Data Structures: Queue

A Queue is a FIFO (First In First Out — the element placed at first can be accessed at first) structure which can be commonly found in many programming languages. This structure is named as “queue” because it resembles a real-world queue — people waiting in a queue (line).

Just like in like real life, the person who was in line first gets served first.

Diagram: Queue

Queue Operations

Given below are the 2 basic operations that can be performed on a queue. Reference the diagram above

  • Enqueue: Insert an element to the end of the queue.
  • Dequeue: Delete the element from the beginning of the queue.

Applications of Queues

  • Used to manage threads in multithreading.
  • Used to implement queuing systems (e.g.: priority queues).

Data Structures: Stack

A Stack is a linear data structure which follows a particular order in which the operations are performed. The order may be LIFO (Last In First Out) or FILO (First In Last Out).

Diagram: Stack

Push: Insert an element on to the top of the stack.

Pop: Delete the topmost element and return it.

Furthermore, the following additional functions are provided for a stack in order to check its status.

  • Peek: Return the top element of the stack without deleting it.
  • isEmpty: Check if the stack is empty.
  • isFull: Check if the stack is full.

Applications of stacks

  • Used for expression evaluation (e.g.: shunting-yard algorithm for parsing and evaluating mathematical expressions).
  • Used to implement function calls in recursion programming.

Data Structures: Array

An array is a collection of items stored at contiguous memory locations. The idea is to store multiple items of the same type together. This makes it easier to calculate the position of each element by simply adding an offset to a base value, i.e., the memory location of the first element of the array.

Diagram: Array

The diagram above demonstrates how each element can be uniquely identified by its index value or by its memory address. Notice that the memory addresses are sequential. This is what is meant by contiguous (touching). This demonstrates how and why order is preserved with arrays.

Data Structures: Doubly Linked List

Doubly Linked List (DLL) is very similar to a Linked List, but contains an extra pointer, typically referred to as previous pointer, together with next pointer and the data.

Diagram: Doubly Linked List

Advantages over singly linked list

  • A doubly linked list can be traversed in both forward and backward direction. 
  • The delete operation for doubly linked lists is more efficient if pointer to the node to be deleted can be supplied. 
  • Quickly insert a new node before a given node. 
  • In a singly linked list, to delete a node, the pointer to the previous node is needed. To get the previous node, the list has to be traversed from the beginning. Whereas the doubly linked list can fetch the previous node using previous pointer. 


  • Every node of the doubly linked list requires extra space for the previous pointer.
  • All operations have additional overhead because of the extra previous pointer. For example, in insertion, we need to modify previous pointers together with next pointers.


A node can be added in four ways:

  • At the front of the DLL 
  • After a given node. 
  • At the end of the DLL 
  • Before a given node.

Data Structures: Linked List

Like arrays, the Linked List is a linear data structure. Unlike arrays, linked list elements are not stored at a contiguous location; the elements are linked using pointers.

Like arrays, the Linked List is a linear data structure. Unlike arrays, linked list elements are not stored at a contiguous location; the elements are linked using pointers.

Diagram: Linked List

Why use a Linked List over an Array?

Arrays can be used to store linear data of similar types, but arrays have the following limitations:
1) Arrays have a fixed size – so we must know the upper limit (ceiling) on the number of elements in advance. Additionally, in most implementations, the memory allocation is equal to the upper limit regardless of usage. 
2) Inserting new elements into an array is expensive because space has to be created for the new elements and to create space, existing elements have to be shifted. 
For example, in a system, if we maintain a sorted list of IDs in an array nums[]. 
nums[] = [100, 101, 105, 200, 206]. 
And if we want to insert a new ID 104, then to maintain the sorted order, we have to move all the elements after 100 (excluding 100). 
Deletion is also expensive with arrays. For example, to delete 101 in nums[], everything after 101 has to be shifted.

Advantages over arrays:

  • Dynamic size 
  • Ease of insertion/deletion


  • Random access is disallowed. We must access elements sequentially starting from the first node and then traverse the list until we reach the element we are seeking. So an efficient binary search is not an option with linked lists in their default implementation.
  • The more segments a list is broken into the more overhead there is for locating the next linked element.
  • Extra memory space for a pointer is required with each element of the list.
  • Not cache friendly. Since array elements are contiguous locations, there is locality of reference which is not the case for linked lists.


