solver.Prod inside solver.Sum??

[OG]

Hi!

I am starting to use or-tools within the Python environment. I've been able to solve some problems but now I'm stuck trying to express one of the constraints. I want to multiply element by element two 2D-square arrays and then sum up all the items. One of the arrays contains already calculated values and the other one contains decision variables. I was trying to use something like solver.sum(Solver.Prod(...)) but it is not working. This is the expression I've coded with no success:

solver.Add(solver.Sum([solver.Prod(M[i,j],D[i][j]) for i in range(n) for j in range(n)]) == d)

M is the array containing the decision variables (0 or 1 values), D the array with the numers, n is the length of the arrays and d is another decision variable.

Any ideas on how to express this constraint?

Thank you in advance. Regards,

OG

[hakank]

What I understand of your problem, you can use "*" instead of Prod(), i.e.

       solver.Add(solver.Sum([M[(i,j)]*D[i][j] for i in range(n)

                           for j in range(n)]) == d)

/hakank

[OG]

Thanks for your answer Hakank. I've tried your option and I get the following error:

TypeError: unsupported operand type(s) for *: 'IntVar' and 'float'

Is there a way to multiply integers with float variables?

[hakank]

If you are using the CP interface (using "from constraint_solver import pywrapcp" or something like that) there is - what I know of - no support to  mix decision variables defined as IntVar() with float values. Perhaps you are using the MIP interface? Then try to define M[(i,j)] using NumVar() instead.

/hakank

[OG]

Yes, I am using the CP interface, so I guess I'll need to reformulate my program. Thank you for your help Hakank.

[Laurent Perron]

Hi

A lot of the API is moved to the IntVar/IntVar[] classes

    Intvar[] hire = solver.MakeIntVarArray(num_workers, 0, 1, "workers");

    IntVar total_cost = hire.ScalProd(cost).Var();

To see what is available, please have a look at the Helper.cs files in this directory:

https://github.com/google/or-tools/tree/master/src/com/google/ortools/constraintsolver

I am sorry we do not have a better documentation.

Thanks

Laurent Perron | Operations Research | lpe...@google.com | (33) 1 42 68 53 00

2017-03-15 10:42 GMT-04:00 Darknight85 <pooya....@gmail.com>:

I am trying to do the exact same thing with the C# interface, but I couldn't find a solver.Sum() method. I have a vector of variables and a vector of coefficients that need to be multiplied and then summed. Any thoughts?

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[Pooya Rezaei]

I was looking at the LinearSolver, but I saw that there are some Helper.cs files for that as well. 

After looking at the examples, I found out how to use LINQ with the or-tools, which is pretty powerful! Just for the record, this can also be written as:

var total_cost = Enmerable.Range(0, num_workers).Select(i => cost[i]*hire[i]).ToArray().Sum();

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