Exponential method in dafny: invariant might not be maintained -


i started learning dafny , learned invariants. i've got code:

function pot(m:int, n:nat): int {   if n==0 1   else if n==1 m   else if m==0 0   else pot(m,n-1) * m }  method pot(m:int, n:nat) returns (x:int) ensures x == pot(m,n) {   x:=1;   var i:=0;   if n==0 {x:=1;}   while i<=n   invariant i<=n;   {     x:=m*x;     i:=i+1;   } }

and given error following: "this loop invariant might not maintained loop." think might need invariant, think code correct other (i guess). appreciated. in advance.

a loop invariant must hold whenever loop branch condition evaluated. on last iteration of loop, i n+1, loop invariant not true then.

changing loop invariant i <= n + 1 or changing loop branch condition i < n fix particular problem.

after that, still have work finish proving method correct. feel free ask further questions if stuck.


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