A linked list is represented by a pointer to the first node of the linked list. The first node is called the head. If the linked list is empty, then the value of the head is NULL
Each node in a list consists of at least two parts: 
1) Data 
2) Pointer (Or Reference) to the next node 

Bitwise Operations

Human readable code is certainly not the point.

What is Bitwise Manipulation?

Bitwise manipulation is when we perform a logical operation against each individual bit of a binary number. The logical connectives a.k.a. the bitwise operators AND, OR, and XOR can be used as masks that can affect the specific bits.

What is the point of Bitwise manipulation?

Human readability is certainly not the point, but speed and efficiency are. Bitwise operations are primitive actions that can be executed directly on the CPU meaning that they stake up less temporary/persistent storage and require less pre-processing overhead to compute. You can think of them like shortcuts.


The AND logical operation can be used to turn off certain bits of a binary number, because:

1 AND 0 is 0

0 AND 0 is 0

Consider you have an input of 1101 1011 and a bitwise mask of 1111 1000. Examine the function table below:

Bitwise AND Mask Results

Notice that the mask only applies to the last three digits. The third to last digit is unchanged only because it was already 0.


The AND logical operation can be used to turn off certain bits of a binary number, because:

1 AND 0 is 0

0 AND 0 is 0

Consider you have an input of 1001 1011 and a bitwise mask of 1110 0000. Examine the function table below:

Bitwise OR Mask Results

Notice that only the first three digits changed (except the first digit which was already a 1). The OR switch changed all the 0 bits to 1’s.


The XOR (Exlusive OR) logical function can be used to reverse certain bits of a binary number, because:

0 XOR 1 is 1

1 XOR 1 is 0

Consider you have an input of 1011 1001 and a bitwise mask of 0000 1111. Examine the function table below:

Bitwise XOR Mask Results

Notice that all four of the bits aligned with the mask are now the opposite of what they once were without exceptions. The bits have essentially been “flipped”.

Logical Shift Left

Performing a logical shift one bit to the left on a binary number means:

  • Moving all the bits of the number one place to the left
  • Discarding the most significant bit
  • Putting a 0 into the empty place on the right

Interestingly this is equivalent to multiply our binary number by 2. Notice that performing a logical shift three bits to the left on a binary number is the same as multiplying the number by 23 = 8.


If we start with the decimal number 14, three logical shifts to the left of its binary form results in the decimal number 112:

Logical Shift Left Results

Logical Shift Right

Performing a logical shift one bit to the right on a binary number means:

  • Moving all the bits of the number one place to the right
  • Discarding the least significant bit
  • Putting a 0 into the empty place on the left

As you might have guessed, this is equivalent to dividing our number by 2. Notice that performing a logical shift two bits to the right on a binary number is the same as dividing the number by 22 = 4.


If we start with the decimal number 112, two logical shifts to the right of its binary form results in the decimal number 28:

Logical Shift Right Results

Regular Expression (Regex) Cheat Sheet

Regex (short for regular expression), is a string of text that allows you to create patterns that help match, locate, and manage text.

Jump to: Regex Tester Tool

What is Regex?

Regular expressions are notations for describing patterns of text and, in effect, make up a special-purpose language for pattern matching. Although there are myriad variants, all share the idea that most characters in a pattern match literal occurrences of themselves, but some metacharacters have special meaning, such as * to indicate some kind of repetition or […] to mean any one character from the set within the brackets.

Beautiful Code by Andy Oram, Greg Wilson

More simply, Regex (short for regular expression), is a string of text that allows you to create patterns that help match, locate, and manage text.

Below is a quick reference Javascript regex cheat sheet. If you need a more in depth refresher or a place to get started I recommend these resources on regex:

LanguageReference Materials
JavacriptRegular expressions – JavaScript | MDN (

JavaScript RegExp Reference (
T-SQL (MS SQL Server)Search Text with Regular Expressions – SQL Server
Management Studio (SSMS) | Microsoft Docs
PythonRegular Expression HOWTO — Python 3.9.6 documentation

Python RegEx (
C LanguagesRegular expressions in C – GeeksforGeeks

An example of using regular expressions in C (

Regex Class (System.Text.RegularExpressions) | Microsoft Docs (C#)
Pearl (PCRE)perlre – Perl regular expressions – Perldoc Browser
Powershellabout Regular Expressions – PowerShell | Microsoft Docs
JavaJava Regular Expressions (

