Notes

Read logs

tail -f log/development.log

less log/development.log
# Shift+F will take you to the end of the file, and continuously display new
# contents. In other words, it behaves just like tail -f.

less +F log/development.log

# to display colors
less -R file.log

Install gems without sudo

rbenv local <version>

Postgres

What's the default superuser username/password for postgres?

sudo -u postgres psql postgres

sudo -u postgres createuser -D -A -P myuser
sudo -u postgres createdb -O myuser mydb

psql myuser -h 127.0.0.1 -d db_name

Elements outline (dev tools)

* {
    outline: 1px solid green;
}

HTTP servers (e.g. testing static sites on mobile devices)

# install (requires nodejs/npm)
npm install http-server -g
#run
http-server -p 8000 --cors

# Python
python -m http.server
# localhost http://127.0.0.1:8000/

Cinnamon restart Ctrl+Alt+Esc (useful when reseting key bindings).

Clear terminal Ctrl+L

Recode, in place, all .srt files in current folder, from Central European (1250) to UTF-8 encoding.

recode cp1250.. ./*.srt

Omxplayer specify subtitle

omxplayer -o hdmi -r file-name.avi --subtitles file-name.srt

Download website

wget -m -k -nH <http://site.url/>

Turn off screen using command line

xset dpms force off

Jekyll post's update date

Update post's modification date in the front matter with a git pre-commit hook.

Create pre-commit file in .git/hooks/ with the following content

#! /bin/sh
# Contents of .git/hooks/pre-commit

git diff --cached --name-status | grep "^M" | while read a b; do
  cat $b | sed "/---.*/,/---.*/s/^update:.*$/update: $(date -u "+%Y-%m-%d")/" &gt; tmp
  mv tmp $b
  git add $b
done

Add an update: field to post's front matter
```
---
title: Post Title
update:
---
```
On git commit the update: field will update

Sublime Text

Ctrl+R goto symbol.
With MarkdownEditing package navigate headings.

F6 toggle spell checking on/off.

Run multiple Firefox versions

download desired firefox version
https://ftp.mozilla.org/pub/firefox/releases/
extract archive and cd into the firefox directory

cd firefox-52.4.0esr/firefox
HOME=`pwd` ./firefox --no-remote

Naming

is_empty        # boolean
is_postable()   # return boolean
post_payment()

Name things based on what they are and not what they look like (css classes).

Javascript example:

fetchFromServer("getSomeTweets", displayTweets);
function displayTweets(err, data) {
  console.log(data.tweets);
}

Machine Learning

classifiers
- support vector machines
neural networks (nets)
- deep learning
reinforcement learning

REST

Client asks for a resource and gets a representation of that resource.

URI

Every URL is a URI, but not every URI is a URL. ISBN numbers for instance are URI's but not URLS ;)

Dynamic Programming, recursion + memoization + guessing

Invariants are truths about objects of that class that should endure for the lifetime of the object.

The MVC pattern allows the following interactions:

A model can change its data depending upon inputs received from the controller.
The changed data is reflected on the views, which are subscribed to changes in the model.
Controllers can send commands to update the model's state, such as when making changes to a document. Controllers can also send commands to modify the presentation of a view without any change to the model, such as zooming in on a graph or chart.
The MVC pattern implicitly includes a change propagation mechanism to notify each component of changes on the other dependent components.
A number of web applications in the Python world implement MVC or a variation of it. We will look at a couple of them, namely Django and Flask, in the coming sections.

Analysis
What needs to be done. Output is a set of requirements.
Visitors of the website need to be able to:

apply for jobs
browse, order products

Design
How things should be done. Output is an implementation specification.
Convert requirements into an implementation specification. (set of classes and interfaces)

name the objects
define the behaviors

Polymorphism

multiple forms
allows objects with different internal structures to have a common interface

Algorithms Coursera

Put pseudo-code into programs to understand algorithms better.

Teddy bear programming — explain the problem to somebody, put it in words.

Do one thing
This implies that the blocks within if statements, else statements, while statements, and so on should be one line long. Probably that line should be a function call.

