Help me with indexing

Specific Domains Optimization (Mathematical)
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@variable(model, x[1:2, 1:2]) makes a 2-by-2 matrix of variables. It looks like you want

@variable(model, P[1:size(branch, 1)])
P[3]  # Corresponds to (4, 5, 0.2)

or

S = [(branch[r, 1], branch[r, 2]) for r = 1:size(branch, 1)]
@variable(model, P[S])
P[(3, 4)]  # Corresponds to (3, 4, 0.1)