如何将一个数除3而不使用*、/、+、-、%等运算符?
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如何将一个数除3而不使用*、/、+、-、%等运算符?
号码可以有签名,也可以没有签名。
当前回答
使用黑客的喜悦魔术数字计算器
int divideByThree(int num)
{
return (fma(num, 1431655766, 0) >> 32);
}
其中fma是在math.h头文件中定义的标准库函数。
其他回答
这是Python中的,基本上,字符串比较和一个状态机。
def divide_by_3(input):
to_do = {}
enque_index = 0
zero_to_9 = (0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
leave_over = 0
for left_over in (0, 1, 2):
for digit in zero_to_9:
# left_over, digit => enque, leave_over
to_do[(left_over, digit)] = (zero_to_9[enque_index], leave_over)
if leave_over == 0:
leave_over = 1
elif leave_over == 1:
leave_over = 2
elif leave_over == 2 and enque_index != 9:
leave_over = 0
enque_index = (1, 2, 3, 4, 5, 6, 7, 8, 9)[enque_index]
answer_q = []
left_over = 0
digits = list(str(input))
if digits[0] == "-":
answer_q.append("-")
digits = digits[1:]
for digit in digits:
enque, left_over = to_do[(left_over, int(digit))]
if enque or len(answer_q):
answer_q.append(enque)
answer = 0
if len(answer_q):
answer = int("".join([str(a) for a in answer_q]))
return answer
使用fma()库函数的解决方案,适用于任何正数:
#include <stdio.h>
#include <math.h>
int main()
{
int number = 8;//Any +ve no.
int temp = 3, result = 0;
while(temp <= number){
temp = fma(temp, 1, 3); //fma(a, b, c) is a library function and returns (a*b) + c.
result = fma(result, 1, 1);
}
printf("\n\n%d divided by 3 = %d\n", number, result);
}
请看我的另一个答案。
这个方法(c#)怎么样?
private int dividedBy3(int n) {
List<Object> a = new Object[n].ToList();
List<Object> b = new List<object>();
while (a.Count > 2) {
a.RemoveRange(0, 3);
b.Add(new Object());
}
return b.Count;
}
使用计数器是一个基本的解决方案:
int DivBy3(int num) {
int result = 0;
int counter = 0;
while (1) {
if (num == counter) //Modulus 0
return result;
counter = abs(~counter); //++counter
if (num == counter) //Modulus 1
return result;
counter = abs(~counter); //++counter
if (num == counter) //Modulus 2
return result;
counter = abs(~counter); //++counter
result = abs(~result); //++result
}
}
也很容易执行一个模数函数,查看注释。
使用cblas,作为OS X加速框架的一部分。
[02:31:59] [william@relativity ~]$ cat div3.c
#import <stdio.h>
#import <Accelerate/Accelerate.h>
int main() {
float multiplicand = 123456.0;
float multiplier = 0.333333;
printf("%f * %f == ", multiplicand, multiplier);
cblas_sscal(1, multiplier, &multiplicand, 1);
printf("%f\n", multiplicand);
}
[02:32:07] [william@relativity ~]$ clang div3.c -framework Accelerate -o div3 && ./div3
123456.000000 * 0.333333 == 41151.957031