C Pitfalls and Defects Chapter 7 Portability Defects 7.7 Truncation During Division

Truncation during division
q = a / b;
r = a % b;
Here, let's assume b is greater than 0.
We want a,b,q,rWhat kind of relationship do you maintain?
1. The most important point, we want q * b + r == a, because this is the relationship that defines the remainder.
2. If we change the sign of a, we hope this will change the sign of q, but this will not change the absolute value of q.
3. When b>0, we want to ensure that r>=0 and r But they cannot be at the same time.
However, the definition of C language only guarantees the first form, and when a>=0 and b>0, guarantees |r|<|b| and r>=0.
0 <= h< HASHSIZE, n is always non-negative, then we simply write as follows:
h = n % HASHSIZE;
However, if n may be negative, and h may be negative at this time, thenIt may not always be appropriate to do so.However, we know that h > -HASHSIZE,
so we can write:
h = n % HASHSIZE;
if (h < 0) {
h += HASHSIZE;
}
Better yet, programs should be designed to avoid situations where the value of n is negative and declare n to be unsignednumber.