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How do I access the left-hand side of a constraint?

Eli Towle
  • Updated

It is sometimes necessary to query a constraint's left-hand side (LHS). Use cases include:

You can access the Sense and right-hand side (RHS) of a linear constraint by querying the corresponding constraint attributes. The left-hand side or linear expression of the constraint can be accessed via:

Printing a constraint definition

The following Python code prints the definition (LHS, sense, and RHS) of a constraint:

import gurobipy as gp

model = gp.Model()
x = model.addVars(5, name="x")
con = model.addConstr(x.sum() <= 1, name="con1")

model.update()
print(f"{model.getRow(con)} {con.Sense} {con.RHS}")

The output:

x[0] + x[1] + x[2] + x[3] + x[4] < 1.0

Note that it is not sufficient to simply print the Constr object itself; this displays no information other than the name of the constraint.

Evaluating a constraint's LHS at the current solution

The following functions/properties return the value of a linear expression evaluated at the current solution:

Thus, in Python, the LHS of a constraint can be computed at the current solution as follows:

import gurobipy as gp

m = gp.read("glass4.mps")
m.optimize()

con = m.getConstrs()[0]
print(m.getRow(con).getValue())

Note that the LHS can only be evaluated at the current solution if Gurobi found at least one feasible solution when optimizing the model.

Changing coefficients on the constraint's LHS

After querying the constraint's left-hand side, you can iterate over the variables and coefficients contained in this linear expression. For example, the following Python script uses the LinExpr.getCoeff(), LinExpr.getVar(), and Model.chgCoeff() methods to increase the coefficients of all variables participating in the model's first constraint by 1:

import gurobipy as gp

m = gp.read("glass4.mps")

con = m.getConstrs()[0]
lexpr = m.getRow(con)
for i in range(lexpr.size()):
    coeff = lexpr.getCoeff(i)
    v = lexpr.getVar(i)
    m.chgCoeff(con, v, coeff + 1)

 

Further information

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