Tutorial by Examples: v

Variable substitutions should only be used inside double quotes. calculation='2 * 3' echo "$calculation" # prints 2 * 3 echo $calculation # prints 2, the list of files in the current directory, and 3 echo "$(($calculation))" # prints 6 Outside of doubl...

The ^ operator will perform a binary XOR in which a binary 1 is copied if and only if it is the value of exactly one operand. Another way of stating this is that the result is 1 only if the operands are different. Examples include: # 0 ^ 0 = 0 # 0 ^ 1 = 1 # 1 ^ 0 = 1 # 1 ^ 1 = 0 # 60 = 0b1111...

The zero value of a map is nil and has a length of 0. var m map[string]string fmt.Println(m == nil) // true fmt.Println(len(m) ==0) // true A nil map has no keys nor can keys be added. A nil map behaves like an empty map if read from but causes a runtime panic if written to. var m map[string]...

You shouldn't call NSLog without a literal format string like this: NSLog(variable); // Dangerous code! If the variable is not an NSString, the program will crash, because NSLog expects an NSString. If the variable is an NSString, it will work unless your string contains a %. NSLog will pars...

We can illustrate this problem with the following pseudo-code function foo() { global $bob; $bob->doSomething(); } Your first question here is an obvious one Where did $bob come from? Are you confused? Good. You've just learned why globals are confusing and considered a bad p...

Subpatterns can be referenced with their relative group number: (?-1) will recurse into the previous group (?+1) will recurse into the next group Also usable with the \g<N> syntax.

In a normal string, the backslash character is the escape character, which instructs the compiler to look at the next character(s) to determine the actual character in the string. (Full list of character escapes) In verbatim strings, there are no character escapes (except for "" which is ...

WITH RECURSIVE ManagersOfJonathon AS ( -- start with this row SELECT * FROM Employees WHERE ID = 4 UNION ALL -- get manager(s) of all previously selected rows SELECT Employees.* FROM Employees JOIN ManagersOfJonathon ON Employees.ID = Manager...

Most databases do not have a native way of generating a series of numbers for ad-hoc use; however, common table expressions can be used with recursion to emulate that type of function. The following example generates a common table expression called Numbers with a column i which has a row for numbe...

WITH RECURSIVE ManagedByJames(Level, ID, FName, LName) AS ( -- start with this row SELECT 1, ID, FName, LName FROM Employees WHERE ID = 1 UNION ALL -- get employees that have any of the previously selected rows as manager SELECT ManagedByJames.Level + 1, ...

# Create the nodes # In a real world scenario we would use at least 3 managers to cover the fail of one manager. docker-machine create -d virtualbox manager docker-machine create -d virtualbox worker1 # Create the swarm # It is possible to define a port for the *advertise-addr* and *listen-ad...

Any fold can be run in the opposite direction with the help of the Dual monoid, which flips an existing monoid so that aggregation goes backwards. newtype Dual a = Dual { getDual :: a } instance Monoid m => Monoid (Dual m) where mempty = Dual mempty (Dual x) `mappend` (Dual y) = Dua...

import Data.Traversable as Traversable data MyType a = -- ... instance Traversable MyType where traverse = -- ... Every Traversable structure can be made a Foldable Functor using the fmapDefault and foldMapDefault functions found in Data.Traversable. instance Functor MyType where ...

Implementations of traverse usually look like an implementation of fmap lifted into an Applicative context. data Tree a = Leaf | Node (Tree a) a (Tree a) instance Traversable Tree where traverse f Leaf = pure Leaf traverse f (Node l x r) = Node <$> traverse f l <*...

A traversal can be run in the opposite direction with the help of the Backwards applicative functor, which flips an existing applicative so that composed effects take place in reversed order. newtype Backwards f a = Backwards { forwards :: f a } instance Applicative f => Applicative (Backward...

The standard (section 23.3.7) specifies that a specialization of vector<bool> is provided, which optimizes space by packing the bool values, so that each takes up only one bit. Since bits aren't addressable in C++, this means that several requirements on vector are not placed on vector<bool...

package main import ( "log" "net/http" ) func main() { // Create a mux for routing incoming requests m := http.NewServeMux() // All URLs will be handled by this function m.HandleFunc("/", func(w http.ResponseWriter, r *http.Reque...

#import <Foundation/Foundation.h> @interface Car:NSObject { NSString *CarMotorCode; NSString *CarChassisCode; } - (instancetype)initWithMotorValue:(NSString *) motorCode andChassisValue:(NSInteger)chassisCode; - (void) startCar; - (void) stopCar; @end @implementation Car...

The following will log the JSON response from the API to the console if the request was successful, otherwise it will log the error. var xhr = new XMLHttpRequest(); xhr.open('GET', 'https://api.twitch.tv/kraken', true); xhr.setRequestHeader('Client-ID', '...'); xhr.onload = function(...

dynamic foo = 123; Console.WriteLine(foo + 234); // 357 Console.WriteLine(foo.ToUpper()) // RuntimeBinderException, since int doesn't have a ToUpper method foo = "123"; Console.WriteLine(foo + 234); // 123234 Console.WriteLine(foo.ToUpper()): // NOW A STRING