Originally posted by: JuanIgnacioAdame

I'm pretty new to OPL, and recently I've been trying to write a simple .mod that I can use to solve quadratic unconstrained binary optimization (QUBO) problems.  Here is the script:

int num_logical_variables = ...;

range Logical_variables = 1..num_logical_variables;

float Q[Logical_variables][Logical_variables] = ...;

dvar int x[Logical_variables] in 0..1;

dexpr float Energy = sum(j in Logical_variables, i in Logical_variables) (Q[j][i]*x[j]*x[i]);

maximize Energy;

// Post processing
tuple result {
  float energy;
}
result energy = <Energy>;

However, I am getting the error "Decision variable (or expression) "Energy" not allowed" and I am not sure why.  Could anyone help me with this?

Thanks!

Hi,

you seem to miss a subject to block

Can you replace

maximize Energy;

by

maximize Energy;

subject to
{

}

?

regards

Originally posted by: JuanIgnacioAdame

That indeed seems to have helped!  However, now I have a different error coming from my .dat file:

time_limit = 9223372036854775807;
num_logical_variables = 10;
Q = [[0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [1, 0, 0, 0, 0, 0, 0, 0, 0, 0], [1, 1, 0, 0, 0, 0, 0, 0, 0, 0], [1, 1, 1, 0, 0, 0, 0, 0, 0, 0], [1, 1, 1, 1, 0, 0, 0, 0, 0, 0], [1, 1, 1, 1, 1, 0, 0, 0, 0, 0], [1, 1, 1, 1, 1, 1, 0, 0, 0, 0], [1, 1, 1, 1, 1, 1, 1, 0, 0, 0], [1, 1, 1, 1, 1, 1, 1, 1, 0, 0], [1, 1, 1, 1, 1, 1, 1, 1, 1, 0]];

I get "Element "time_limit" not defined."

Thanks for your help!

Hi,

you simply need to add

int time_limit = ...;

in your .mod

regards

Originally posted by: JuanIgnacioAdame

Alright, things seems to be working pretty well now; thanks!

Hi

more useful links at

https://www.ibm.com/developerworks/community/forums/html/topic?id=0d0b2396-3b48-4638-b032-3b9ea74f1a11&ps=25

regards

Originally posted by: JuanIgnacioAdame

Thanks again, will definitely keep those links in mind for general troubleshooting in the future.

I actually have another question in the meantime.  I've managed to get the optimal value of the objective function printed, but I don't know how to print the actual solution itself.  So far I have:

tuple solution {
    int solution;
}

{solution} Solution = {<x[j]> | j in Logical_variables};

It should be a vector of size |Logical_variables|, but in fact it's only returning a single scalar.  Do you know what I could be doing wrong?

Thanks again.

Hi,

execute
{
writeln("Solution=",Solution);
writeln("x=",x);
}

will give

Solution= {<1>}
x= [1 1 1 1 1 1 1 1 1 1]

which looks normal.

regards

Originally posted by: JuanIgnacioAdame

Thanks again for these very quick and helpful responses!