Solve string data
Answered
Hi, I am using gurobipy to solve models. I am new to gurobipy and could successfully run a sample code and read an lp file. However, I would like to use a database to pass the data to the solver and would like to know how I accomplish that. The read method does not accept a string and therefore, I am failing to do so. I would appreciate it if you could provide me with a link to the right document.
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Hi Mohamadreza,
Good practice when building a model is to separate the logical model from the data model.
We demonstrate this in our python examples where we build and solve the classic diet problem: diet.py
We then show how to separate the data and model into different python files: diet2.py and dietmodel.py
We also show how to populate the data using a SQLite database: diet3.py
Could you please review these and let us know if you need any clarification?
- Riley
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Thank you Riley for your response.
Unfortunately, the files linked did not provide me with the right solution. The sample I have is given below:
import gurobipy as gp
# Define the model
model = gp.Model()
# Define the problem data as a string
data_string = """ Maximize
3 x + 2 y
Subject To
Constraint1: 2 x + y <= 10
Constraint2: x + 3 y <= 12
Bounds
x >= 0
y >= 0
End
"""
# Pass the problem data string to the model using the read() method
model.read(datastring=data_string)
# Optimize the model
model.optimize()
# Print the optimal solution
print("Optimal solution:")
for v in model.getVars():
print(v.varName, v.x)
print("Objective value:", model.objVal)When the modelMaximize
3 x + 2 y
Subject To
Constraint1: 2 x + y <= 10
Constraint2: x + 3 y <= 12
Bounds
x >= 0
y >= 0
Endis read from a file such as model.lp, the code works perfectly and prints the results. However, in the code snippet I have provided you with, I want to do exactly the same; I have a string of my model which in a real application is selected from my database and then passed to be solved, etc. The format is as given. I would not like to use model methods, etc. but pass a string to the read method.I hope it clarifies my point.thanks0 -
Hi Mohamadreza,
The functionality you are after does not exist, there is only one way to read a problem in LP file format, and that is to read from a file.
However we can create a function that does what you want, by writing the string to a temporary file then using gurobipy to read the file, as an intermediate step:
import gurobipy as gp
from tempfile import NamedTemporaryFile
def read_from_string(string):
with NamedTemporaryFile(mode="w", suffix=".lp") as f:
f.write(string)
f.flush()
return gp.read(f.name)You can then use it like so:
prob = """
Maximize
.01 Pennies + .05 Nickels + .1 Dimes + .25 Quarters + 1 Dollars
Subject To
Copper: .06 Pennies + 3.8 Nickels + 2.1 Dimes + 5.2 Quarters + 7.2 Dollars -
Cu = 0
Nickel: 1.2 Nickels + .2 Dimes + .5 Quarters + .2 Dollars -
Ni = 0
Zinc: 2.4 Pennies + .5 Dollars - Zi = 0
Manganese: .3 Dollars - Mn = 0
Bounds
Cu <= 1000
Ni <= 50
Zi <= 50
Mn <= 50
Integers
Pennies Nickels Dimes Quarters Dollars
End
"""
m = read_from_string(prob)
m.optimize()- Riley
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Ah all right. Since I need to solve huge models, it will take a lot more time to first write and then read from a file. Therefore, I was wondering if there is another approach.
Thank you for your response.
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Hi Mohamadreza,
It might be worth investigating whether your database can store files. A compressed MPS file (with extension .mps.bz2) can be directly read by Gurobi, and would be much smaller than the equivalent string.
- Riley
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