All posts by cjohnson318

Access Elements in a Django JSONField List

I set up my Django project to use MySQL, so that I could use a list in a JSONField. Little did I know that accessing elements of that list by their index would be another problem entirely. There are two confounding things. First, in Jinja2, you can access a for-loop index with the loop.index or loop.index0 keywords, but in a Django project, you need to use forloop.counter or forloop.counter0`. The trailing zero on those keywords specify a zero-indexed counter rather than a one-indexed counter. The second thing is that you need to provide a custom template tag in order to cleanly access elements in a JSONField list by their index.

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Using the State Machine Compiler with Python

Today I learned about a state machine compiler and code generator. It provides a small DSL that you can use to describe a state machine and the transitions between the states, which will be compiled to create a number of classes, and then all you have to do is provide the code for the actions. Here, actions, states, and transitions are defined terms that are described in the documentation for the smc tool.

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Getting CSRF to work with Rails and Angular

I had trouble posting to my Rails server from an Angular controller until I found this SO post. In /app/controllers/application_controller.rb I added the following code.

class ApplicationController < ActionController::Base
  protect_from_forgery with: :exception

  after_action :set_csrf_cookie_for_ng

  def set_csrf_cookie_for_ng
    cookies['XSRF-TOKEN'] = form_authenticity_token if protect_against_forgery?
  end

  protected

    # In Rails 4.2 and above
    def verified_request?
      super || valid_authenticity_token?(session, request.headers['X-XSRF-TOKEN'])
    end
end

Getting Started with Watercolor

In this post, I’ll discuss options for building a watercolor kit, and provide some introductory reading material.

My Background

I started working in watercolor in 2016. I heard the quote by Annie Dillard, “How we spend our days is, of course, how we spend our lives.” and thought that I should make an effort to carve out more time for art, before I don’t have any time left to carve. I decided to work more in watercolor because, for me, it strikes a balance between range of expression, time, and space. Also, although I’ve done art my whole life, I’ve tended to shy away from color because of it’s difficulty. Focusing on watercolor has allowed/forced me to learn more about color.

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Functional Digression at Work

One of my coworkers asked how to filter a list, but return the “pass” and “fail” items in separate lists. For example, filter takes some data and a condition, and returns a list of the items that pass the condition, but what if we returned two lists? One of my other coworkers came up with a neat solution using reduce, and I wrote something using generics. Here’s the operation I’m calling “fork”, implemented in Swift3.

import Foundation

func fork<T>(_ data: [T], completion: (T)->Bool) -> ([T],[T]) {
    var pass = [T]()
    var fail = [T]()
    for item in data {
        completion(item) ? pass.append(item) : fail.append(item)
    }
    return (pass,fail)
}

var (pass, fail) = fork([0,1,2,3,4]) { $0 % 2 == 0 }

Deterministic Finite Automata in Swift

I’ve been reading Understanding Computation by Tom Suart, and it reminds me of my first CS class in grad school where I learned about and struggled to implement finite automata. I think one major lesson from that experience was to forget about the pictures of circles and loops on the blackboard, and figure out the bare minimum needed to represent the idea. In the case of finite automata, the bare minimum was that there was a state, and a set of rules that modified the state; each rule was composed of a current state, an input, and a next state. I decided to implement a simple finite automata in Swift using a list of Ints as states, and a single Int as input.

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Very Very Verbose Cosine Similarity

This material was a teaching aid for a crash course I gave at work about cosine similarity. Cosine similarity is a blunt instrument used to compare two sets of text. If two the two texts have high numbers of common words, then the texts are assumed to be similar. The ultimate goal is to plug two texts into a function and get an easy to understand number out that describes how similar the texts are, and cosine similarity is one way to skin that cat.

Please note, there are plenty of other very fast implementations for cosine similarity, but this one was written for educational purposes.

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