Does Different slack per node when using AddDimensionWithVehicleCapacity Exists

[youssef kaichouh]

I'm solving a CVRP with heterogeneous fleet using AddDimensionWithVehicleCapacity method, and i'm trying to set a slackVar range per index (to implement Reload Vrp)
routing.AddDimensionWithVehicleCapacity(
        demand_callback_index,
        0,  # null capacity slack
        data['vehicle_capacities'],  # vehicle maximum capacities
        True,  # start cumul to zero
        'Capacity')
    capacity_dimension = routing.GetDimensionOrDie(capacite)
for node in nodes:
            index = manager.NodeToIndex(node)
            capacity_dimension.SlackVar(index).SetRange(0, slack_of_node(node))
But the slack variable seems to stay always zero 
I'm missing something plz ?

[Laurent Perron]

You can always constrain the slack var manually. 

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[youssef kaichouh]

What do you mean please ? it isn't done by capacity_dimension.SlackVar(index).SetRange(0, slack_of_node(node)) ? 

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[youssef kaichouh]

I did replace capacity_dimension.SlackVar(index).SetRange(0, slack_of_node(node)) by
solver.Add(capacity_dimension.SlackVar(index) >= 0)
solver.Add(capacity_dimension.SlackVar(index) <= slack_of_node(node))
And the slack is always considered to be zero 
Any help please 

[Laurent Perron]

The null capacity slack is an upper bound of all slack variables. Setting the range will not extend the domain of the slack variables. 

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[youssef kaichouh]

I see, but how can i do please set the slack variable in (0, x) for some nodes with x depending on the node ? is it possible with a DimensionWithVehicleCapacity ?

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[Laurent Perron]

DimensionWithVehicleCapacity -> max cumulative transit on the path of each vehicle

For node n: 

index = manager.NodeToIndex(n)

slack_var = dimension.SlackVar(index)

slack_var.SetRange(0, max)

Laurent Perron | Operations Research | lpe...@google.com | (33) 1 42 68 53 00

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[youssef kaichouh]

This is exactly what i did by capacity_dimension.SlackVar(index).SetRange(0, slack_of_node(node)) and it's not working, the slack remains 0 

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[blind.line]

You cannot start with slack=0 and then increase it

You must start with non-zero slack and then decrease/increase as needed. 

See this issue, https://github.com/google/or-tools/issues/2264

If you choose to create the dimension with zero slack, you will trigger an optimization that fixes the slack to zero always. 

James

On Jun 5, 2022, at 14:10, youssef kaichouh <youssef.k...@gmail.com> wrote:



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[youssef kaichouh]

yes It did work this way, thank you!