Element constraint sets incorrect index value.

[xgillard]

Using JaCop 4.6.0 on Java.

The following code snippet showcases an example where jacop's element constraint yields an incorrect result. This issue might be related to #27, but I don't believe so. This is why I created this second issue for you to track.

Store cp = new Store();

IntVar   index = new IntVar(cp, -3, 3);
IntVar[]  vars = new IntVar[]{new IntVar(cp, 0, 0)};
IntVar   value = new IntVar(cp, -4, 2);

cp.setLevel(cp.level + 1);
cp.impose(Element.choose(index, vars, value));
if ( !cp.consistency() ) {
    System.err.println("Ahem, this is wrong !");
}

// This is what gets printed:
// index = _1 = 1
// vars  = [_2 = 0]
// value = _3 = 0
System.out.println("index = "+ index);
System.out.println("vars  = "+ Arrays.toString(vars));
System.out.println("value = "+ value);

// 1st assertion fails because index = {1}. Still vars[1] is not feasible : it is out of bounds
assert index.value()   == 0;
assert vars[0].value() == 0;
assert value.value()   == 0;

[radsz]

Thank you for making an effort to report a problem.

For element the first element in the list has index 1 not 0. ;). Therefore, it is correct.
If you prefer to have things indexed from zero then you need to add extra parameter to Element.
cp.impose(Element.choose(index, vars, value, -1));
This -1 changes starting index from 1 to 0. If I do then your test does not fail.

Again, thank you for making an effort to report a problem. JaCoP participates in solver competition and solves thousands of different problems so usually we have no simple issues like that. Therefore, if you find those type of simple problems most of the time is use of API in the wrong place or misunderstanding of the constraint contract.

I hope you will pass those initial difficulty in using CP/JaCoP and enjoying using our library. I recommend looking at many different examples we provide. It helps a lot at the beginning.