Tutorial by Examples: f

Differences in subsetting syntax A data.table is one of several two-dimensional data structures available in R, besides data.frame, matrix and (2D) array. All of these classes use a very similar but not identical syntax for subsetting, the A[rows, cols] schema. Consider the following data stored i...

Category theory is a modern mathematical theory and a branch of abstract algebra focused on the nature of connectedness and relation. It is useful for giving solid foundations and common language to many highly reusable programming abstractions. Haskell uses Category theory as inspiration for some o...

A category C consists of: A collection of objects called Obj(C) ; A collection (called Hom(C)) of morphisms between those objects. If a and b are in Obj(C), then a morphism f in Hom(C) is typically denoted f : a -> b, and the collection of all morphism between a and b is denoted hom(a,b) ; A...

Using Kestrel you can specify port using next approaches: Defining ASPNETCORE_URLS environment variable. Windows SET ASPNETCORE_URLS=https://0.0.0.0:5001 OS X export ASPNETCORE_URLS=https://0.0.0.0:5001 Via command line passing --server.urls parameter dotnet run --server.urls=http...

You can select elements with different selectors : by tag : "div" by class : ".class" by id : "#id" by attribute : "[color=blue]" multiple selectors (OR): "div1, div2, class1" multiple selectors (AND): "div1 div2 class1"

<div class="navbar navbar-inverse navbar-fixed-top"> <div class="container"> <div class="navbar-header"> <!--- vvv Hamburger icon that gets shown when window reaches a certain scale vvv ---> <button type...

In Scala operators (such as +, -, *, ++, etc.) are just methods. For instance, 1 + 2 can be written as 1.+(2). These sorts of methods are called 'infix operators'. This means custom methods can be defined on your own types, reusing these operators: class Matrix(rows: Int, cols: Int, val data: Seq[...

The case modifier causes the Scala compiler to automatically generate common boilerplate code for the class. Implementing this code manually is tedious and a source of errors. The following case class definition: case class Person(name: String, age: Int) ... will have the following code automati...

There are several ways to format and get a string as a result. The .NET way is by using String.Format or StringBuilder.AppendFormat: open System open System.Text let hello = String.Format ("Hello {0}", "World") // return a string with "Hello World" let builder...

F|forbidden Similar to Deny, this flag forces the server to immediately return a 403 Forbidden status code to the requesting browser or client for the request. Example: Deny access to requests that end with exe: RewriteRule .exe$ - [F] G|gone If a requested resource was available in the past,...

CLP(FD) constraints are provided by all serious Prolog implementations. They allow us to reason about integers in a pure way. ?- X #= 1 + 2. X = 3. ?- 5 #= Y + 2. Y = 3.

Sometimes you want to destructure key under a map which might not be present in the map, but you want a default value for the destructured value. You can do that this way: (def my-map {:a 3 :b 4}) (let [{a :a b :b :keys [c d] :or {a 1 c 2}} my-map] (println ...

Destructurling works in many places, as well as in the param list of an fn: (defn my-func [[_ a b]] (+ a b)) (my-func [1 2 3]) ;= 5 (my-func (range 5)) ;= 3 Destructuring also works for the & rest construct in the param list: (defn my-func2 [& [_ a b]] (+ a b)) (my-func2 1 ...

Destructuring also gives you the ability to interpret a sequence as a map: (def my-vec [:a 1 :b 2]) (def my-lst '("smthg else" :c 3 :d 4)) (let [[& {:keys [a b]}] my-vec [s & {:keys [c d]} my-lst] (+ a b c d)) ;= 10 It is useful for defining functions with named p...

Predicates that produce side effects leave the realm of pure logic. These are for example: writeq/1 read/1 format/2 Side effects are phenomena that cannot be reasoned about within the program. For example, deletion of a file or output on the system terminal.

CLP(FD) constraints are completely pure relations. They can be used in all directions for declarative integer arithmetic: ?- X #= 1+2. X = 3. ?- 3 #= Y+2. Y = 1.

In oracle, the difference (in days and/or fractions thereof) between two DATEs can be found using subtraction: SELECT DATE '2016-03-23' - DATE '2015-12-25' AS difference FROM DUAL; Outputs the number of days between the two dates: DIFFERENCE ---------- 89 And: SELECT TO_DATE( '201...

The MEDIAN function since Oracle 10g is an easy to use aggregation function: SELECT MEDIAN(SAL) FROM EMP It returns the median of the values Works on DATETIME values too. The result of MEDIAN is computed by first ordering the rows. Using N as the number of rows in the group, Oracle calcula...

Enum classes can also declare members (i.e. properties and functions). A semicolon (;) must be placed between the last enum object and the first member declaration. If a member is abstract, the enum objects must implement it. enum class Color { RED { override val rgb: Int = 0xFF0000 ...

Sometimes, we may open a file which we do not have permission to write in Vim without using sudo. Use this command to save a read-only file edited in Vim. :w !sudo tee > /dev/null % Which you could map to :w!! in your .vimrc: cmap w!! w !sudo tee > /dev/null % You will be presented a ...