Dafny 选择排序降序
Dafny selection sort descending
我有以下方法以降序执行选择排序。但是while循环中的very first invariant据说循环不维护,为什么会这样?
method sortDesc (a : array<int>)
modifies a;
requires a != null;
ensures forall m,n :: 0 <= m < n < a.Length ==> a[m] >= a[n];
requires a.Length > 1;
ensures multiset(a[..]) == multiset(old(a[..]));
{
var index := a.Length - 1;
while(index > -1)
invariant forall m,n :: 0 <= m <= index && index < n < a.Length ==> a[m] >= a[n]; // all elements that we have not sorted are bigger than any element we have sorted !!!!!!!!!!!!
invariant a.Length > index > -1;
invariant forall k,l :: index < k < l < a.Length ==> a[k] >= a[l]; //the part we have gone through is already sorted!!
invariant multiset(a[..]) == multiset(old(a[..])); // USED TO MAKE SURE ALL THE STARTING ELEMENTS ARE STILL THERE!!!!!
decreases index;
{
var minIndex := findMinBetween(a, index, a.Length);
if (a[index] < a[minIndex])
{
a[index], a[minIndex] := a[minIndex],a[index]; //SWAP ELEMENTS!!!
}
if (index == 0)
{
return;
}
index := index - 1;
}
}
也许比较 a[index] < a[minIndex]
是错误的方式。您正在尝试将小东西向右移动,因此如果 a[minindex]
小于 a[index]
您需要将它 (a[minindex]
) 向右移动。
我有以下方法以降序执行选择排序。但是while循环中的very first invariant据说循环不维护,为什么会这样?
method sortDesc (a : array<int>)
modifies a;
requires a != null;
ensures forall m,n :: 0 <= m < n < a.Length ==> a[m] >= a[n];
requires a.Length > 1;
ensures multiset(a[..]) == multiset(old(a[..]));
{
var index := a.Length - 1;
while(index > -1)
invariant forall m,n :: 0 <= m <= index && index < n < a.Length ==> a[m] >= a[n]; // all elements that we have not sorted are bigger than any element we have sorted !!!!!!!!!!!!
invariant a.Length > index > -1;
invariant forall k,l :: index < k < l < a.Length ==> a[k] >= a[l]; //the part we have gone through is already sorted!!
invariant multiset(a[..]) == multiset(old(a[..])); // USED TO MAKE SURE ALL THE STARTING ELEMENTS ARE STILL THERE!!!!!
decreases index;
{
var minIndex := findMinBetween(a, index, a.Length);
if (a[index] < a[minIndex])
{
a[index], a[minIndex] := a[minIndex],a[index]; //SWAP ELEMENTS!!!
}
if (index == 0)
{
return;
}
index := index - 1;
}
}
也许比较 a[index] < a[minIndex]
是错误的方式。您正在尝试将小东西向右移动,因此如果 a[minindex]
小于 a[index]
您需要将它 (a[minindex]
) 向右移动。