问题
These are assertions for an algorithm to find the minimum of an array b[h.k]:
Precondition: h <= k < b.length
Postcondition: b[x] is the minimum of b[h...k]
Is this the correct loop for this invariant?
invariant: b[x] is the minimum of b[h...t]
int x = t; int t = h;
// {inv: b[x] is the minimum of b[h...t]}
while (t != k) {
t = t+1;
if (b[t] < b[x])
{ x = t;}
}
回答1:
You can find the minimum of an array this way (pseudocode):
// assume b.length > 0
min = b[0]
for i=1 to b.length
if b[i] < min
min = b[i]
To restrict it to b[h, ..., k]
:
min = b[h]
for i=h+1 to k
if b[i] < min
min = b[i]
So you basically just change the upper and lower bound of the loop
Since h<=k<b.length
, b[h]
is valid and executing the loop from the next element until k
iterates over the reqiured elements (if h==k
, the loop is empty)
UPDATE: as you are consistently failing with the implementation of the pseudocode into java, I'll translate it for you:
// assume: int b[]; int h; int k; h<=k<=b.length and b.length>0
// find min == b[i] such that b[i]<=b[j] for all h<=j<=k
int min = b[h];
for (int i=h+1; i<k; i=i+1) {
if (b[i] < min) {
min = b[i];
}
}
// here: min contains the (first) minimum element within b[h, ..., k]
Note: you could write i=i+1
as ++i
as well
来源:https://stackoverflow.com/questions/10202004/how-would-i-write-a-loop-for-this-invariant