Difference between (addVars & addVar) and (addConstrs & addConstr)
Answered
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The addVar method adds a single variable while the addVars method adds multiple variables at once. The addVars method is a convenience method, since you can get the exact same result by using a \(\texttt{for}\)-loop and the addVar method.
import gurobipy as gp
m = gp.Model()
x = m.addVars(3, name="x")
m.update()
print(x)generates 3 variables named \(\texttt{"x[0]","x[1]","x[2]"}\), which can be accessed via \(\texttt{x[0],x[1],x[2]}\).
The equivalent code using addVar would be
import gurobipy as gp
m = gp.Model()
x = {}
for i in range(3):
x[i] = m.addVar(name="x[%d]"%i)
m.update()
print(x)The same difference holds for addConstr and addConstrs. There should be no significant performance difference between the two respective methods.
Usually, adding sets of optimization variables through addVars results in a more concise code.
For constraints, it is recommended to use addConstr if the generated constraints are very complex, i.e., when many \(\texttt{for}\)-loops and \(\texttt{if}\)-clauses are needed to iterate through the indices. Otherwise, using addConstrs to add a set of constraints simultaneously results in a more concise code.
You could browse through the Python examples to get a feeling when to use which method. Our webinar on gurobipy should also bring some clarity.
Best regards,
Jaromił0 -
Thanks for a great explanation!
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Jaromił Najman already gave a details answer. Thanks.
I just wanted to say one little point (that I keep in mind when it comes to addConstrs vs addConstr)
If you see the following model, the 3rd constraint is a single constraint (use addConstr) whereas 2nd constraint equation will become len(A) numbers of constraints. When we enumerate Equation 2 using index of decision variable x_ij, we get multiple constraints; in such case, lets use addConstrs.
https://or.stackexchange.com/questions/1508/common-structures-in-gurobi-python
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