Add constraint using model.addConstr() using variables in Python

[David Lanier]

Hello,

I am new to using Gurobi and Python. I am working on a course project implementing an IP branch and bound algorithm using Python. As i am creating branching nodes from the parent node, I want to copy the parent model and then add constraints. I want to pass the lhs, rhs, and name using addConstr ( lhs, sense, rhs=None, name="" ) as variables created in my Python script. This method doesn't seem to accept those types of arguments so I try to use the LinExpr() method and it doesnt seem to work for my needs either. See below for a snippet of code and the message I get:

    #Create branching nodes

    mleft = m

    mright = m

    

    # Create new constraints for each node based on the non-integer decision variable value and re-solve

    mleftLHS = LinExpr('x' + str(i + 1))

    mleftRHS = LinExpr(DecisionVariableValues[i])

    mleftConstraintID = LinExpr('c' + str(ConstraintCount))

    print mleftLHS

    print mleftRHS

    print mleftConstraintID

    mleft.addConstr(mleftLHS, GRB.LESS_EQUAL, mleftRHS, mleftConstraintID)

Error:

  File "C:/Users/Dave Lanier/Documents/IE 7521/Project 1/HW1v2.py", line 58, in <module>

    mleftLHS = LinExpr('x' + str(2))

  File "linexpr.pxi", line 43, in gurobipy.LinExpr.__init__ (../../src/python/gurobipy.c:27292)

 IndexError: string index out of range

Any suggestions on how to overcome this or a better option for creating the new constraints? Full script is attached

[Greg Glockner]

Check the constructor for the LinExpr object - it does not use a string.