Select some, but not all, variables in getVars()

[alel...@gmail.com]

Hi,

I am trying to select some of the variables returned by the solver, but I keep getting errors.

As shown below, the model returns 2 w variables (w_0 and w_1), 6 Z variables (Z_0,...,Z_5), and an intercept 'b'.

(In red, the values I would like to retrieve).

  

print(model.getVars()) 

 

Optimal objective 1.127659574e+01
[<gurobi.Var w_0 (value 0.19148936170212763)>, <gurobi.Var w_1 (value -0.148936170212766)>, <gurobi.Var Z_0 (value 5.787234042553191)>, <gurobi.Var Z_1 (value 0.0)>, <gurobi.Var Z_2 (value 0.0)>, <gurobi.Var Z_3 (value 2.723404255319149)>, <gurobi.Var Z_4 (value 0.0)>, <gurobi.Var Z_5 (value 2.76595744680851)>, <gurobi.Var b (value 3.4255319148936176)>]

 In order to retrieve the intercept, I type: 

b = b.X 

 print(b)

And I successfully get the value 3.4255319148936176

But I haven't managed to get the coefficients w[0] and w[1] ("w" being a list and n = 2).

I have tried: 

for i in range(n):
     w = [x for x in model.getVars() if x.VarName.find('w_%d' %i)]

 

for i in range(n):
     w = [v.x for v in m.getVars()] 

and other countless things, but none of them print 0.19148936170212763 and -0.148936170212766. The second one prints me the values of all the variables in getVars(), even if the range is only n=2.

Any hint on how to do it?

Thank you!

[Daniel Espinoza]

You can keep a reference to your variables when you create them, and then query directly on them the X attribute (rather than `finding them` by strings)

Best,

Daniel

[Alex Laval]

This is how they are created. In the example above, I created an example with n=2 for the w variables, but n could be as large as 100. In that case, I fear that querying the X attribute on each 100 references would be far less efficient. Isn't there any other way?

w = []

    for i in range(n):

        w.append(m.addVar(lb = -GRB.INFINITY, ub = GRB.INFINITY, name = 'w_{}'.format(i)))

    m.update()

    

    z = []

    for j in range (N):

        z.append(m.addVar(lb = -GRB.INFINITY, vtype=GRB.CONTINUOUS, name = 'Z_{}'.format(j), obj = 1))

    m.update()

Thanks! 

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[Alex Laval]

Nevermind! Got it!

It worked with this simple thing:

for i in range(n):

        w[i]=w[i].X

Thanks though!