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Fundamental programming with ruby examples

This is repo keeps examples with description modern principles, patterns.

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Ruby Gotchas

Unusual behavior of the ruby - a small details that hide from our sight for hours of hardcore debugging.

by Dave Aronson

Meta Programming

Metaprogramming is the writing of computer programs that write or manipulate other programs (or themselves) as their data, or that do part of the work at compile time that would otherwise be done at runtime. In many cases, this allows programmers to get more done in the same amount of time as they would take to write all the code manually, or it gives programs greater flexibility to efficiently handle new situations without recompilation. Or, more simply put: Metaprogramming is writing code that writes code during runtime to make your life easier.

Functional Programming

Using a language in a functional style implies you have access to a few key features:

  • Immutable values: once a “variable” is set, it cannot be changed. In Ruby, this means you effectively have to treat variables like constants.
  • No side-effects: when passed a given value, a function must always return the same result. This goes hand in hand with having immutable values; a function can never take a value and change it, as this would be causing a side-effect that is tangential to returning a result.
  • Higher-order functions: these are functions that allow functions as arguments, or use functions as the return value. This is, arguably, one of the most critical features of any functional language.
  • Currying: enabled by higher-order functions, currying is transforming a function that takes multiple arguments into a function that takes one argument. This goes hand in hand with partial function application, which is transforming a multi-argument function into a function that takes less arguments then it did originally.
  • Recursion: looping by calling a function from within itself. When you don’t have access to mutable data, recursion is used to build up and chain data construction. This is because looping is not a functional concept, as it requires variables to be passed around to store the state of the loop at a given time.
  • Lazy-evaluation, or delayed-evaluation: delaying processing of values until the moment when it is actually needed. If, as an example, you have some code that generated list of Fibonacci numbers with lazy-evaluation enabled, this would not actually be processed and calculated until one of the values in the result was required by another function, such as puts.


Note about parallelism and concurrency: The primary difference between using processes versus threads is the way that memory is handled. At a high level, processes copy memory, while threads share memory. This makes process spawning slower than thread spawning, and leads to processes consuming more resources once running. Overall, threads incur less overhead than processes. This Thread API is a Ruby API. I've hinted that the different Ruby implementations have different underlying threading behaviours.

Green Threads

Ruby 1.9 replaced green threads with native threads. However, the GIL is still preventing parallelism. That being said, concurrency has been improved through better scheduling. The new scheduler makes context-switch decisions more efficient, by essentially moving them to a separate native thread, known as the timer thread.

GIL - Global Interpreter Lock

MRI has a global interpreter lock (GIL). It's a lock around the execution of Ruby code. This means that in a multi-threaded context, only one thread can execute Ruby code at any one time.

So if you have 8 threads busily working on a 8-core machine, only one thread and one core will be busy at any given time. The GIL exists to protect Ruby internals from race conditions that could corrupt data. There are caveats and optimizations, but this is the gist.

This has some very important implications for MRI. The biggest implication is that Ruby code will never run in parallel on MRI. The GIL prevents it.

See example

Mutex - Mutual Execution

Mutexes provide a mechanism for multiple threads to synchronize access to a critical portion of code. In other words, they help bring some order, and some guarantees, to the world of multi-threaded chaos.

The name 'mutex' is shorthand for 'mutual exclusion.' If you wrap some section of your code with a mutex, you guarantee that no two threads can enter that section at the same time.

See example


Fibers are primitives for implementing light weight cooperative concurrency in Ruby. Basically they are a means of creating code blocks that can be paused and resumed, much like threads. The main difference is that they are never preempted and that the scheduling must be done by the programmer and not the VM. As opposed to other stackless light weight concurrency models, each fiber comes with a small 4KB stack. This enables the fiber to be paused from deeply nested function calls within the fiber block.

See example

Rails - Thread-safety in the framework

The problem with this is that there is no simple way to say with absolute certainty whether an app as a whole is thread-safe.

  • Global variables are global. This means that they are shared between threads. If you weren’t convinced about not using global variables by now, here’s another reason to never touch them. If you really want to share something globally across an app, you are more than likely better served by a constant (but see below), anyway.

