如何从 stdlib 为 qsort 编写比较函数? [英] How to write a compare function for qsort from stdlib?
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问题描述
我有一个结构:
struct pkt_
{
double x;
double y;
double alfa;
double r_kw;
};
typedef struct pkt_ pkt;
这些结构的表格:
pkt *tab_pkt;
tab_pkt = malloc(ilosc_pkt * sizeof(pkt));
我要做的是通过 tab_pkt.alfa
和 tab_pkt.r
对 tab_pkt
进行排序:
What I want to do is to sort tab_pkt
by tab_pkt.alfa
and tab_pkt.r
:
qsort(tab_pkt, ilosc_pkt, sizeof(pkt), porownaj);
porownaj 是一个比较函数,但是怎么写呢?这是我的草图":
Where porownaj is a compare function, but how to write it? Here is my "sketch" of it:
int porownaj(const void *pkt_a, const void *pkt_b)
{
if (pkt_a.alfa > pkt_b.alfa && pkt_a.r_kw > pkt_b.r_kw) return 1;
if (pkt_a.alfa == pkt_b.alfa && pkt_a.r_kw == pkt_b.r_kw) return 0;
if (pkt_a.alfa < pkt_b.alfa && pkt_a.r_kw < pkt_b.r_kw) return -1;
}
推荐答案
这样的东西应该可以工作:
Something like this should work:
int porownaj(const void *p_a, const void *p_b)
{
/* Need to store arguments in appropriate type before using */
const pkt *pkt_a = p_a;
const pkt *pkt_b = p_b;
/* Return 1 or -1 if alfa members are not equal */
if (pkt_a->alfa > pkt_b->alfa) return 1;
if (pkt_a->alfa < pkt_b->alfa) return -1;
/* If alfa members are equal return 1 or -1 if r_kw members not equal */
if (pkt_a->r_kw > pkt_b->r_kw) return 1;
if (pkt_a->r_kw < pkt_b->r_kw) return -1;
/* Return 0 if both members are equal in both structures */
return 0;
}
远离愚蠢的把戏,比如:
Stay away from silly tricks like:
return pkt_a->r_kw - pkt_b->r_kw;
它返回非规范化的值,难以阅读,无法正常处理浮点数,有时甚至会出现棘手的极端情况,即使是整数值也无法正常工作.
which return un-normalized values, are confusing to read, won't work properly for floating point numbers, and sometimes have tricky corner cases that don't work properly even for integer values.
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