result not correct

Answered

ghada ouerfelli

• February 02, 2022 14:56

from gurobipy import *
#input data
a=[5 ,6 ,4 ,6 ,7]#
k=['1','2','3'] # bins
C=20 # capacity of bin
#key parameters
N=range(len(a))
M=range(len(k))
#create a model
m=Model('Bin packing')
# create variables
x=m.addVars(N,M,lb=0.0,vtype=GRB.BINARY,name='x')
y=m.addVars(M,lb=0.0,vtype=GRB.BINARY,name='y')
#create constraints
m.addConstrs(x.sum(i,'*')==1 for i in N)
m.addConstrs(x.sum('*',k)*a[i]<=C*y[k] for i in N  for k in M)
#create objective function
obj_fn=quicksum(y[k] for k in M)
m.setObjective(obj_fn, GRB.MINIMIZE)
m.optimize()

the result is: Best objective 3.000000000000e+00, best bound 3.000000000000e+00, gap 0.0000%

and it should be 2 bins 

0

• 

  Jaromił Najman

  Gurobi Staff

  • February 03, 2022 08:24

  You packing constraint is incorrect. It should read

  m.addConstrs( quicksum(x[i,k]*a[i] for i in N) <= C*y[k]  for k in K)

  This should provide the correct answer.

  Best regards, 
  Jaromił

  0
• 

  ghada ouerfelli

  • February 03, 2022 10:43

  

  

  

  Thanks a lot Sir 

  0

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