Decision variables are the unknown information in a problem. Decision variables differ from normal C++ variables in that you can place constraints on the values of decision variables. For this reason, decision variables are also known as constrained variables. In this example, the decision variables are x and y.

Each decision variable has a domain of possible values. In this example, the domain of decision variable x is [5..12], or all integers from 5 to 12. The domain of decision variable y is [2..17], or all integers from 2 to 17.