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# [Frama-c-discuss] YASE lexicogrphical_compare

• Subject: [Frama-c-discuss] YASE lexicogrphical_compare
• From: Christoph.Weber at first.fraunhofer.de (Christoph Weber)
• Date: Thu Oct 23 08:25:10 2008

```Hello,
today I have something more complex.

I tried to specify a function lexicogrphical_compare. This function shall compare two arrays, and return true if the first array is smaller than the other.

First I need a conditional assignment. I did not find any other condition and even this one is not recognized.
/*@
logic integer min(int a, int b) =
(a < b) ? a : b;

*/

/*@
requires last > first;
requires \valid_range (first1, 0, last1-first1-1);
requires \valid_range (first2, 0, last2-first2-1);
behavior is_less:
assumes \result == true;
ensures \exists integer k2; \forall integer k1;
0 <= k1 <= k2 < min(last1-first1, last2-first2)
==> ((first1[k2] < first2[k2]) ^^ (last1-first1 < last2-first2)) && first1[k1] <= first2[k1];

behavior is_not_less:
assumes \result == false;
0 <= k1 <= k2 < min(last1-first1, last2-first2)
==> first1[k2] >= first2[k2] || first1[k1] >= first2[k1];

*/
bool lexicographical_compare_int_array (int* first1, int* last1, int* first2, int* last2 )
{
//@ ghost int* a = first1;
//@ ghost int* b = first2;
//@ ghost int length1 = last1-first1;
//@ ghost int length2 = last2-first2;

/*@
loop invariant a <= first1 <= last1;
loop invariant first2 - b == first1 - a;
loop invariant b <= first2 <= last2;

loop invariant \forall integer k; 0 <= k < first1-a ==> a[k] == b[k];
*/
for ( ; first1 != last1 && first2 != last2; ++first1, ++first2)
{
if (*first1 < *first2)
return true;
if (*first2 < *first1)
return false;
}
return first1 == last1 && first2 != last2;
}

The Question is, what has to be done, that Jessie understands what I want to describe? And is my loop invariant strong/specific enough.

Merci d'avance.

Christoph
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