c - 不使用加法运算符将两个数字相加

Question

In c ~ is 1's complement operator. This is equivalent to: ~a = -b + 1 So, a - ~b -1 = a-(-b + 1) + 1 = a + b – 1 + 1 = a + b

谁能给我解释一下？

Answer 1

从小学数学我们知道

a = -(-a);

从二进制补码我们知道

-a = (~a) + 1  (invert and add one)

所以我们知道

a + b 
= a - (-b)      elementary math
= a - (~b + 1)  twos complement
= a - (~b) - 1   distribute the negative (elementary math)