Adding new variables to existing model

Answered

June 20, 2022 14:24

Dear all,

I am working on the following model:

m = Model('Test')
first_set = list(range(3))
second_set = list(range(3))
x = m.addVars(first_set, second_set, vtype=GRB.CONTINUOUS) #9
y = m.addVars(second_set, vtype=GRB.BINARY) #3

m.addConstrs(x[i,j] <= y[j] for i in first_set for j in second_set)
m.addConstrs(quicksum(x[i,j] for j in second_set) >= 1 for i in first_set)

m.setObjective(quicksum(y[k] for k in second_set), GRB.MINIMIZE)
m.optimize()

However, if I 'simply' want to add some variables, the model becomes infeasible:

first_set.append(3)
for j in second_set:
    x[3,j] = m.addVar(3,j, vtype=GRB.CONTINUOUS)

m.update()
m.optimize()
for v in m.getVars():
    print(v.VarName, v.X)

Why can I observe this behaviour and how can I circumvent it? Thanks in advance!

Best wishes,
Jonas

Comments

8 comments

Official comment
Simranjit Kaur

Gurobi Staff

November 16, 2025 23:34
This post is more than three 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.
Marika Karbstein

Gurobi Staff

June 20, 2022 14:54
Hi Jonas,

The lines
```
for j in second_set:
    x[3,j] = m.addVar(3,j, vtype=GRB.CONTINUOUS)
```
add variables with lower bound 3 and upper bound j=0/1/2 which is infeasible.

In general, the article How do I determine why my model is infeasible? might help.

Best regards,
Marika
0
Marika Karbstein

Gurobi Staff

June 20, 2022 15:02 Edited
You probably wanted to do something like
```
x[3] = m.addVars(second_set, vtype=GRB.CONTINUOUS)
```
which adds multiple variables without defining specific bounds?
0
Jonas Grundler

June 21, 2022 08:27
Hi Marika,

thanks, this helps!
However, the new variables are not forced to fulfill the following constraint:
```
m.addConstrs(quicksum(x[i,j] for j in second_set) >= 1 for i in first_set)
```
How can I change this? Thanks!

Best wishes,
Jonas
0
Marika Karbstein

Gurobi Staff

June 21, 2022 08:42
In your example, it was not clear that "3" is an index of first_set and that you need a variable for each pair of first_set, second_set.

Did you try
```
x = m.addVars(first_set, second_set, vtype=GRB.CONTINUOUS)
```
?

Then your constraints should work.
0
Jonas Grundler

June 22, 2022 07:03
Yes, I tried, but unfortunately it does not work. Do I have to use the 'column' attribute here? Or is there a more convenient way?
0
Marika Karbstein

Gurobi Staff

June 22, 2022 07:40 Edited
Ok, I see.
Yes, you need to use the column argument like in the feasopt.py example (see also Modify a model) and add the variables individually with addVar().
0
Jonas Grundler

June 22, 2022 08:02
Okay, I see. Thanks a lot!
0

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Adding new variables to existing model

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