Lesson: Regular Expressions (The Java™ Tutorials > Essential Java Classes) (
Linux (Bash)How to Use Regular Expressions (regexes) on Linux (

Using Grep + Regex (Regular Expressions) to Search Text in Linux | DigitalOcean

Advanced Bash regex with examples – Linux Tutorials – Learn Linux Configuration
Other Resources and Reference MaterialComparison of regular-expression engines – Wikipedia

An introduction to regular expressions – O’Reilly (
Regex Reference Materials

Quick Reference Cheat Sheet

Character classes
.any character except newline
\w\d\sword, digit, whitespace
\W\D\Snot word, digit, whitespace
[abc]any of a, b, or c
[^abc]not a, b, or c
[a-g]character between a & g
^abc$start / end of the string
\b\Bword, not-word boundary
Escaped characters
\.\*\\escaped special characters
\t\n\rtab, linefeed, carriage return
Groups & Lookaround
(abc)capture group
\1backreference to group #1
(?:abc)non-capturing group
(?=abc)positive lookahead
(?!abc)negative lookahead
Quantifiers & Alternation
a*a+a?0 or more, 1 or more, 0 or 1
a{5}a{2,}exactly five, two or more
a{1,3}between one & three
a+?a{2,}?match as few as possible
ab|cdmatch ab or cd
Regex Cheat Sheet

Regex Tester Tool

Disclaimer: For educational and demonstrative purposes only. Always test your regular expressions before applying anything to a production system. Results from the above tool are not guaranteed.

How To Use:

  1. Type a regular expression in the Regex input box. (Leading and ending slashes are added automatically)
  2. Type a sample string to match against in the other box.
  3. Check out the resulting matches.
  4. Tweak your regex expression until you get the expected results to test your knowledge.

Software Architectural Patterns

What is Software Architecture?

The job of a Building Architect is typically to design buildings, structures and civil infrastructure. Not too dissimilarly, the job of a Software Architect is to design the systems, services and infrastructure of computing systems. More importantly, just like as building architectural planning is typically the first step in any major construction project, so too is software architecture (albeit, one of the two is better suited to an agile methodology).

What is an Architectural Pattern?

An architectural pattern is a general, reusable solution to a commonly occurring problem in software architecture within a given context. The architectural patterns address various issues in software engineering, such as computer hardware performance limitations, high availability and minimization of a business risk.

Wikipedia: Architectural Pattern

You can think of an Architectural Pattern as a sort of “template” that you can use as a first step when designing the architecture of your system or application; it is not, in and of itself, an architecture. Rather, an architectural pattern is generally considered “strictly described and commonly available”. They’re designed to be broad and represent high level solutions to general software engineering problems that are reoccurring.

Just like there are many different “styles” of Building Architecture (i.e. Classical, Industrial, Victorian, Tudor, Art Deco, ect.) Software Architecture has “Patterns”.

Why use an established Architectural Pattern?

It’s good to learn from your mistakes. It’s better to learn from other people’s mistakes.

Warren Buffett, CEO of Berkshire Hathaway

Like I said before, an architectural pattern is a starting point; a template. Starting with the model that most closely fits your project’s needs has advantages:

  • More optimized systems – by using architectural patterns, we build transferrable models that can be reused, thus making them scalable and extensible.
  • Early design changes – most architectural patterns are flexible and provide you the opportunity to examine your project holistically so that you can work out errors or fundamental changes that need to be made before technical debt is accrued.
  • Simplification – not just for your sake but for the sake of collaboration among all the stakeholders involved. The faster stakeholders can form a mutual understanding, the faster communication, negotiation, and consensus. Obfuscation never solved anything.

Common Architectural Patterns

Below are just some of today’s most commonly used patterns:

  • Layered
  • Multi-Tier
  • Pipe and Filter
  • Client Server
  • Event-Driven
  • Microservices

There are many many other architectural patterns out there and this only represents a small subset of those. I may cover some of these plus others in more detail in the future via a separate post.

Below are high level conceptual diagrams for each of the above.

Layered Architecture

Multi-Tiered Architecture

Pipe and Filter Architecture

Client Server Architecture

Event-Driven Architecture

Microservices Architecture