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# [Frama-c-discuss] Newbie question

```For information, here is the max_seq example with sufficient annotations
to prove the code.

Notice:
- the use of \at(p,Pre) in the loop invariant, needed because p is
modified by the loop
(the previous invariants were *wrong*)
- the premisse 0 <= j < i instead of 0 <= j <= i in the quantified loop
invariant

Hope this helps,

- Claude

/*@ requires n > 0;
requires \valid(p+ (0..n-1));
assigns \nothing;
ensures \forall integer i; 0 <= i <= n-1 ==> \result >= p[i];
ensures \exists integer e; 0 <= e <= n-1 &&  \result == p[e];
*/
int max_seq(int* p, int n) {

int res = *p;
int i   = 0;

//@ ghost int e = 0;
/*@ loop invariant 0 <= i <= n;
@ loop invariant 0 <= e <= n-1;
@ loop invariant p == \at(p,Pre)+i;
@ loop invariant \forall integer j; 0 <= j < i ==> res >=
*(\at(p,Pre)+j);
@ loop invariant \valid(\at(p,Pre)+e) && \at(p,Pre)[e] == res;
@ loop variant n-i;
@*/
for(i = 0; i < n; i++) {
if (res < *p) {
res = *p;
//@ ghost e = i;
}
p++;
}
return res;
}

--
Claude March?                          | tel: +33 1 72 92 59 69
INRIA Saclay - ?le-de-France           | mobile: +33 6 33 14 57 93
Parc Orsay Universit?                  | fax: +33 1 74 85 42 29
4, rue Jacques Monod - B?timent N      | http://www.lri.fr/~marche/
F-91893 ORSAY Cedex                    |

```