How to declare dynamic variables inside FOR loop in OR-TOOLS

Question

I'm getting started with OR-TOOLS from Google and I couldn't figure out how to declare variables dynamically.

A very simple example in OR-TOOLS websites in given as follows

x = solver.NumVar(0, 10, 'x')
y = solver.NumVar(0, 10, 'y')
solver.Add(x + 7 * y <= 17.5)
solver.Maximize(x + 10 * y)`

Because I am modelling a Benders decomposition, I'm trying to declare some variables inside a loop. Thus, I could dynamically create its variables.

I've tried something like:

for i in range(3):
    x[i] = solver.NumVar(0, 10, 'x[i]')
    y[i] = solver.IntVar(0, 10, 'y[i]')
    solver.Add(x[i] + 7 * y[i] <= 17.5)

However, that clearly doesn't work.

Could someone help me please? Thanks!

Answer 1

This approach surely works and is probably shown in lots of examples.

Without testing, your idea could look like:

# prepare some data-structure to hold variables returned
x = [None] * 3
y = [None] * 3

for i in range(3):
    x[i] = solver.NumVar(0, 10, 'x[{}]'.format(i))
    y[i] = solver.IntVar(0, 10, 'y[{}]'.format(i))
    solver.Add(x[i] + 7 * y[i] <= 17.5)  # invariant is important!
                                         # we only access available indices

Nothing wrong with that conceptionally.

Your variables will now be ready to be used / consumed by access your lists.