Error reading LP format file - Neighboring tokens
ユーザーの入力を待っています。Hi there,
I've been trying and trying to run a LP file but I always receive the following expressions: - Error reading LP format file (path) at line 3. Malformed term in expression. Neighboring tokens: " ( 7.5 r_1 + 15 r_2 + 16.5 r_3 + ". Error 10012: Unable to read file -
Here is the first piece of the LP file where I get the error:
Minimize
30 r_1 + 45 r_2 + 40 r_3 + 30 r_4 + 10 r_5 + 30 p
Subject To
v: ( 7.5 r_1 + 15 r_2 + 16.5 r_3 + 2.5 r_4 + 5 r_5 ) * 360 = d
I've also tried changing that constraint with the following:
v: ( 7.5 r_1 + 15 r_2 + 16.5 r_3 + 2.5 r_4 + 5 r_5 ) * 360 / d = 1
But I always have the same error. How can I solve the issue?
P.S. I've just noticed that it's forbidden to divide by a variable in Gurobi, but I don't know how I can change that constraint
Thanks in advance for your support!
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Gurobi Staff
I really don't recommend building your model as an LP file. It's time-consuming, and you will run into a lot of formatting issues like this. The issues here are:
- The \(360\) should be distributed to each term inside the parentheses
- Variables like \(d\) should appear on the left-hand side of the constraint
Gurobi's Python API is straightforward to use and less error-prone. Here is a simple Python script that builds the same model:
import gurobipy as gp
from gurobipy import GRB
m = gp.Model()
r = m.addVars(5, name="r")
p = m.addVar(name="p")
d = m.addVar(name="d")
m.addConstr(
360 * (7.5 * r[0] + 15 * r[1] + 16.5 * r[2] + 2.5 * r[3] + 5 * r[4]) == d, name="v"
)
m.setObjective(
30 * r[0] + 45 * r[1] + 40 * r[2] + 30 * r[3] + 10 * r[4] + 30 * p, GRB.MINIMIZE
)
m.write("model.lp")
m.optimize()The LP file contains the following contents:
Minimize
30 r[0] + 45 r[1] + 40 r[2] + 30 r[3] + 10 r[4] + 30 p
Subject To
v: 2700 r[0] + 5400 r[1] + 5940 r[2] + 900 r[3] + 1800 r[4] - d = 0
Bounds
EndIf you're interested in trying out the Python interface, I recommend:
- The How do I install Gurobi for Python? article
- The Linear Programming Formulation with Gurobi Python API video
- Gurobi's functional code examples (mip1.py is a good start)
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Dear Eli,
thank you for your reply. Unfortunately I had already corrected those issues and built the model as a LP file, before reading your kind reply.
Anyway, Gurobi says: "Solution count 0", "Model is infeasible", without finding any optimal solution.
So does it mean that the model has no formatting issue? Even though I must correct the model itself on my paper sheet.
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Yes, that means Gurobi successfully read the LP file, but the model is infeasible.
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Okay.
By the way, these two webpages:
https://www.gurobi.com/documentation/10.0/refman/py_model_computeiis.html
They explain the way to detect the subset of constraints which render the model not feasible, by computing an Irreducible Inconsistent Subsystem (IIS).
However I don't understand where and how I can compute it and, moreover, would it be fine to compute it with the LP file?
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Are you solving the model with Gurobi's command-line tool (\(\texttt{gurobi_cl}\))? If so, you can set the ResultFile parameter to an ILP file. This will direct Gurobi to compute an IIS and write it to disk. For example:
gurobi_cl model.lp resultfile=model.ilp
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