div()
标准C函数
我认为你应该查看来自<stdlib.h>
的div()
函数。(它们是标准C函数,并且在所有版本的标准中都有定义,尽管链接到了POSIX规范。)
C11标准§7.22.6.2指定:
div
…函数在单个操作中计算numer / denom
和numer % denom
。
请注意,C11在§6.5.5中指定整数除法(C99类似):
当整数相除时,/
运算符的结果是代数商,任何小数部分都会被舍弃。105)
105)这通常称为“向零取整”。
但是C90(§6.3.5)更加灵活但不太实用:
当整数相除且除法不精确时。如果两个操作数都是正数,则/
运算符的结果是小于代数商的最大整数,%
运算符的结果为正数。如果任何一个操作数为负,则/
运算符的结果是小于或等于代数商的最大整数或大于或等于代数商的最小整数,%
运算符的符号也是实现定义的。
floor_div()
使用div()
计算请求的floor_div()
的计算代码简洁整洁。
int floor_div(int a, int b)
{
assert(b != 0);
div_t r = div(a, b);
if (r.rem != 0 && ((a < 0) ^ (b < 0)))
r.quot--;
return r.quot;
}
测试代码
下面代码中的打印格式非常精确地针对了样本数据。(使用%4d
和%-4d
会更好,但更加费力)。此代码打印长度为89个字符加换行符的行;更一般的布局将打印长度为109的行。两者都无法避免SO上的水平滚动条。
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
static int floor_div(int a, int b)
{
assert(b != 0);
div_t r = div(a, b);
if (r.rem != 0 && ((a < 0) ^ (b < 0)))
r.quot--;
return r.quot;
}
static void test_floor_div(int n, int d)
{
assert(d != 0);
printf( "%3d/%-2d = %-3d (%3d)", +n, +d, floor_div(+n, +d), +n / +d);
printf("; %3d/%-3d = %-4d (%4d)", +n, -d, floor_div(+n, -d), +n / -d);
if (n != 0)
{
printf("; %4d/%-2d = %-4d (%4d)", -n, +d, floor_div(-n, +d), -n / +d);
printf("; %4d/%-3d = %-3d (%3d)", -n, -d, floor_div(-n, -d), -n / -d);
}
putchar('\n');
}
int main(void)
{
int numerators[] = { 0, 1, 2, 4, 9, 23, 291 };
enum { NUM_NUMERATORS = sizeof(numerators) / sizeof(numerators[0]) };
int denominators[] = { 1, 2, 3, 6, 17, 23 };
enum { NUM_DENOMINATORS = sizeof(denominators) / sizeof(denominators[0]) };
for (int i = 0; i < NUM_NUMERATORS; i++)
{
for (int j = 0; j < NUM_DENOMINATORS; j++)
test_floor_div(numerators[i], denominators[j]);
putchar('\n');
}
return 0;
}
测试输出
0/1 = 0 ( 0); 0/-1 = 0 ( 0)
0/2 = 0 ( 0); 0/-2 = 0 ( 0)
0/3 = 0 ( 0); 0/-3 = 0 ( 0)
0/6 = 0 ( 0); 0/-6 = 0 ( 0)
0/17 = 0 ( 0); 0/-17 = 0 ( 0)
0/23 = 0 ( 0); 0/-23 = 0 ( 0)
1/1 = 1 ( 1); 1/-1 = -1 ( -1); -1/1 = -1 ( -1); -1/-1 = 1 ( 1)
1/2 = 0 ( 0); 1/-2 = -1 ( 0); -1/2 = -1 ( 0); -1/-2 = 0 ( 0)
1/3 = 0 ( 0); 1/-3 = -1 ( 0); -1/3 = -1 ( 0); -1/-3 = 0 ( 0)
1/6 = 0 ( 0); 1/-6 = -1 ( 0); -1/6 = -1 ( 0); -1/-6 = 0 ( 0)
1/17 = 0 ( 0); 1/-17 = -1 ( 0); -1/17 = -1 ( 0); -1/-17 = 0 ( 0)
