how to add new coloumns into the left side of constraints?

Answered

Ariel W

• October 25, 2024 10:45

for example, i design original constraints like:

c1: a1*x1+a2*x2=1
c2: a3*x1+a4*x2=1
c3: x1+x2 ≤ 5

while new constraints will be

c1: a1*x1+a2*x2 + a5*x3 + a6*x4=1
c2: a3*x1+a4*x2+ a7*x3 + a8*x4=1
c3: x1+x2 + x3 + x4 ≤ 5

maybe add more xi

I WANT to extract the left side these constrainst and add new elements to it to generate new constrainsts, but Gurobi mentions that "no lhs of LinExpr" or no"Expr", how can i achive the purpose to add new columns to the model?

old_expr = cons[0].lhs

old_expr = cons[0].getAttr('Expr')

Related to

• Python
• Attribute

0

• 

  Jaromił Najman

  Gurobi Staff

  • October 25, 2024 13:40
  • Edited

  You can get the LHS of a linear constraint by calling the getRow() method, which will return a LinExpr object.

  However, what you need in your case is the Column object, which you fill with the respective coefficients and constraints via the addTerms method and then you can call the addVar method and pass this Column object to add the variable to the constraint of interest with pre-defined coefficients.

  In your particular example, this would be something like

  colx3 = gp.Column()
  colx4 = gp.Column()
  colx3.addTerms([a5, a7, 1.0], [c1, c2, c3])
  colx4.addTerms([a6, a8, 1.0], [c1, c2, c3])
  
  x3 = model.addVar(..., column = colx3)
  x4 = model.addVar(..., column = colx4)

  Please note that the Column object works only with linear constraints.

  Best regards, 
  Jaromił

   

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