Book: "Mathematics for Computer Science" by Eric Lehman

Probability

P(A and B) = P(A) * P(B)
P(A or B) = P(A) + P(B) - P(A and B)

Independent events
Probability of getting heads on two coin flips

P(HH) = P(H) * P(H) = 1/2 * 1/2 = 1/4

Gamblers fallacy: after getting n heads in a row the probability of getting another heads is still 1/2 (is not affected by the previous independent draws).

Probability of getting tails on the first flip, heads on the second and tails on the third

P(THT) = P(T) * P(H) * P(T) = 1/2 * 1/2 * 1/2 = 1/8

Circle

Circumference: C = 2πr
Area: A = πr^2

Electricity

Electricity is the movement of electrons.
Electrons create charge.

Voltage is the difference in charge between two points.
Current is the rate at which charge is flowing (flow of charge).
Resistance is a material's tendency to resist the flow of charge (current).

Javascript and Jquery

The scope of Javascript functions is defined at the time the function is created.
The scope is determined at the time the function is created, but the values of the variables in the scope are only retrieved at the time the function is called.

this is a reference to the object that the function is created inside.

getElementBy returns a live nodelist.
querySelector returns a static nodelist.

Docker

A Docker image is the output of a docker build. The build process runs each of the instructions within a Dockerfile.

docker build

docker run &lt;image&gt;
  -it interactive terminal
  -d detached
  -p port
docker stop
docker start

docker exec -it   # interact with containers running in the background

mount x11 socket
export display
volume

Jessica Frazelle https://blog.jessfraz.com/

Google Clean Code Talks by Miško Hevery

Never return a null, instead return a Null Object, e.g. an empty list.

Don't mix instantiation with business logic, use factories, builders.

Do as little work in constructors as possible.
Ideally only assignments in constructors.

Law of Demeter
Asking for something you don't need directly only to get to what you really want.

Only ask for objects you directly need (operate on).
You are not supposed to know about things you don't directly need.
Dependency injection, parent object does not make a child, it asks for child. So if child needs a new dependency the parent is not aware.
e.g. House is not responsible for construction. If it needs a door it asks for one. The HouseFactory is responsible for construction. video

Four biggest untestables

Location of new operators
Work in constructors
Global State (Singletons)
Law of Demeter violations

Sandi Metz

Novice programmers don’t yet have the skills to write simple code.

The message-based perspective yields more flexible applications than does the class-based perspective. Changing the fundamental design question from “I know I need this class, what should it do?” to “I need to send this message, who should respond to it?” is the first step in that direction.

You don’t send messages because you have objects, you have objects because you send messages.

What vs how
Ask for 'what' you want instead of telling 'how' to do it.

Class is just one way for an object to acquire a public interface; the public interface an object obtains by way of its class may be one of several that it contains.
It’s not what an object is that matters (class), it’s what it does (interface).

Good design preserves maximum flexibility at minimum cost by putting off decisions at every opportunity, deferring commitments until more specific requirements arrive.

The true purpose of testing, just like the true purpose of design, is to reduce costs.

Incoming messages should be tested for the state they return. Outgoing command messages should be tested to ensure they get sent. Outgoing query messages should not be tested.

Injecting Dependencies as Roles

Inheritance is for specialization not for sharing code.
Inject an object to play the role of the thing that varies.

Null Object pattern

By returning a null object (i.e. an empty list) there is no need to verify that the return value is in fact a list. The calling function may simply iterate the list as normal, effectively doing nothing. It is, however, still possible to check whether the return value is a null object (e.g. an empty list) and react differently if desired.

Code smells are surface symptoms of deeper problems in the code.

God object
feature envy
long method
parameter creep
cyclomatic complexity (too many branches or loops, convoluted logic difficult to follow)
overly long or short identifiers

Refactoring

fix complex code first
fix code smells
fix low-hanging fruits (code style, conventions)

Resources

Object-Oriented JavaScript by Marcus Phillips - Udacity, YouTube

Javascript Design Patterns - Udacity

Parenting101x The Science of Parenting - Edx

JavaScript Domain-Driven Design - Safari