qsort bsearch 函数用法

àì夳堔傛蜴生んèń 2022-08-10 15:59 204阅读 0赞

http://www.cnblogs.com/syxchina/archive/2010/07/29/2197382.html

http://baike.baidu.com/view/982231.htm?fr=aladdi

# bsearch 二分法 #

用 法: void \*bsearch(const void \*key, const void \*base, size\_t nelem, size\_t width, int(\*fcmp)(const void \*, const \*));

语法:

\#include < [stdlib.h][]>

void \*bsearch( const void \*key, const void \*buf, size\_t num, size\_t size, int (\*compare)(const void \*, const void \*) );

参数：第一个找的关键字。第二个：要查找的[数组][Link 1]。第三个：指定[数组][Link 1]中元素的数目。第四个：每个元素的长度（以[字符][Link 2]为单位）。第五个：指向比较函数的[指针][Link 3].：key指向所要查找的元素.要查

功能： 函数用折半查找法在从 [数组][Link 1]元素buf\[0\]到buf\[num-1\] 匹配参数key。如果函数compare 的第一个参数小于第二个参数，返回负值；如果等于返回零值；如果大于返回正值。 [数组][Link 1]buf 中的元素应以升序排列。函数bsearch()的返回值是指向匹配项，如果没有发现匹配项，返回NULL

qsort 　　功 能: 使用快速排序例程进行排序

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用 法: void qsort(void \*base, int nelem, int width, int (\*fcmp)(const void \*,const void \*));

使用qsort()排序并用 bsearch()搜索是一个比较常用的组合，使用方便快捷。

其中base是排序的一个集合 [数组][Link 1]，num是这个数组元素的个数，width是一个元素的大小，comp是一个比较函数。

比如：对一个长为1000的 [数组][Link 1]进行排序时，int a\[1000\]; 那么base应为a，num应为 1000，width应为 sizeof(int)，comp函数随自己的命名。

qsort(a,1000,sizeof(int),comp);

其中comp函数应写为：

上面是由小到大排序，return \*(int \*)b - \*(int \*)a; 为由大到小排序

对一个 [二维数组][Link 4]进行排序：

int a\[1000\]\[2\]; 其中按照a\[0\]的大小进行一个整体的排序，其中a\[1\]必须和a\[0\]一起移动交换。

//即第一行和第二行（a\[0\]和a\[1\]分别代表第一行和第二行的首地址）

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各参数：1 待排序数组首地址 2 数组中待排序元素数量 3 各元素的占用空间大小 4 指向函数的指针

用于确定排序的顺序 排序方法有很多种， 选择排序，冒泡排序，归并排序，快速排序等。 看名字都知道快速排序 是目前公认的一种比较好的排序算法（我没听书速度比这快的了，特殊场合例外），比选择排序，冒泡排序都要快。这是因为他速度很快，所以系统也在库里实现这个算法，便于我们的使用。 这就是qsort。

qsort 要求提供一个 比较函数，是为了做到通用性更好一点。比如你不仅仅的是要排序一个数字而已，可能你要用来排序几个数字 ，比如有一个结构 struct num \{ int a; int b; \}; 然后我有一个num 类型的数组， num dddd\[100\]; 我想给 dddd这个数组排序，那怎么办？ 我想让 a +b 最大的num元素排在数组的最前面，那又怎么办？ 这都可以通过定义比较函数来做到的。 比较函数的作用就是给qsort指明 元素的大小是怎么比较的。 像这样的比较函数 inline int MyCmp(const void\* a, const void\* b) 都是有两个元素 作为参数，返回一个int 值， 如果 比较函数返回大于0，qsort就认为 a>b , 如果比较函数返回等于0 qsort就认为a 和b 这两个元素相等，返回小于零 qsort就认为 ab),你比较函数却返回一个 -1 （小于零的）那么qsort认为a<本文中排序都是采用的从小到大排序>

一、对int类型数组排序

int num\[100\];

Sample: int cmp ( const void \*a , const void \*b )

\{ return \*(int \*)a - \*(int \*)b; \}

qsort(num,100,sizeof(num\[0\]),cmp);

上面是由小到大排序，return \*(int \*)b - \*(int \*)a; 为由大到小排序。

二、对char类型数组排序（同int类型）

char word\[100\];

Sample: int cmp( const void \*a , const void \*b )

\{ return \*(char \*)a - \*(int \*)b; \}

qsort(word,100,sizeof(word\[0\]),cmp);

三、对double类型数组排序（特别要注意）

double in\[100\];

int cmp( const void \*a , const void \*b )

\{ return \*(double \*)a > \*(double \*)b ? 1 : -1; \}

qsort(in,100,sizeof(in\[0\]),cmp)；

四、对结构体一级排序

struct In \{ double data; int other; \}s\[100\]

//按照data的值从小到大将结构体排序,关于结构体内的排序关键数据data的类型可以很多种，

//参考上面的例子写

int cmp( const void \*a ,const void \*b)

\{ return (\*(In \*)a).data > (\*(In \*)b).data ? 1 : -1; \}

qsort(s,100,sizeof(s\[0\]),cmp);

五、对结构体二级排序

struct In \{ int x; int y; \}s\[100\];

//按照x从小到大排序，当x相等时按照y从大到小排序

int cmp( const void \*a , const void \*b )

struct In \*c = (In \*)a; struct In \*d = (In \*)b;

if(c->x != d->x)

return c->x - d->x;

else

return d->y - c->y;

qsort(s,100,sizeof(s\[0\]),cmp);

六、对字符串进行排序

struct In \{ int data; char str\[100\]; \}s\[100\];

//按照结构体中字符串str的字典顺序排序

int cmp ( const void \*a , const void \*b )

return strcmp( (\*(In \*)a)->str , (\*(In \*)b)->str );

qsort(s,100,sizeof(s\[0\]),cmp);

七、计算几何中求凸包的cmp

int cmp(const void \*a,const void \*b)

//重点cmp函数，把除了1点外的所有点，旋转角度排序

struct point \*c=(point \*)a;

struct point \*d=(point \*)b;

if( calc(\*c,\*d,p\[1\]) < 0)

return 1;

else if( !calc(\*c,\*d,p\[1\]) && dis(c->x,c->y,p\[1\].x,p\[1\].y) < dis(d->x,d->y,p\[1\].x,p\[1\].y))

//如果在一条直线上，则把远的放在前面

return 1;

else return -1;

PS: 其中的qsort函数包含在的头文件里，strcmp包含在的头文件里

[stdlib.h]: http://baike.baidu.com/view/1347718.htm
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