Howto add an objective with a loop?

[ben-strasser]

Thanks for the nice library!

How do I add an objective with a loop? I currently have:

lp = LpProblem("tsp", LpMinimize)

objective = LpConstraint()
for i in range(k):
    for j in range(k):
        objective += d[i,j] * x[i,j]
lp.setObjective(objective)

It seems to work. lp.solve() does what I want. However, it seems wrong to use LpConstraint to create an objective. Also the objective function looks wrong when I dump the lp with print(lp). It states:

tsp:
MINIMIZE
2*x0,1 + 1*x0,2 + 5*x1,0 + 4*x1,2 + 4*x2,0 + 3*x2,1 + 0 = 0

which is clearly a constraint because of the =0 printed.

When I do

lp.setObjective(lpSum((d[i,j] * x[i,j] for i in range(k) for j in range(k) if i != j)))

The print changes to:

tsp:
MINIMIZE
2*x0,1 + 1*x0,2 + 5*x1,0 + 4*x1,2 + 4*x2,0 + 3*x2,1 + 0

So the two are definitely not the same.

What is the intended way to set such an objective function using a loop without lpSum?

The reason I do not want to use lpSum is because my logic is way more complex than in the toy example above and even in the toy example, IMO, the one-liner with lpSum is difficult to read.