1/23 = 0 ( 0); 1/-23 = -1 ( 0); -1/23 = -1 ( 0); -1/-23 = 0 ( 0)
2/1 = 2 ( 2); 2/-1 = -2 ( -2); -2/1 = -2 ( -2); -2/-1 = 2 ( 2)
2/2 = 1 ( 1); 2/-2 = -1 ( -1); -2/2 = -1 ( -1); -2/-2 = 1 ( 1)
2/3 = 0 ( 0); 2/-3 = -1 ( 0); -2/3 = -1 ( 0); -2/-3 = 0 ( 0)
2/6 = 0 ( 0); 2/-6 = -1 ( 0); -2/6 = -1 ( 0); -2/-6 = 0 ( 0)
2/17 = 0 ( 0); 2/-17 = -1 ( 0); -2/17 = -1 ( 0); -2/-17 = 0 ( 0)
2/23 = 0 ( 0); 2/-23 = -1 ( 0); -2/23 = -1 ( 0); -2/-23 = 0 ( 0)
4/1 = 4 ( 4); 4/-1 = -4 ( -4); -4/1 = -4 ( -4); -4/-1 = 4 ( 4)
4/2 = 2 ( 2); 4/-2 = -2 ( -2); -4/2 = -2 ( -2); -4/-2 = 2 ( 2)
4/3 = 1 ( 1); 4/-3 = -2 ( -1); -4/3 = -2 ( -1); -4/-3 = 1 ( 1)
4/6 = 0 ( 0); 4/-6 = -1 ( 0); -4/6 = -1 ( 0); -4/-6 = 0 ( 0)
4/17 = 0 ( 0); 4/-17 = -1 ( 0); -4/17 = -1 ( 0); -4/-17 = 0 ( 0)
4/23 = 0 ( 0); 4/-23 = -1 ( 0); -4/23 = -1 ( 0); -4/-23 = 0 ( 0)
9/1 = 9 ( 9); 9/-1 = -9 ( -9); -9/1 = -9 ( -9); -9/-1 = 9 ( 9)
9/2 = 4 ( 4); 9/-2 = -5 ( -4); -9/2 = -5 ( -4); -9/-2 = 4 ( 4)
9/3 = 3 ( 3); 9/-3 = -3 ( -3); -9/3 = -3 ( -3); -9/-3 = 3 ( 3)
9/6 = 1 ( 1); 9/-6 = -2 ( -1); -9/6 = -2 ( -1); -9/-6 = 1 ( 1)
9/17 = 0 ( 0); 9/-17 = -1 ( 0); -9/17 = -1 ( 0); -9/-17 = 0 ( 0)
9/23 = 0 ( 0); 9/-23 = -1 ( 0); -9/23 = -1 ( 0); -9/-23 = 0 ( 0)
23/1 = 23 ( 23); 23/-1 = -23 ( -23); -23/1 = -23 ( -23); -23/-1 = 23 ( 23)
23/2 = 11 ( 11); 23/-2 = -12 ( -11); -23/2 = -12 ( -11); -23/-2 = 11 ( 11)
23/3 = 7 ( 7); 23/-3 = -8 ( -7); -23/3 = -8 ( -7); -23/-3 = 7 ( 7)
23/6 = 3 ( 3); 23/-6 = -4 ( -3); -23/6 = -4 ( -3); -23/-6 = 3 ( 3)
23/17 = 1 ( 1); 23/-17 = -2 ( -1); -23/17 = -2 ( -1); -23/-17 = 1 ( 1)
23/23 = 1 ( 1); 23/-23 = -1 ( -1); -23/23 = -1 ( -1); -23/-23 = 1 ( 1)
291/1 = 291 (291); 291/-1 = -291 (-291); -291/1 = -291 (-291); -291/-1 = 291 (291)
291/2 = 145 (145); 291/-2 = -146 (-145); -291/2 = -146 (-145); -291/-2 = 145 (145)
291/3 = 97 ( 97); 291/-3 = -97 ( -97); -291/3 = -97 ( -97); -291/-3 = 97 ( 97)
291/6 = 48 ( 48); 291/-6 = -49 ( -48); -291/6 = -49 ( -48); -291/-6 = 48 ( 48)
291/17 = 17 ( 17); 291/-17 = -18 ( -17); -291/17 = -18 ( -17); -291/-17 = 17 ( 17)
291/23 = 12 ( 12); 291/-23 = -13 ( -12); -291/23 = -13 ( -12); -291/-23 = 12 ( 12)