Tutorial by Examples: r

Checking if running inside a virtual environment

Sometimes the shell prompt doesn't display the name of the virtual environment and you want to be sure if you are in a virtual environment or not. Run the python interpreter and try: import sys sys.prefix sys.real_prefix Outside a virtual, environment sys.prefix will point to the system p...

Python Language • Virtual environments

Accessing Parameters

When executing a Bash script, parameters passed into the script are named in accordance to their position: $1 is the name of the first parameter, $2 is the name of the second parameter, and so on. A missing parameter simply evaluates to an empty string. Checking for the existence of a parameter can...

Bash • Scripting with Parameters

How do foldFree and iterM work?

There are some functions to help tear down Free computations by interpreting them into another monad m: iterM :: (Functor f, Monad m) => (f (m a) -> m a) -> (Free f a -> m a) and foldFree :: Monad m => (forall x. f x -> m x) -> (Free f a -> m a). What are they doing? First l...

Haskell Language • Free Monads

IComparable as an Example of Implementing an Interface

Interfaces can seem abstract until you seem them in practice. The IComparable and IComparable<T> are great examples of why interfaces can be helpful to us. Let's say that in a program for a online store, we have a variety of items you can buy. Each item has a name, an ID number, and a price. ...

C# Language • Interfaces

Your first Sinatra app

# app.rb require 'sinatra' get '/' do 'Hello, Universe!' end Install Sinatra: gem install sinatra Run the app: ruby app.rb That's it! Access your app at http://localhost:4567

sinatra • Getting started with sinatra

Ajax Abort a Call or Request

var xhr = $.ajax({ type: "POST", url: "some.php", data: "name=John&location=Boston", success: function(msg){ alert( "Data Saved: " + msg ); } }); //kill the request xhr.abort()

jQuery • Ajax

Including LaTeX Preample Commands

There are two possible ways of including LaTeX preamble commands (e.g. \usepackage) in a RMarkdown document. 1. Using the YAML option header-includes: --- title: "Including LaTeX Preample Commands in RMarkdown" header-includes: - \renewcommand{\familydefault}{cmss} - \usepacka...

R Language • Creating reports with RMarkdown

Including bibliographies

A bibtex catalogue cna easily be included with the YAML option bibliography:. A certain style for the bibliography can be added with biblio-style:. The references are added at the end of the document. --- title: "Including Bibliography" author: "John Doe" output: pdf_documen...

R Language • Creating reports with RMarkdown

A simple authentication POST request with Jsoup

A simple POST request with authentication data is demonstrated below, note that the username and password field will vary depending on the website: final String USER_AGENT = "Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36"; ...

Jsoup • Logging into websites with Jsoup

A more comprehensive authentication POST request with Jsoup

Most websites require a much more complicated process than the one demonstrated above. Common steps for logging into a website are: Get the unique cookie from the initial login form. Inspect the login form to see what the destination url is for the authentication request Parse the login form t...

Jsoup • Logging into websites with Jsoup

Parsing JavaScript Generated Page with Jsoup and HtmUnit

page.html - source code <html> <head> <script src="loadData.js"></script> </head> <body onLoad="loadData()"> <div class="container"> <table id="data" border="1"> <...

Jsoup • Parsing Javascript Generated Pages

Structure of a Simple ASP Page

<%@ Language="VBScript" CodePage = 65001 %> <% Option Explicit Response.Charset = "UTF-8" Response.CodePage = 65001 %> <!doctype html> <html> <head> <title>My First Classic ASP Page</title> </head> <body&gt...

asp-classic • Getting started with asp-classic

Multiline string literals

VB now allows string literals that split over multiple lines. Old syntax: Dim text As String = "Line1" & Environment.NewLine & "Line2" New syntax: Dim text As String = "Line 1 Line 2"

Visual Basic .NET Language • Visual Basic 14.0 Features

Using the default Form instance

VB.NET offers default Form instances. The developer does not need to create the instance as it is created behind the scenes. However, it is not preferable to use the default instance all but the simplest programs. Public Class Form1 Public Sub Foo() MessageBox.Show("Bar") ...

Visual Basic .NET Language • Working with Windows Forms

Using uWSGI to run a flask application

The built-in werkzeug server certainly is not suitable for running production servers. The most obvious reason is the fact that the werkzeug server is single-threaded and thus can only handle one request at a time. Because of this we want to use the uWSGI Server to serve our application instead. In...

Flask • Deploying Flask application using uWSGI web server with Nginx

Installing nginx and setting it up for uWSGI

Now we want to install nginx to serve our application. sudo apt-get install nginx # on debian/ubuntu Then we create a configuration for our website cd /etc/nginx/site-available # go to the configuration for available sites # create a file flaskconfig with your favourite editor flaskconfig...

Flask • Deploying Flask application using uWSGI web server with Nginx

Enable streaming from flask

Flask has that feature which lets you stream data from a view by using generators. Let's change the app.py file add from flask import Response add from datetime import datetime add from time import sleep create a new view: @app.route("/time/") def time(): def streamer(): ...

Flask • Deploying Flask application using uWSGI web server with Nginx

CRTP to avoid code duplication

The example in Visitor Pattern provides a compelling use-case for CRTP: struct IShape { virtual ~IShape() = default; virtual void accept(IShapeVisitor&) const = 0; }; struct Circle : IShape { // ... // Each shape has to implement this method the same way ...

C++ • Curiously Recurring Template Pattern (CRTP)

Pitfall: using == to compare strings

A common mistake for Java beginners is to use the == operator to test if two strings are equal. For example: public class Hello { public static void main(String[] args) { if (args.length > 0) { if (args[0] == "hello") { System.out.println(&q...

Java Language • Common Java Pitfalls

Creating zip archive from directory

System.IO.Compression.ZipFile.CreateFromDirectory("myfolder", "archive.zip") Create archive.zip file containing files which are in myfolder. In example paths are relative to program working directory. You can specify absolute paths.

Visual Basic .NET Language • File/Folder Compression