Logarithm with parameterized variables on Python

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Iason Liagkas

2023年04月09日 11:28

Hello guys, I am trying to program a constraint of y=ln(x) but x,y have parameters. Could you help me with this code by an example? Do I need Python 10 for this? I am on 9.5 currently.
Say I want to write:
model.addGenConstrLog(x[a,b],y[c,d] for a in A for b in B for c in C for d in D), what is the correct code for this? how could I put parameterized variables in the logarithmic function?

Kind regards
Iason

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Eli Towle

Gurobi Staff

2023年04月10日 18:20
Given Var objects \(\texttt{x}\) and \(\texttt{y}\), you can add the constraint \( y = \textrm{ln}(x) \) using Model.addGenConstrLog() as follows:
```
x = model.addVar(name="x")
y = model.addVar(name="y")

model.addGenConstrLog(x, y)
```
To add multiple general constraints, you can iterate over your indices and add the constraints one by one. Here is another example that models the \(n\) constraints \( y_i = \textrm{ln}(x_i) \) for \(i = 0, \ldots, n-1\):
```
n = 5
x = model.addVars(n, name="x")
y = model.addVars(n, name="y")

for i in range(n):
    model.addGenConstrLog(x[i], y[i])
```
The two arguments to Model.addGenConstrLog() should both be Var objects. What exactly are you trying to model, and how do you define \(\texttt{x}\) and \(\texttt{y}\)?
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Iason Liagkas

2023年04月15日 09:04
No I know how to define x, and y. They are multi-parameter variables and from what you say I have to put the for outside of the logarithm function. That was something I was not aware of. You have answered my question Eli.

Kind regards
Iason
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