qsort语义

解决方案 2007年9月20日星期四00:09:12 -0700，gauss010写道： 假设我有一个char [M] [N]类型的对象A.每个A [i]是一个包含字符串的缓冲区 ，我想使用 strcmp函数对A的M个字符串进行排序。 qsort函数的描述说我必须传递指向要排序的数组的第一个对象的指针。 这导致我写下以下内容： char A [M] [N]; int cmp（const void * a，const void * b）{ int v = strcmp（*（char（*）[N]）a，*（char（*）[N]）b）; strcmp指向const字符，而不是指向数组。 如果strcmp在这里有一个可见的原型，你可以只是 int cmp（const void * a，const void * b）{return strcmp（a，b）; } K& R2使用（int（*）（const void *，const void *））strcmp作为qsort的 参数，但我认为标准委员会已将 视为UB（而const void * s必须与const char * s具有相同的 表示，这是特别不正常的 实现可以以不同的方式将它们传递给函数...） if（v< 0）return -1; else if（v == 0）返回0; 否则返回1; 为什么你不能回到v？ } void sortA（void）{ qsort（& A [0]，sizeof A / sizeof * A，sizeof * A，cmp）; } 但我也希望能够使用我的比较功能来排序其他 第二个数组维度大小不同的字符串数组。 例如，我可能想用它对 $的对象进行相同的排序b $ b类型char [2 * M] [2 * N]。然后我想像这样重写： char A [M] [N]，B [2 * M] [2 * N]; int cmp（const void * a，const void * b）{ int v = strcmp（a，b）; if（v< 0）return -1 ; 否则if（v == 0）返回0; 否则返回1; } void sortA（void）{ qsort（& A [0] [0]，sizeof A / sizeof * A，sizeof * A，cmp）; } void sortB（void）{ qsort（& B [0] [0]，sizeof B / sizeof * B，sizeof * B，cmp）; } 但是现在我并没有严格遵循qsort 函数的描述。我现在不是*真的*传递一个指向第一个对象的指针 要排序的数组，而不是我将指针传递给第一个 要排序的数组的第一个对象的字节。 你传递的是void *，它们实际上是指向char的指针，上面有 的句法盐。当然，它们指向第一个字节。 qsort（）知道它们的对象大小的唯一方法是b / b 指向的是，通过第二个字节和第三个 参数。 - Army1987（将NOSPAM替换为电子邮件） 如果你是从Windows机器发送电子邮件，请关闭微软的 傻瓜??智能行情？特征。这样你就可以避免通过你的邮件洒垃圾字符了。 - Eric S. Raymond和Rick Moen 也许使用： int compare（const void * aa，const void * bb） { const char * const * a = aa; const char * const * b = bb; 返回strcmp（a，b）; } 使用qsort比较C字符串。 On Thu，2007年9月20日12:02:28 -0700，user923005写道： 也许使用： int compare（const void * aa，const void * bb） { const char * const * a = aa; const char * const * b = bb; 返回strcmp（a，b）; } 来比较C字符串使用快速排序。 为什么？ strcmp需要两个const char * s ... - Army1987（将NOSPAM替换为电子邮件） 如果您要从Windows计算机发送电子邮件，请关闭Microsoft的 愚蠢的??智能行情？特征。这样你就可以避免通过你的邮件洒垃圾字符了。 - Eric S. Raymond和Rick Moen Suppose I have an object A of type char[M][N]. Each A[i] is a buffercontaining a string, and I want to sort the M strings of A using thestrcmp function. The description of the qsort function says that Imust pass a pointer to the first object of the array to be sorted.This leads me to write the following:char A[M][N];int cmp(const void *a, const void *b) {int v = strcmp(*(char (*)[N])a, *(char (*)[N])b);if (v < 0) return -1;else if (v == 0) return 0;else return 1;}void sortA(void) {qsort(&A[0], sizeof A / sizeof *A, sizeof *A, cmp);}But I also want to be able to use my comparison function to sort otherarrays of strings whose second array dimension is of a different size.For example, I may want to use it to do the same sort on an object oftype char[2*M][2*N]. Then I think of rewriting it like this:char A[M][N], B[2*M][2*N];int cmp(const void *a, const void *b) {int v = strcmp(a,b);if (v < 0) return -1;else if (v == 0) return 0;else return 1;}void sortA(void) {qsort(&A[0][0], sizeof A / sizeof *A, sizeof *A, cmp);}void sortB(void) {qsort(&B[0][0], sizeof B / sizeof *B, sizeof *B, cmp);}But now I''m not strictly following the description of the qsortfunction. I''m now not *really* passing a pointer to the first objectof the array to be sorted, rather I''m passing a pointer to the firstbyte of the first object of the array to be sorted.Imagining how I would implement the qsort function myself, I know thatthis will work fine, but abusing the description of the qsort functionlike this makes me uncomfortable.Do you think it''s OK to write such code? 