## 📅 Reducing Overscheduling

At a Nextmv tech talk a couple weeks ago, I showed a least absolute deviations (LAD) regression model using OR-Tools. This isn’t new – I pulled the formulation from Rob Vanderbei’s “Local Warming” paper, and I’ve shown similar models at conference talks in the past using other modeling APIs and solvers. There are a couple reasons I keep coming back to this problem. One is that it’s a great example of how to build a machine learning model using an optimization solver....

## 🖍 Visualizing Decision Diagrams

I attended DPSOLVE 2023 recently and found lots of good inspiration for the next version of Nextmv’s Decision Diagram (DD) solver, Hop. It’s a few years old now, and we learned a lot applying it in the field. Hop formed the basis for our first routing models. While those models moved to a different structure in our latest routing code, the first version broke ground combining DDs with Adaptive Large Neighborhood Search (ALNS), and its use continues to grow organically....

## 🚀 Blogging is back, baby!

I’ve been a mostly absent blogger for the past few years. I could make excuses. They might sound like, “I was busy finishing my dissertation!” or “I founded a company and have a toddler!” or “The static site generator I used was abandoned!” Whatever they might be, these excuses would certainly end in exclamation points. But, ultimately, for several years it just felt like blogging was dead. Its space was usurped by Tweets, LinkedIn hustle posts, long form Medium content aimed at attracting talent, and other content trends....

## 🔲 Magic Squares and Big-Ms

Note: This post was updated to work with Python 3 and PySCIPOpt. The original version used Python 2 and python-zibopt. It has also been edited for clarity. Back in October of 2011, I started toying with a model for finding magic squares using SCIP. This is a fun modeling exercise and a challenging problem. First one constructs a square matrix of integer-valued variables. from pyscipopt import Model # [...snip...] m = Model() matrix = [] for i in range(size): row = [m....

## ⏳️ Know Your Time Complexities - Part 2

In response to this post, Ben Bitdiddle inquires: I understand the concept of using a companion set to remove duplicates from a list while preserving the order of its elements. But what should I do if these elements are composed of smaller pieces? For instance, say I am generating combinations of numbers in which order is unimportant. How do I make a set recognize that [1,2,3] is the same as [3,2,1] in this case?...

## ⏳️ Know Your Time Complexities

This is based on a lightning talk I gave at the LA PyLadies October Hackathon. I’m actually not going to go into anything much resembling algorithmic complexity here. What I’d like to do is present a common performance anti-pattern that I see from novice programmers about once every year or so. If I can prevent one person from committing this error, this post will have achieved its goal. I’d also like to show how an intuitive understanding of time required by operations in relation to the size of data they operate on can be helpful....

## 🎰 Deterministic vs. Stochastic Simulation

I find I have to build simulations with increasing frequency in my work and life. Usually this indicates I’m faced with one of the following situations: The need for a quick estimate regarding the quantitative behavior of some situation. The desire to verify the result of a computation or assumption. A situation which is too complex or random to effectively model or understand. Anyone familiar at all with simulation will recognize the last item as the motivating force of the entire field....

## 🔮 NetworkX and Python Futures

Note: This post was updated to work with NetworkX and for clarity. It’s possible this will turn out like the day when Python 2.5 introduced [coroutines][coroutines]. At the time I was very excited. I spent several hours trying to convince my coworkers we should immediately abandon all our existing Java infrastructure and port it to finite state machines implemented using Python coroutines. After a day of hand waving over a proof of concept, we put that idea aside and went about our lives....

## 👉 Affine Scaling in R

I recently stumbled across an implementation of the affine scaling interior point method for solving linear programs that I’d coded up in R once upon a time. I’m posting it here in case anyone else finds it useful. There’s not a whole lot of thought given to efficiency or numerical stability, just a demonstration of the basic algorithm. Still, sometimes that’s exactly what one wants. solve.affine <- function(A, rc, x, tolerance=10^-7, R=0....

## 🐪 Reformed JAPHs: Transpiler

Note: This post was edited for clarity. For the final JAPH in this series, I implemented a simple transpiler that converts a small subset of Scheme programs to equivalent Python programs. It starts with a Scheme program that prints 'just another scheme hacker'. (define (output x) (if (null? x) "" (begin (display (car x)) (if (null? (cdr x)) (display "\n") (begin (display " ") (output (cdr x))))))) (output (list "just" "another" "scheme" "hacker")) The program then tokenizes that Scheme source, parses the token stream, and converts that into Python 3....