问题描述
我想在数组上使用高阶 Count(P)
函数,例如:
Count(even,a)
或 Count(higher_than_10,a)
,其中第一个参数是谓词,第二个参数是数组。
也就是说,计算这个 P 命题在一个数组上发生了多少次。
Dafny 有没有办法做到这一点?我认为这种函数是存在的,但可能它们的语法发生了变化或其他什么。
谢谢
我已经看过了:
-https://stackoverflow.com/questions/35167124/dafny-and-counting-of-occurrences
-https://stackoverflow.com/questions/51379857/polymorphism-in-dafny
-https://gitter.im/dafny-lang/community?at=5d90c402086a72719e848f24
-https://www.imperial.ac.uk/events/104961/higher-order-functions-in-the-verification-aware-programming-language-dafny-k-rustan-m-leino/
解决方法
这是定义此类函数的一种方法。
function method CountHelper<T>(P: T -> bool,a: array<T>,i: int): int
requires 0 <= i <= a.Length
reads a
decreases a.Length - i
{
if i == a.Length
then 0
else (if P(a[i]) then 1 else 0) + CountHelper(P,a,i+1)
}
function method Count<T>(P: T -> bool,a: array<T>): int
reads a
{
CountHelper(P,0)
}
method Main()
{
var a := new int[10] (i => i);
var evens := Count(x => x % 2 == 0,a);
print evens,"\n";
var bigs := Count(x => x >= 5,a);
print bigs,"\n";
}
运行时,它按预期打印 5
两次。
$ dafny /compile:3 count.dfy
Dafny 3.0.0.20820
Dafny program verifier finished with 3 verified,0 errors
Running...
5
5