Tutorial by Examples: ed

Speed up serializers queries

Let's say we have model Travel with many related fields: class Travel(models.Model): tags = models.ManyToManyField( Tag, related_name='travels', ) route_places = models.ManyToManyField( RoutePlace, related_name='travels', ) coordinate = models.F...

django-rest-framework • Serializers

Color-coded 2D line plots with color data in third dimension

In MATLAB versions prior to R2014b, using the old HG1 graphics engine, it was not obvious how to create color coded 2D line plots. With the release of the new HG2 graphics engine arose a new undocumented feature introduced by Yair Altman: n = 100; x = linspace(-10,10,n); y = x.^2; p = plot(x,y,'r...

MATLAB Language • Undocumented Features

Using Single-case Discriminated Unions as Records

Sometimes it is useful to create union types with only one case to implement record-like types: type Point = Point of float * float let point1 = Point(0.0, 3.0) let point2 = Point(-2.5, -4.0) These become very useful because they can be decomposed via pattern matching in the same way as tu...

F# • Discriminated Unions

Using ngStorage to share data

Firstly, include the ngStorage source in your index.html. An example injecting ngStorage src would be: <head> <title>Angular JS ngStorage</title> <script src = "http://ajax.googleapis.com/ajax/libs/angularjs/1.3.14/angular.min.js"></script> &l...

AngularJS • Sharing Data

Getting a shell into a running (detached) container

Log into a running container A user can enter a running container in a new interactive bash shell with exec command. Say a container is called jovial_morse then you can get an interactive, pseudo-TTY bash shell by running: docker exec -it jovial_morse bash Log into a running container with a s...

Docker • Running containers

Nested Actors

It is also possible to create nested actors, i.e. grand-children, like this: // here we pass in the ActorRefFactory explicitly as an example val a = actor(system, "fred")(new Act { val b = actor("barney")(new Act { whenStarting { context.parent ! ("hello from &quot...

akka • Actor DSL

Checked and Unchecked

C# statements executes in either checked or unchecked context. In a checked context, arithmetic overflow raises an exception. In an unchecked context, arithmetic overflow is ignored and the result is truncated. short m = 32767; short n = 32767; int result1 = checked((short)(m + n)); //will ...

C# Language • Checked and Unchecked

Draw a red cross using l (relative line) path commands

SVG • Paths

RequiredAttribute

The Required attribute specifies that a property is required. An error message can be specified on using the ErrorMessage property on the attribute. First add the namespace: using System.ComponentModel.DataAnnotations; And apply the attribute on a property. public class Product { [Require...

asp.net-mvc • Data annotations

Safe downcasting

Suppose that you have a pointer to an object of a polymorphic class: Shape *ps; // see example on defining a polymorphic class ps = get_a_new_random_shape(); // if you don't have such a function yet, you // could just write ps = new Square...

C++ • Polymorphism

Abstract derived types

An extensible derived type may be abstract type, abstract :: base_type end type Such a derived type may never be instantiated, such as by type(base_type) t1 allocate(type(base_type) :: t2) but a polymorphic object may have this as its declared type class(base_type), allocatable :: t1 o...

Fortran • Object Oriented Programming

Displaying numbers with required precision

var a:Number=0.123456789; trace(a); // 0.123456789 trace(a.toPrecision(4)); // 0.1235 trace(a.toFixed(4)); // 0.1235 trace(a.toExponential(4)); // 1.2345e-1 trace(a.toString(16)); // 0 - works for integer part only var b:Number=12345678.9876543; // a bigg...

ActionScript 3 • Working with numerical values

Leaked variables in list comprehension

Python 2.x2.3 x = 'hello world!' vowels = [x for x in 'AEIOU'] print (vowels) # Out: ['A', 'E', 'I', 'O', 'U'] print(x) # Out: 'U' Python 3.x3.0 x = 'hello world!' vowels = [x for x in 'AEIOU'] print (vowels) # Out: ['A', 'E', 'I', 'O', 'U'] print(x) # Out: 'hello world!' ...

Python Language • Incompatibilities moving from Python 2 to Python 3

Laravel 5 App on Shared Hosting on Linux Server

By default Laravel project's public folder exposes the content of the app which can be requested from anywhere by anyone, the rest of the app code is invisible or inaccessible to anyone without proper permissions. After developing the application on your development machine, it needs to be pushed t...

Laravel • Deploy Laravel 5 App on Shared Hosting on Linux Server

Write Data to a File

To write the contents of a string to a file: Dim toWrite As String = "This will be written to the file." System.IO.File.WriteAllText("filename.txt", toWrite) WriteAllText will open the specified file, write the data, and then close the file. If the target file exists, it is ...

Visual Basic .NET Language • File Handling

Using an uninitialized local variable

int a; std::cout << a; // Undefined behavior! This results in undefined behavior, because a is uninitialised. It is often, incorrectly, claimed that this is because the value is "indeterminate", or "whatever value was in that memory location before". However, it is th...

C++ • Undefined Behavior

call/N predicates

The call/N family of predicates can call arbitrary Prolog goals at run time: ?- G=true, call(G). true. ?- G=(true,false), call(G). false.

Prolog Language • Higher-Order Programming

!/0 and related predicates

Predicates that impede or prohibit a declarative reading of Prolog programs are extra-logical. Examples of such predicates are: !/0 (->)/2 and if-then-else (\+)/1 These predicates can only be understood procedurally, by taking into account the actual control flow of the interpreter, and a...

Prolog Language • Extra-logical Predicates

Named Arguments

local function A(name, age, hobby) print(name .. "is " .. age .. " years old and likes " .. hobby) end A("john", "eating", 23) --> prints 'john is eating years old and likes 23' -- oops, seems we got the order of the arguments wrong... -- this happ...

Lua • Functions

LaTeX Editors

While you can create LaTeX documents using any editor and compiling using the console, there exist several plugins for widely used editors to simplify creating LaTeX documents, and there are specialized LaTeX editors. An exhaustive list of LaTeX editors is available on TeX.SE (the StackExchange site...

latex • Getting started with latex