How to lock variables

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Timur Kayhan

July 17, 2022 10:27

Hi all,

for an algorithm I need to fixate a few variables.

For instance, x_ij should be fixated to x_34 and then afterwards the problem will be solved with the gurobi solver.

How do I do that?

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4 comments

Official comment
Simranjit Kaur

Gurobi Staff

November 16, 2025 23:32
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Jonasz Staszek

Community Moderator

July 18, 2022 06:14 Edited
Hi Timur,

what do you exactly mean by "fixate"? Do you need to assign a constant value to this variable? Or perhaps you want to keep only \(x_{3,4}\) out of all \(x_{i, j} \) for some \(i \in I, j \in J \) and remove the others?

Please let us know - we will then gladly help you further.

Best regards
Jonasz
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Timur Kayhan

July 18, 2022 11:36
Hi Jonasz,

lets assume x_ij is a binary variable.

I want to keep only x_34 = 1 out of all x_ij for all i and j.

Best regards,

Timur
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Jonasz Staszek

Community Moderator

July 18, 2022 13:44
I would then either add a constraint or modify the bounds of the respective variable:
```
model.addLConstr(x[3,4] == 1)
# or alternatively
x[3,4].LB = 1
x[3,4].UB = 1
```
Do you even need the variables pertaining to \( x_{i,j} \) s.t. \( i \neq 3, j \neq 4\) in such a situation? If not, I would abstain from generating them in the first place.

Best regards
Jonasz
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