解决方案 On Thu, 20 Sep 2007 00:09:12 -0700, gauss010 wrote:Suppose I have an object A of type char[M][N]. Each A[i] is a buffercontaining a string, and I want to sort the M strings of A using thestrcmp function. The description of the qsort function says that Imust pass a pointer to the first object of the array to be sorted.This leads me to write the following: char A[M][N]; int cmp(const void *a, const void *b) { int v = strcmp(*(char (*)[N])a, *(char (*)[N])b);strcmp takes pointers to const chars, not to arrays.If strcmp has a visible prototype here, you could justint cmp(const void *a, const void *b) { return strcmp(a, b); }K&R2 uses (int (* )(const void *, const void *))strcmp as anargument to qsort, but I think the standard committee has sincedeemed it UB (whereas const void *s are required to have the samerepresentations as const char*s, a particularly perverseimplementation could pass them to functions in different ways...) if (v < 0) return -1; else if (v == 0) return 0; else return 1;Why couldn''t you just return v? } void sortA(void) { qsort(&A[0], sizeof A / sizeof *A, sizeof *A, cmp); }But I also want to be able to use my comparison function to sort otherarrays of strings whose second array dimension is of a different size.For example, I may want to use it to do the same sort on an object oftype char[2*M][2*N]. Then I think of rewriting it like this: char A[M][N], B[2*M][2*N]; int cmp(const void *a, const void *b) { int v = strcmp(a,b); if (v < 0) return -1; else if (v == 0) return 0; else return 1; } void sortA(void) { qsort(&A[0][0], sizeof A / sizeof *A, sizeof *A, cmp); } void sortB(void) { qsort(&B[0][0], sizeof B / sizeof *B, sizeof *B, cmp); }But now I''m not strictly following the description of the qsortfunction. I''m now not *really* passing a pointer to the first objectof the array to be sorted, rather I''m passing a pointer to the firstbyte of the first object of the array to be sorted.You''re passing void *, which are essentially pointers to char withsyntactic salt on them. Of course they point to the first byte.The only way for qsort() to know how large the objects they areintended to point to are, is through the second and thirdargument.--Army1987 (Replace "NOSPAM" with "email")If you''re sending e-mail from a Windows machine, turn off Microsoft''sstupid a??Smart Quotesa?? feature. This is so you''ll avoid sprinkling garbagecharacters through your mail. -- Eric S. Raymond and Rick Moen Maybe use:int compare (const void *aa, const void *bb){const char * const *a = aa;const char * const *b = bb;return strcmp (a, b);}to compare C strings using qsort. On Thu, 20 Sep 2007 12:02:28 -0700, user923005 wrote:Maybe use:int compare (const void *aa, const void *bb){ const char * const *a = aa; const char * const *b = bb; return strcmp (a, b);}to compare C strings using qsort.Why?strcmp takes two const char *s...--Army1987 (Replace "NOSPAM" with "email")If you''re sending e-mail from a Windows machine, turn off Microsoft''sstupid a??Smart Quotesa?? feature. This is so you''ll avoid sprinkling garbagecharacters through your mail. -- Eric S. Raymond and Rick Moen 这篇关于qsort语义的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！ 上岸，阿里云！