问题试图使用C qsort函数
#include <stdio.h>
#include <stdlib.h>
float values[] = { 4, 1, 10, 9, 2, 5, -1, -9, -2,10000,-0.05,-3,-1.1 };
int compare (const void * a, const void * b)
{
return ( (int) (*(float*)a - *(float*)b) );
}
int main ()
{
int i;
qsort (values, 13, sizeof(float), compare);
for (i = 0; i < 13; i++)
{
printf ("%f ",values[ i ]);
}
putchar('\n');
return 0;
}
的结果是:
-9.000000 -3.000000 -2.000000 -1.000000 -1.100000 -0.050000 1.000000 2.000000 4.000000 5.000000 9.000000 10.000000 10000.000000
-9.000000 -3.000000 -2.000000 -1.000000 -1.100000 -0.050000 1.000000 2.000000 4.000000 5.000000 9.000000 10.000000 10000.000000
由于-1和-1.1的顺序改变这是错误的。
我相信,它正在发生,因为我的比较功能。
It's wrong because the order of -1 and -1.1 is changed. I believe it is happening because my "compare" function.
我该如何解决这个问题?
How can I fix this?
感谢
您比较函数被打破了。它说,例如, -1.0
等于(相当于)为 -1.1
,因为(INT)((-1.0) - (-1.1))
是零。换句话说,你自己告诉的qsort
的相对顺序 -1.0
和 -1.1
无所谓。你为什么感到惊讶,在最终订购这些值进行排序?
Your comparison function is broken. It says, for example, that -1.0
is equal (equivalent) to -1.1
, since (int) ((-1.0) - (-1.1))
is zero. In other words, you yourself told qsort
that the relative order of -1.0
and -1.1
does not matter. Why are you surprised that in the resultant ordering these values are not sorted?
在一般情况下,你应该避免减去从另一个比较数值。这是行不通的。对于浮点类型它可能会产生了好几个不同的原因,其中之一的你只看到自己的IM precise结果。对于整数类型可能溢出。
In general, you should avoid comparing numerical values by subtracting one from another. It just doesn't work. For floating-point types it might produce imprecise results for quite a few different reasons, one of which you just observed yourself. For integer types it might overflow.
通用成语比较两个数值 A
和 B
为的qsort
看起来(A&GT; b) - (A&LT; b)
。记住它并使用它。你的情况,这将是
The generic idiom for comparing two numerical values a
and b
for qsort
looks as (a > b) - (a < b)
. Remember it and use it. In your case that would be
int compare (const void * a, const void * b)
{
float fa = *(const float*) a;
float fb = *(const float*) b;
return (fa > fb) - (fa < fb);
}
在C code那么它可能使绝对意义上定义一个宏
In C code it might make perfect sense to define a macro
#define COMPARE(a, b) (((a) > (b)) - ((a) < (b)))
和使用它来代替明确地阐明了比较。
and use it instead of spelling out the comparisons explicitly.