Choco abs of an IntVar not working

Question

[I'm using Choco 3.3.3]

I have an IntVar a and an int b. I want to save the difference into an IntVar[] array d. I've done this at another point in the code in exactly the same way and it worked without a problem, but here I just don't get it to work.

d = VF.boundedArray("d", num_ts, -20, 20, solver);
for(int t=0; t < num_ts; t++){
    IntVar a = VF.bounded("a", 0, 10, solver);
    solver.post(ICF.sum(aa[t], a)); //values are [2,2,2,1,2,2]
    int b = bb[t][j]; //values are [2,3,2,2,2,2]
    IntVar c = VF.offset(a, -b);
    ...//see below
}

When I just use c (d[t] = c;), the result when printing out the values of d is [0, -1, 0, -1, 0, 0], but I need the absolut of that, so [0,1,0,1,0,0]

These are the different things that I've tried and the results of d:

d[t] = VF.abs(c); //result [0,0,0,0,0,0]
solver.post(ICF.times(c, -1, d[t])); //result [-7,-6,-7,-7,-7,-7]
solver.post(ICF.sum(new IntVar[]{VF.minus(a), VF.fixed(b, solver)}, d[t])); //result [-7,-6,-7,-7,-7,-7]
solver.post(ICF.arithm(a, "+", d[t], "=", b)); //result [-7,-6,-7,-7,-7,-7]
solver.post(ICF.distance(VF.fixed(b, solver), a, "=", d[t])); //result [-20, -20, -20, -20, -20, -20]

Can anyone tell me what I'm doing wrong? I am especially perplexed as to where [-7,-6,-7,-7,-7,-7] are coming from...

Answer 1

I am not sure I understood what you want to do and what you really tried but my guess is that you did not "solve" the model (or at least you did not propagate the constraints). As shown in the javadoc, variable.getValue() returns the LOWER BOUND in case the variable is not instantiated (it throws an exception when passing -ea to the JVM arguments). So add -ea, make sure you call solver.findSolution() before asking for the variable value and see if its working. If not, please send an executable code so that we can reproduce the problem. Did it solved your problem?

Jean-Guillaume, https://www.cosling.com/