I have my decision variable x which is indexed on the list N.

I want the constraint that x[i+1] <= x[i] or x[i] <= x[i-1]. But how do I do this in Pyomo without going out of range with my index?

model.x = Var(N, within=NonNegativeReals) def constraint1(model, n): return model.x[n+1] <= model.x[n] model.constraint1 = Constraint(N, rule=constraint1)

This thus doesn’t work. Anyone an idea how to do this?

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## Answer

You could use `Constraint.Skip`

to avoid accessing an invalid index (see e.g. here). For example:

def constraint1(model, n): if n == 0: return Constraint.Skip else: return model.x[n] <= model.x[n - 1]

I don’t know how your `N`

is defined, but this should give you an idea.