How to add modulo constraints in gurobi

回答済み

Vaibhav Dixit

2024年09月08日 13:56

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Hi Gurobi Community,

I'm trying to add a constrains which represents that \sum( x[i] for i in lst) is odd.

I have approached it in two ways but in both ways I have found errors.

First way:

model.addConstr(gp.quicksum(x[i] for i in lst) %2  == 1)

 

Second way:

By defining new integer vars odd_value[.]

model.addConstr(gp.quicksum(x[i] for i in lst) == odd_value[cnt])
model.addConstr(odd_value[cnt] % 2 == 1)

In both cases I get ,

TypeError: unsupported operand parent(s) for %: '<class 'gurobipy.LinExpr'>' and 'Integer Ring'

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• 正式なコメント

  

  Simranjit Kaur

  • Gurobi Staff

  2026年11月13日 01:20

  This post is more than two 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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  Riley Clement

  • Gurobi Staff

  2024年09月08日 23:40

  Hi Vaihab,

  If I understand correctly, you are wanting to ensure that the following sum is odd.

  gp.quicksum(x[i] for i in lst)

  This sum is odd if, and only, if there is an even number 1 less than it.  If we introduce an integer variable named aux then 2*aux will be even.  Once this variable is added you can form the following constraint

  model.addVar(vtype="I", lb=-float("inf"))
  model.addConstr(gp.quicksum(x[i] for i in lst) == 1 + 2*aux)
  

  - Riley

   

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  Vaibhav Dixit

  2024年09月09日 04:33

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  Hi,

  Thanks, its working now. So basically to ensure  that "gp.quicksum(x[i] for i in lst)"  is odd , the aux variable takes its values ​​accordingly, there is no need to add more constraints corresponding to the aux variable.

  So in same way I can define many such aux variables if 'x'  is 2D array.

  x=model.addVars(len(lst),len(lst), vtype="I")
  aux=model.addVars(len(lst),vtype="I", lb=-float("inf"))

  for j in range(len(lst)):
      model.addConstr(gp.quicksum(x[i,j] for i in lst) == 1 + 2*aux[j])

  pls correct me, if I'm wrong.

  -Vaibhav

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  Riley Clement

  • Gurobi Staff

  2024年09月09日 04:39 編集済み

  Correct!

  Only thing I have to add is that if x are non-negative then the sum can never be negative so you don't need the lower bound of -infinity on the integer variables, and can let them take their default lower bound of 0.

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