Design Patterns – An introduction

Design patterns are a quintessential topic that all entry level software developers are intimidated by. This post is my attempt at explaining the concepts in a simple way. I will avoid going into detailed examples since there are a lot of great linksvideos and books that do that well. I’d like to explain the core ideas behind design patterns in a concise way. This post will provide a high level summary about the types of design patterns and when they are used.

 

What?

A design pattern is a general, reusable solution to a commonly occurring problem software design.
There are three main categories design patterns can be divided into:
  1. Creational
  2. Structural
  3. Behavioral

Why?

They speed up development by reusing existing paradigms (tested, proven to be efficient).
int main(){

}

How?

Drawing a picture of a few objects takes time. But if we use stencils of objects to compose the drawing, it can be done much faster.
A design pattern is like a template (stencil) to solve problems in software development.

 

Creational patterns

are used for creating instances of classes and objects. They have two main ideas
  1. Encapsulate the information about which concrete class to use at creation
  2. Hide the nitty gritty details of creation and composition of the concrete classes and objects
Object creational patterns delegate object creation to another object.
Class creational patterns use the principle of inheritance to delegate object creation to subclasses. Some patterns that fall in the category of creational patterns are:
  • Factory pattern allows a class to delegate instantiation to its subclasses
  • Abstract factory pattern provides an interface to create a family of objects without specifying the concrete classes
  • Builder pattern encapsulates object construction from object representation
  • Prototype pattern clones from a fully initialized instance
  • Singleton pattern ensures that a class has only one instance during program execution and provide a global point of access to that instance

 

Structural patterns

simplify composition of entities (their relationships and functionality). Some common patterns in this category are:
  • Adapter pattern matches interface of a class to what the client expects
  • Bridge pattern decouples the interface from implementation to provide ‘loose coupling’
  • Composite pattern is a tree structure of objects with the same interface. Some objects are simple, while others are complex but they all have a uniform interface
  • Decorator pattern provides a way to extend functionality of a class during runtime without changing the interface
  • Facade pattern provides a higher level simplified interface to a set of interfaces in a subsystem
  • Flyweight pattern enables sharing by allowing a lot of objects share common properties
  • Proxy pattern provides a placeholder for another object to control access

 

Behavioral patterns

provide flexibility in communication across objects. Some common patterns in this category are:

  • Chain of responsibility passes the request along a chain of receivers until an object handles it
  • Command pattern encapsulates a request into an object
  • Iterator pattern provides a way to access the elements of a collection of objects without exposing the underlying details
  • Mediator pattern defines how different objects interact without referring to each other explicitly
  • Null object pattern provides a default object to avoid null references
  • Observer pattern defines a one to many dependency across objects so that a state change in the one object causes a notification to be sent to all it’s dependents
  • State pattern allows an object to change behavior when there is an internal state change
  • Strategy pattern defines a family of algorithms that are encapsulated to be interchangeable. Thus the algorithm can vary independently of the clients that use them
  • Template pattern lets subclasses redefine (override) some (specific) steps of an algorithm without affecting the high level structure of that algorithm
  • Visitor pattern allows a new operation to be defined without changing the class definition of the entities on which it operates
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Deep Learning

Recently the term deep learning has become a buzz word in the field of technology. People are excited about using deep learning to develop products that solve problems (including those in the field of Artificial Intelligence a.k.a AI) which seemed impossible (or too futuristic) in the past.
A few days ago, there was news about how a computer system trained to play Go using deep learning beat a top human Go player that caught my attention. Until now Go (unlike chess) was a game that computers couldn’t always beat good human players at. So I was intrigued. What is this deep learning? I set out to figure it out using some papers, lectures and this book.

“If you can’t explain it simply, you don’t understand it well enough.”

Albert Einstein.


So this post is an attempt to explain some of these concepts and to ensure that I understand them well.
Let’s start with concisely stating what deep learning is and then go into further details.

What?

Deep Learning is an approach to Machine Learning based on the knowledge of
  • learning algorithms
  • statistics
  • applied mathematics

Why now?

Deep Learning has recently been revived in popularity due to increased computation ability of CPUs and general purpose GPUs, huge amount of datasets (quite a bit of it labeled) and new techniques to train deeper neural networks.

How?

Let’s take a step back to understand how deep learning fits with AI.
  • Classical algorithms in Computer Science map representations to output.
  • The goal of AI is to learn how to generate these representations in order to convert them to output, without human intervention.
  • There are many factors that humans use to understand / classify the observed input data into output.
  • Usually, humans are required to disentangle these features and remove the redundant ones to specify them as representations in classical algorithms.
Deep Learning organizes these features in a hierarchy where simpler concepts combine to generate a complex one.
Deep learning uses multiple layered neural networks to understand inputs and extract abstract concepts (a.k.a. features) from it. These abstract concepts are combined hierarchically to generate complex representations that in turn generate output.
It is also worth noting that Deep Learning does not attempt to simulate the human brain. Instead it takes the knowledge we have about learning hierarchical representations automatically from data and uses it to build computer systems that solve tasks requiring intelligence.