Mostafa
  • Member for 2 years, 7 months
  • Last seen this week
Counter-intuitive results in OR
14 votes

One nice similar result I have seen in the Open shop scheduling problem, where Three is easy, two is hard! In the paper Gribkovskaia et al. (2006), they write: "In discrete optimization the ...

View answer
How to model a mixed-integer linear programming formulation in Python using Gurobi?
Accepted answer
12 votes

Here is the complete implementation for the above-mentioned model. from gurobipy import * import numpy as np # Parameters needed are: # (1) the total number of jobs (n). Here I denote it by "...

View answer
CPLEX exceeds time limit issue
8 votes

That may be related to the "presolve" phase of the optimization procedure. In large instances, the time for "presolve" may be too large, that makes the total time larger than the specified time-limit. ...

View answer
Breakthroughs in Operations Research since 2010
7 votes

INFORMS recongnizes the best contributions in OR/MS with the Frederick W. Lanchester prize, that is: "The Lanchester prize is awarded for the best contribution to operations research and the ...

View answer
Difference between exploration and exploitation in Simulated Annealing algorithm
6 votes

Those two are also called Diversification (Exploration) and Intensification (Exploitation). In SA, Diversification relates to the larger values of the probability of accepting an inferior neighbor ...

View answer
How to compare two different formulations of a problem?
6 votes

I would like to add some criteria for the computational comparison, that I think is appropriate and common. As mentioned, the experiments should be performed on standard benchmarks, and if available, ...

View answer
How to model a mixed-integer linear programming formulation in Python using Gurobi?
6 votes

To complete the answer, I will add some other things that may be useful. To change the parameters of the solver gurobi, e.g. setting the time-limit to 600 seconds: m.setParam('TimeLimit', 600) To ...

View answer
Is it abnormal for a model to take 8+ hours to solve?
5 votes

It is highly dependent to the problem, and to the size of the instance you are solving. For example, in many scheduling problems I have dealt with, the optimal solution cannot be found within 8 hours ...

View answer
Counter-intuitive results in OR
5 votes

A very interesting paper entitled: Analysis of flow shop scheduling anomalies has been recently published by Panwalkar & Koulamas (2020)1, that introduces the following anomalies: 1- Type 1 ...

View answer
Scheduling problem data generation
5 votes

One type of data that is likely to exist in many real-world applications is the so-called "ordered" data. In an ordered setting, if the processing time of a job $j$ is larger than that of another job $...

View answer
Python does not identify the academic version of CPLEX
Accepted answer
5 votes

Thanks to all for the great suggestions. The thing that should be done is to install CPLEX setup.py in the site-packages of the python. But it is not enough. Actually, what worked was the suggestion ...

View answer
How can I find the optimal assignments for this MILP problem heuristically?
4 votes

It seems to me that this problem can be reduced to a variant of the generalized assignment problem (GAP). If you set $U_{\max} = 1, \forall u$, then your problem is indeed GAP with equal weights for ...

View answer
NP-hardness of a special case of multiple choice knapsack problem
Accepted answer
3 votes

I think the problem is NP-hard since: it will reduce to the 0-1 Knapsack problem with an equality constraint; and, changing $\leq$ to $=$ in the 0-1 Knapsack constraint does not change its ...

View answer
How is this problem called in literature?
3 votes

The setting where processing times are variables dependent on the allocated resources, rather than constants, is usually called scheduling with controllable processing times. A very good survey of ...

View answer
How to implement a local search with different operators?
3 votes

I suggest you take a look at Variable Neighborhood Search (VNS) method. The Wikipedia page has some useful pseudo codes. My answer to OP EDIT: First, I think there is no good or bad algorithm, it ...

View answer
How to linearize a weighted maximum coverage problem?
2 votes

I think there is no continuous form of the maximal covering location problem (MCLP) with a guarantee of providing the same optimal solution as the integer program. However, in addition to the ...

View answer
Formulate relationship between four binary variables
2 votes

Oh, I got the solution! I add the constraint $$y_h + y_{h'} = x_h + x_{h'} + 1 - z.$$ Now, I should enforce $z$ to be 1 if $y_h + y_{h'} = 0$, and 0 otherwise. For that I add: $$y_h + y_{h'} \leq M(...

View answer
Is this another variant of Job Shop Scheduling Problem?
2 votes

I can refer you to two papers that you may get some hints from. The first one is by Lee et al. (2012)1, proposing an MILP for the flexible job-shop problem with ‘AND’/‘OR’ precedence constraints, and ...

View answer