AngularJS

Overview

AngularJS is a JavaScript-based open-source front-end web application framework. It is maintained by Google and a community of individual developers and corporations to address many of the challenges encountered in developing single-page applications. The framework works by first reading the HTML page, which has embedded into it additional custom tag attributes. Angular interprets those attributes as directives to bind input or output parts of the page to a model that is represented by standard JavaScript variables. The values of those JavaScript variables can be manually set within the code, or retrieved from static or dynamic JSON resources.

History

AngularJS was originally developed in 2009 by Misko Hevery at Brat Tech LLC as the software behind an online JSON storage service, that would have been priced by the megabyte, for easy-to-make applications for the enterprise. This venture was located in Mountain View, California. The company had been involved in the development of AngularJS for approximately two years prior to the release of the first public beta version of the framework.

A screenshot of a web application built using AngularJS

Design Goals

The AngularJS framework works by first reading the HTML page, which has embedded into it additional custom tag attributes. Angular interprets those attributes as directives to bind input or output parts of the page to a model that is represented by standard JavaScript variables. The values of those JavaScript variables can be manually set within the code, or retrieved from static or dynamic JSON resources.

Architecture

AngularJS is built on the belief that declarative programming should be used to create user interfaces and connect software components, while imperative programming is better suited to defining an application's business logic. The framework adapts and extends traditional HTML to better serve dynamic content through two-way data-binding that allows for the automatic synchronization of models and views. As a result, AngularJS de-emphasizes explicit DOM manipulation with the goal of improving testability and performance.

Directives

Directives in AngularJS are a way to extend the functionality of HTML. Directives are markers on a DOM element (such as an attribute, element name, comment or CSS class) that tell AngularJS's HTML compiler to attach a specified behavior to that DOM element or even transform the DOM element and its children.

Expressions

Expressions in AngularJS are JavaScript-like code snippets that are usually placed in bindings such as == Introduction ==

Expression templates are a sophisticated programming technique used in C++ to optimize the performance of mathematical operations on complex data structures. They leverage the power of template metaprogramming to transform expressions into efficient, inline code, reducing the overhead typically associated with operator overloading. This technique is particularly beneficial in scientific computing, where operations on large matrices or vectors are frequent and performance is critical.

Background and Motivation

The concept of expression templates arose from the need to address the inefficiencies in operator overloading. In traditional operator overloading, each operation on objects like matrices or vectors creates temporary objects, leading to unnecessary memory allocations and deallocations. This can significantly degrade performance, especially in computationally intensive applications. Expression templates solve this problem by eliminating temporary objects and enabling the compiler to generate optimized code that performs the operations directly.

How Expression Templates Work

Expression templates work by representing expressions as a tree of objects, where each node corresponds to an operation. These objects are lightweight and do not perform any computations themselves. Instead, they store references to their operands and the operation to be performed. When the final result of the expression is needed, the entire expression tree is evaluated in a single pass, directly computing the result without creating intermediate temporaries.

Template Metaprogramming

Template metaprogramming is the foundation of expression templates. It allows the creation of templates that can manipulate types and values at compile time. This capability is exploited to build expression trees that represent complex mathematical expressions. The use of templates ensures that the expression is fully resolved at compile time, allowing the compiler to generate highly optimized code.

Operator Overloading

In the context of expression templates, operator overloading is used to construct the expression tree. Each overloaded operator returns an object representing the operation, rather than performing the operation immediately. This deferred execution model is key to the efficiency of expression templates, as it postpones computation until the entire expression is known.

Complex mathematical expression represented as a tree structure with nodes for operations and operands.

Implementation Details

The implementation of expression templates involves several key components:

Expression Classes

Expression classes are the building blocks of expression templates. Each class represents a specific operation, such as addition or multiplication, and stores references to its operands. These classes are typically lightweight and designed to be combined in a hierarchical manner to form complex expressions.

Template Specialization

Template specialization is used to handle different types of operations and operands. By specializing templates for specific operations, it is possible to optimize the evaluation of expressions for particular data types or structures. This allows for fine-tuned performance improvements in specific scenarios.

Lazy Evaluation

Lazy evaluation is a crucial aspect of expression templates. It ensures that computations are only performed when the final result is required. This approach minimizes unnecessary computations and memory usage, contributing to the overall efficiency of the technique.

Advantages of Expression Templates

Expression templates offer several advantages over traditional operator overloading:

**Performance**: By eliminating temporary objects and enabling inline computation, expression templates significantly improve performance, particularly in applications involving large data sets.

**Flexibility**: The use of templates allows for flexible and reusable code, which can be adapted to different types of operations and data structures.

**Compile-time Optimization**: Expression templates leverage the compiler's optimization capabilities, resulting in highly efficient code.

Challenges and Limitations

Despite their advantages, expression templates also present certain challenges:

**Complexity**: The implementation of expression templates can be complex, requiring a deep understanding of template metaprogramming and operator overloading.

**Debugging Difficulty**: The use of templates can obscure the code, making debugging more challenging. Error messages can be cryptic and difficult to interpret.

**Compiler Support**: Not all compilers fully support the advanced features required for expression templates, which can limit their portability.

Applications

Expression templates are widely used in scientific computing and numerical libraries, where performance is critical. They are particularly beneficial in applications involving:

**Matrix and Vector Operations**: Expression templates optimize operations on large matrices and vectors, reducing computational overhead.

**Finite Element Analysis**: In finite element analysis, expression templates can improve the efficiency of complex mathematical computations.

**Signal Processing**: Expression templates are used in signal processing applications to optimize the performance of mathematical transformations.

Conclusion

Expression templates represent a powerful technique for optimizing mathematical computations in C++. By leveraging template metaprogramming and operator overloading, they eliminate the inefficiencies associated with temporary objects and enable the generation of highly optimized code. While they present certain challenges, their benefits in terms of performance and flexibility make them an invaluable tool in the arsenal of modern C++ programming.

Filters

Filters format the value of an expression for display to the user. They can be used in view templates, controllers, services and directives. There are some built-in filters provided by AngularJS like as Currency, Date, JSON, Limit, Lowercase, Number, OrderBy, Uppercase etc. You can also create your own filters.

Scope

Scopes in AngularJS are objects that refer to the model. They act as a glue between controller and view. Scopes can watch expressions and propagate events.

Services

AngularJS services are substitutable objects that are wired together using dependency injection (DI). You can use services to organize and share code across your app.

Dependency Injection

AngularJS has a built-in dependency injection subsystem that helps the developer by making the application easier to develop, understand, and test.

Templates

In AngularJS, templates are written with HTML that contains AngularJS-specific elements and attributes. AngularJS combines the template with information from the model and controller to render the dynamic view that a user sees in the browser.

Routing

Routing is the concept of switching views. AngularJS routes enable you to create different URLs for different content in your application. A route simply binds a URL to a template.

MVC

AngularJS incorporates the basic principles behind the original MVC software design pattern into how it builds client-side web applications.

Two-way Data Binding

AngularJS uses two-way data-binding to handle the synchronization between the model and the DOM. When data in the model changes, the view reflects the change, and when data in the view changes, the model is updated as well.

Testing

AngularJS is designed with testability in mind, so it encourages behavior-view separation, comes pre-bundled with mocks, and takes full advantage of dependency injection. It also comes with end-to-end scenario runner which eliminates test flakiness by understanding the inner workings of AngularJS.