result not correct

Answered

ghada ouerfelli

February 02, 2022 14:56

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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 

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• Official comment

  

  Simranjit Kaur

  • Gurobi Staff

  November 16, 2025 23:44

  This post is more than three years old. Some information may not be up to date. For current information, please check the Gurobi Documentation or Knowledge Base. If you need more help, please create a new post in the community forum, or try Gurobot, our chatbot interface offering instant, expert-level support.
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  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

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  ghada ouerfelli

  February 03, 2022 10:43

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  Thanks a lot Sir 

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