Tutorial by Examples

A Left Outer Join (also known as a Left Join or Outer Join) is a Join that ensures all rows from the left table are represented; if no matching row from the right table exists, its corresponding fields are NULL. The following example will select all departments and the first name of employees that ...

A table may be joined to itself, with different rows matching each other by some condition. In this use case, aliases must be used in order to distinguish the two occurrences of the table. In the below example, for each Employee in the example database Employees table, a record is returned containi...

The increment and decrement operators exist in prefix and postfix form. int a = 1; int b = 1; int tmp = 0; tmp = ++a; /* increments a by one, and returns new value; a == 2, tmp == 2 */ tmp = a++; /* increments a by one, but returns old value; a == 3, tmp == 2 */ tmp = --b; ...

min, max, and sorted all need the objects to be orderable. To be properly orderable, the class needs to define all of the 6 methods __lt__, __gt__, __ge__, __le__, __ne__ and __eq__: class IntegerContainer(object): def __init__(self, value): self.value = value def __rep...

Closures (also known as blocks or lambdas) are pieces of code which can be stored and passed around within your program. let sayHi = { print("Hello") } // The type of sayHi is "() -> ()", aka "() -> Void" sayHi() // prints "Hello" Like other fun...

The basic closure syntax is { [capture list] (parameters) throws-ness -> return type in body }. Many of these parts can be omitted, so there are several equivalent ways to write simple closures: let addOne = { [] (x: Int) -> Int in return x + 1 } let addOne = { [] (x: Int) -> Int in...

Functions may accept closures (or other functions) as parameters: func foo(value: Double, block: () -> Void) { ... } func foo(value: Double, block: Int -> Int) { ... } func foo(value: Double, block: (Int, Int) -> String) { ... } Trailing closure syntax If a function's last parameter ...

class MyClass { func sayHi() { print("Hello") } deinit { print("Goodbye") } } When a closure captures a reference type (a class instance), it holds a strong reference by default: let closure: () -> Void do { let obj = MyClass() // Captures a strong re...

In Python, variables inside functions are considered local if and only if they appear in the left side of an assignment statement, or some other binding occurrence; otherwise such a binding is looked up in enclosing functions, up to the global scope. This is true even if the assignment statement is ...

A for loop iterates over a sequence, so altering this sequence inside the loop could lead to unexpected results (especially when adding or removing elements): alist = [0, 1, 2] for index, value in enumerate(alist): alist.pop(index) print(alist) # Out: [1] Note: list.pop() is being used t...

You can define a new class using the class keyword. class MyClass end Once defined, you can create a new instance using the .new method somevar = MyClass.new # => #<MyClass:0x007fe2b8aa4a18>

A class can have only one constructor, that is a method called initialize. The method is automatically invoked when a new instance of the class is created. class Customer def initialize(name) @name = name.capitalize end end sarah = Customer.new('sarah') sarah.name #=> 'Sarah' ...

There are several special variable types that a class can use for more easily sharing data. Instance variables, preceded by @. They are useful if you want to use the same variable in different methods. class Person def initialize(name, age) my_age = age # local variable, will be destroyed ...

We have three methods: attr_reader: used to allow reading the variable outside the class. attr_writer: used to allow modifying the variable outside the class. attr_accessor: combines both methods. class Cat attr_reader :age # you can read the age but you can never change it attr_writer...

Ruby has three access levels. They are public, private and protected. Methods that follow the private or protected keywords are defined as such. Methods that come before these are implicitly public methods. Public Methods A public method should describe the behavior of the object being created. T...

math.log(x) gives the natural (base e) logarithm of x. math.log(math.e) # 1.0 math.log(1) # 0.0 math.log(100) # 4.605170185988092 math.log can lose precision with numbers close to 1, due to the limitations of floating-point numbers. In order to accurately calculate logs close to 1, ...

In Python 2.6 and higher, math.copysign(x, y) returns x with the sign of y. The returned value is always a float. Python 2.x2.6 math.copysign(-2, 3) # 2.0 math.copysign(3, -3) # -3.0 math.copysign(4, 14.2) # 4.0 math.copysign(1, -0.0) # -1.0, on a platform which supports signed zero ...

There is a problem when using optional arguments with a mutable default type (described in Defining a function with optional arguments), which can potentially lead to unexpected behaviour. Explanation This problem arises because a function's default arguments are initialised once, at the point whe...

Python 2.x2.3 raw_input will wait for the user to enter text and then return the result as a string. foo = raw_input("Put a message here that asks the user for input") In the above example foo will store whatever input the user provides. Python 3.x3.0 input will wait for the user ...

Python 3.x3.0 In Python 3, print functionality is in the form of a function: print("This string will be displayed in the output") # This string will be displayed in the output print("You can print \n escape characters too.") # You can print escape characters too. Pyth...