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Tag: Computing

Low-tech transparency
I recently received two large data files from a client, with names like foo.xlsx and foo.csv. Presumably these are redundant; the latter is probably an export of the former. I did a spot check and that seems to be the case. Then I had a bright idea: use pandas to …
Shells, quoting, and one-liners
The goal this post is to show how to call Perl and Python one-liners from a shell. We want to be able to use bash on Linux, and cmd or PowerShell on Windows, ideally with the same code in all three shells. Bash Bash interprets text inside double quotes but …
Redundant Residue Number Systems
You can uniquely represent a large number by its remainders when divided by smaller numbers, provided the smaller numbers have no factors in common, and carry out arithmetic in this representation. Such a representation is called a Residue Number System (RNS). In the 1960’s people realized RNSs could be useful …
How much can Boolean expressions be simplified?
In the previous post we looked at how to minimize Boolean expressions using a Python module qm. In this post we’d like to look at how much the minimization process shortens expressions. Witn n Boolean variables, you can create 2^n terms that are a product of distinct variables. You can …
Minimizing boolean expressions
This post will look at how to take an expression for a Boolean function and look for a simpler expression that corresponds to the same function. We’ll show how to use a Python implementation of the Quine-McCluskey algorithm. Notation We will write AND like multiplication, OR like addition, and use …
Edsger Dijkstra, blogger
I’ve been thinking about Edsger Dijkstra lately because I suspect some of the ideas he developed will be useful for a project I’m working on. While searching for some of Dijkstra’s writings I ran across the article Edsger Dijkstra: The Man Who Carried Computer Science on His Shoulders. It occurred …
The word problem
Most people have heard of word problems, but not as many have heard of the word problem. If you’re imagining that the word problem is some superlatively awful word problem, I can assure you it’s not. It’s both simpler and weirder than that. The word problem is essentially about whether …
Opening Windows files from bash and eshell
I often work in a sort of amphibious environment, using Unix software on Windows. As you can well imagine, this causes headaches. But I’ve found such headaches are generally more manageable than the headaches from alternatives I’ve tried. On the Windows command line, you can type the name of a …
Inverse Gray code
The previous post looked at Gray code, a way of encoding digits so that the encodings of consecutive integers differ in only bit. This post will look at how to compute the inverse of Gray code. The Gray code of a non-negative integer n is given by def gray(n): return …
Gray code
Suppose you want to list the numbers from 0 to N in such a way that only one bit at a time changes between consecutive numbers. It’s not clear that this is even possible, but in fact it’s easy using Gray code, a method named after Frank Gray. To convert …
How much do you really use?
I’ve been doing a little introspection lately about what software I use, not at an application level but at a feature level. LaTeX It started with looking at what parts of LaTeX I use. I wrote about this in April, and I revisited it this week in response to some …
Org entities
This morning I found out that Emacs org-mode has its own markdown entities, analogous to HTML entities or LaTeX commands. Often they’re identical to LaTeX commands. For example, \approx is the approximation symbol ≈, exactly as in LaTeX. So what’s the advantage of org-entities? In fact, how does Emacs even …
A different kind of computational survival
Last year I wrote a post about being a computational survivalist, someone able to get their work done with just basic command line tools when necessary. This post will be a different take on the same theme. I just got a laptop from an extremely security-conscious client. I assume it …
Symbol pronunciation
I was explaining to someone this evening that I’m in the habit of saying “bang” rather than “exclamation point.” Here’s a list of similar nicknames for symbols. These nicknames could complement the NATO phonetic alphabet if you needed to read symbols out loud, say over the phone. You might, for …
Accelerating an alternating series
This post looks at an algorithm by Cohen et al [1] to accelerate the convergence of an alternating series. This method is much more efficient than the classical Euler–Van Wijngaarden method. For our example, we’ll look at the series which converges slowly to -π²/12. The first algorithm in [1] for …
Hexadecimal floating point
Programming language support for hexadecimal integers is very common. Support for hexadecimal floating point numbers is not. It’s a common convention to put 0x in front of a number to indicate that it is an integer written as an integer literal. For example, 0x12 is not a dozen, but a …
Overview of NIST post-quantum encryption finalists
If and when large-scale quantum computing becomes practical, most public key encryption algorithms currently in use would be breakable. Cryptographers have known this since Peter Shor published his quantum factoring algorithm in 1994. In 2017 researchers submitted 69 algorithms to the NIST Post-Quantum Cryptography Standardization Process. In 2019 NIST chose …
How to compute the square root of a complex number
Suppose you’re given a complex number z = x + iy and you want to find a complex number w = u + iv such that w² = z. If all goes well, you can compute w as follows: ℓ = √(x² + y²) u = √((ℓ + x)/2) v …