| |
| <!-- wchar.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>wcscasecmp</b> (<var>const wchar_t *ws1, const wchar_T *ws2</var>)<var><a name="index-wcscasecmp-526"></a></var><br> |
| <blockquote><p>This function is like <code>wcscmp</code>, except that differences in case are |
| ignored. How uppercase and lowercase characters are related is |
| determined by the currently selected locale. In the standard <code>"C"</code> |
| locale the characters Ä and ä do not match but in a locale which |
| regards these characters as parts of the alphabet they do match. |
| |
| <p class="noindent"><code>wcscasecmp</code> is a GNU extension. |
| </p></blockquote></div> |
| |
| <!-- string.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>strncmp</b> (<var>const char *s1, const char *s2, size_t size</var>)<var><a name="index-strncmp-527"></a></var><br> |
| <blockquote><p>This function is the similar to <code>strcmp</code>, except that no more than |
| <var>size</var> characters are compared. In other words, if the two |
| strings are the same in their first <var>size</var> characters, the |
| return value is zero. |
| </p></blockquote></div> |
| |
| <!-- wchar.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>wcsncmp</b> (<var>const wchar_t *ws1, const wchar_t *ws2, size_t size</var>)<var><a name="index-wcsncmp-528"></a></var><br> |
| <blockquote><p>This function is the similar to <code>wcscmp</code>, except that no more than |
| <var>size</var> wide characters are compared. In other words, if the two |
| strings are the same in their first <var>size</var> wide characters, the |
| return value is zero. |
| </p></blockquote></div> |
| |
| <!-- string.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>strncasecmp</b> (<var>const char *s1, const char *s2, size_t n</var>)<var><a name="index-strncasecmp-529"></a></var><br> |
| <blockquote><p>This function is like <code>strncmp</code>, except that differences in case |
| are ignored. Like <code>strcasecmp</code>, it is locale dependent how |
| uppercase and lowercase characters are related. |
| |
| <p class="noindent"><code>strncasecmp</code> is a GNU extension. |
| </p></blockquote></div> |
| |
| <!-- wchar.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>wcsncasecmp</b> (<var>const wchar_t *ws1, const wchar_t *s2, size_t n</var>)<var><a name="index-wcsncasecmp-530"></a></var><br> |
| <blockquote><p>This function is like <code>wcsncmp</code>, except that differences in case |
| are ignored. Like <code>wcscasecmp</code>, it is locale dependent how |
| uppercase and lowercase characters are related. |
| |
| <p class="noindent"><code>wcsncasecmp</code> is a GNU extension. |
| </p></blockquote></div> |
| |
| <p>Here are some examples showing the use of <code>strcmp</code> and |
| <code>strncmp</code> (equivalent examples can be constructed for the wide |
| character functions). These examples assume the use of the ASCII |
| character set. (If some other character set—say, EBCDIC—is used |
| instead, then the glyphs are associated with different numeric codes, |
| and the return values and ordering may differ.) |
| |
| <pre class="smallexample"> strcmp ("hello", "hello") |
| ⇒ 0 /* <span class="roman">These two strings are the same.</span> */ |
| strcmp ("hello", "Hello") |
| ⇒ 32 /* <span class="roman">Comparisons are case-sensitive.</span> */ |
| strcmp ("hello", "world") |
| ⇒ -15 /* <span class="roman">The character </span><code>'h'</code><span class="roman"> comes before </span><code>'w'</code><span class="roman">.</span> */ |
| strcmp ("hello", "hello, world") |
| ⇒ -44 /* <span class="roman">Comparing a null character against a comma.</span> */ |
| strncmp ("hello", "hello, world", 5) |
| ⇒ 0 /* <span class="roman">The initial 5 characters are the same.</span> */ |
| strncmp ("hello, world", "hello, stupid world!!!", 5) |
| ⇒ 0 /* <span class="roman">The initial 5 characters are the same.</span> */ |
| </pre> |
| <!-- string.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>strverscmp</b> (<var>const char *s1, const char *s2</var>)<var><a name="index-strverscmp-531"></a></var><br> |
| <blockquote><p>The <code>strverscmp</code> function compares the string <var>s1</var> against |
| <var>s2</var>, considering them as holding indices/version numbers. Return |
| value follows the same conventions as found in the <code>strverscmp</code> |
| function. In fact, if <var>s1</var> and <var>s2</var> contain no digits, |
| <code>strverscmp</code> behaves like <code>strcmp</code>. |
| |
| <p>Basically, we compare strings normally (character by character), until |
| we find a digit in each string - then we enter a special comparison |
| mode, where each sequence of digits is taken as a whole. If we reach the |
| end of these two parts without noticing a difference, we return to the |
| standard comparison mode. There are two types of numeric parts: |
| "integral" and "fractional" (those begin with a '0'). The types |
| of the numeric parts affect the way we sort them: |
| |
| <ul> |
| <li>integral/integral: we compare values as you would expect. |
| |
| <li>fractional/integral: the fractional part is less than the integral one. |
| Again, no surprise. |
| |
| <li>fractional/fractional: the things become a bit more complex. |
| If the common prefix contains only leading zeroes, the longest part is less |
| than the other one; else the comparison behaves normally. |
| </ul> |
| |
| <pre class="smallexample"> strverscmp ("no digit", "no digit") |
| ⇒ 0 /* <span class="roman">same behavior as strcmp.</span> */ |
| strverscmp ("item#99", "item#100") |
| ⇒ <0 /* <span class="roman">same prefix, but 99 < 100.</span> */ |
| strverscmp ("alpha1", "alpha001") |
| ⇒ >0 /* <span class="roman">fractional part inferior to integral one.</span> */ |
| strverscmp ("part1_f012", "part1_f01") |
| ⇒ >0 /* <span class="roman">two fractional parts.</span> */ |
| strverscmp ("foo.009", "foo.0") |
| ⇒ <0 /* <span class="roman">idem, but with leading zeroes only.</span> */ |
| </pre> |
| <p>This function is especially useful when dealing with filename sorting, |
| because filenames frequently hold indices/version numbers. |
| |
| <p><code>strverscmp</code> is a GNU extension. |
| </p></blockquote></div> |
| |
| <!-- string.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>bcmp</b> (<var>const void *a1, const void *a2, size_t size</var>)<var><a name="index-bcmp-532"></a></var><br> |
| <blockquote><p>This is an obsolete alias for <code>memcmp</code>, derived from BSD. |
| </p></blockquote></div> |
| |
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