Tutorial by Examples: d

When an expression contains multiple operators, it can potentially be read in different ways. For example, the mathematical expression 1 + 2 x 3 could be read in two ways: Add 1 and 2 and multiply the result by 3. This gives the answer 9. If we added parentheses, this would look like ( 1 + 2 )...

Updating one row UPDATE customers SET email='[email protected]' WHERE id=1 This query updates the content of email in the customers table to the string [email protected] where the value of id is equal to 1. The old and new contents of the database table are illustrated below on the left an...

Given a file sample: hello world ahello here hello_there A normal grep for "hello" returns: $ grep hello sample hello world ahello here hello_there Using -w allows to select those lines containing matches that form whole words: $ grep -w hello sample hello world

DBContract.java //Define the tables and columns of your local database public final class DBContract { /*Content Authority its a name for the content provider, is convenient to use the package app name to be unique on the device */ public static final String CONTENT_AUTHORITY =...

A module is an importable file containing definitions and statements. A module can be created by creating a .py file. # hello.py def say_hello(): print("Hello!") Functions in a module can be used by importing the module. For modules that you have made, they will need to be in t...

This object, Shape has a property image that depends on numberOfSides and sideWidth. If either one of them is set, than the image has to be recalculated. But recalculation is presumably long, and only needs to be done once if both properties are set, so the Shape provides a way to set both propertie...

Suppose you have many changes in one or more files but from each file you only want to commit some of the changes, you can select the desired changes using: git add -p or git add -p [file] Each of your changes will be displayed individually, and for each change you will be prompted to choose...

A simple context tree (containing some common values that might be request scoped and included in a context) built from Go code like the following: // Pseudo-Go ctx := context.WithValue( context.WithDeadline( context.WithValue(context.Background(), sidKey, sid), time.Now().A...

A database is created with the following SQL command: CREATE DATABASE myDatabase; This would create an empty database named myDatabase where you can create tables.

Matplotlib supports both object-oriented and imperative syntax for plotting. The imperative syntax is intentionally designed to be very close to Matlab syntax. The imperative syntax (sometimes called 'state-machine' syntax) issues a string of commands all of which act on the most recent figure or a...

props are used to pass data and methods from a parent component to a child component. Interesting things about props They are immutable. They allow us to create reusable components. Basic example class Parent extends React.Component{ doSomething(){ console.log("Parent comp...

defaultProps allows you to set default, or fallback, values for your component props. defaultProps are useful when you call components from different views with fixed props, but in some views you need to pass different value. Syntax ES5 var MyClass = React.createClass({ getDefaultProps: ...

componentWillReceiveProps(nextProps) This is the first function called on properties changes. When component's properties change, React will call this function with the new properties. You can access to the old props with this.props and to the new props with nextProps. With these variables, you c...

Modern A weak reference looks like one of these: @property (weak) NSString *property; NSString *__weak variable; If you have a weak reference to an object, then under the hood: You're not retaining it. When it gets deallocated, every reference to it will automatically be set to nil Obje...

Register a production/sandbox account at https://dashboard.stripe.com/register Insert below code into your webpage where you want to have a checkout button. <form action="/charge" method="POST"> <script src="https://checkout.stripe.com/checkout.js" cl...

A word of caution: AsyncTask has many gotcha's apart from the memory leak described here. So be careful with this API, or avoid it altogether if you don't fully understand the implications. There are many alternatives (Thread, EventBus, RxAndroid, etc). One common mistake with AsyncTask is to c...

when the object is saved/updated, check the associations and save/update any object that require it (including save/update the associations in many-to-many scenario).

when the object is deleted, delete all the objects in the association.

when the object is deleted, delete all the objects in the association. In addition to that, when an object is removed from the association and not associated with another object (orphaned), also delete it.

when an object is save/update/delete, check the associations and save/update/delete all the objects found. In additional to that, when an object is removed from the association and not associated with another object (orphaned), also delete it.