  • Class variables. For the purpose of a discussion about threads, class variables are not much different from global variables. They are shared across threads just the same way.The problem isn’t so much about using class variables, but about mutating them. And if you are not going to mutate a class variable, in many cases a constant is again a better choice.

  • Class instance variables. But maybe you’ve read that you should always use class instance variables instead of class variables in Ruby. Well, maybe you should, but they are just as problematic for threaded programs as class variables.

  • Memoization by itself is not a thread safety issue. However, it can easily become one for a couple of reasons:

    • It is often used to store data in class variables or class instance variables (see the previous points).
    • The ||= operator is in fact two operations, so there is a potential context switch happening in the middle of it, causing a race condition between threads.
    • It would be easy to dismiss memoization as the cause of the problem, and tell people just to avoid class variables and class instance variables. However, the issue is more complex than that.
    • In this issue, Evan Phoenix squashes a really tricky race condition bug in the Rails codebase caused by calling super in a memoization function. So even though you would only be using instance variables, you might end up with race conditions with memoization.

Make sure memoization makes sense and a difference in your case. In many cases Rails actually caches the result anyway, so that you are not saving a whole lot if any resources with your memoization method. Don’t memoize to class variables or class instance variables. If you need to memoize something on the class level, use thread local variables (Thread.current[:baz]) instead. Be aware, though, that it is still kind of a global variable. So while it’s thread-safe, it still might not be good coding practice.

See example

Code and articles were taken from resources:


In computer programming, SOLID (single responsibility, open-closed, Liskov substitution, interface segregation and dependency inversion) is a mnemonic acronym introduced by Michael Feathers for the "first five principles" named by Robert C. Martin in the early 2000s that stands for five basic principles of object-oriented programming and design. The intention is that these principles, when applied together, will make it more likely that a programmer will create a system that is easy to maintain and extend over time. The principles of SOLID are guidelines that can be applied while working on software to remove code smells by causing the programmer to refactor the software's source code until it is both legible and extensible. It is part of an overall strategy of agile and Adaptive Software Development.

SRP - Single responsibility principle

A class should have only a single responsibility.

Every class should have a single responsibility, and that responsibility should be entirely encapsulated. All its services should be narrowly aligned with that responsibility, this embrace the high cohesion.

See example

OCP - Open/closed principle

Software entities should be open for extension, but closed for modification. That is, such an entity can allow its behaviour to be extended without modifying its source code.

See example

LSP - Liskov substitution principle

Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.

It states that, in a computer program, if S is a subtype of T, then objects of type T may be replaced with objects of type S (i.e., objects of type S may substitute objects of type T) without altering any of the desirable properties of that program (correctness, task performed, etc.)

See example

ISP - Interface segregation principle

Many client-specific interfaces are better than one general-purpose interface.

States that no client should be forced to depend on methods it does not use. ISP splits interfaces which are very large into smaller and more specific ones so that clients will only have to know about the methods that are of interest to them. Such shrunken interfaces are also called role interfaces. ISP is intended to keep a system decoupled and thus easier to refactor, change, and redeploy.

See example

DIP - Dependency inversion principle

One should Depend upon Abstractions, Do not depend upon concretions.

Refers to a specific form of decoupling software modules. When following this principle, the conventional dependency relationships established from high-level, policy-setting modules to low-level, dependency modules are inverted (i.e. reversed), thus rendering high-level modules independent of the low-level module implementation details.

See example

Code and articles were taken from resources:

Design patterns

Creational pattern

In software engineering, creational design patterns are design patterns that deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. The basic form of object creation could result in design problems or in added complexity to the design. Creational design patterns solve this problem by somehow controlling this object creation. Creational design patterns are composed of two dominant ideas. One is encapsulating knowledge about which concrete classes the system uses. Another is hiding how instances of these concrete classes are created and combined.

Read wiki

Abstract factory pattern

The abstract factory pattern provides a way to encapsulate a group of individual factories that have a common theme without specifying their concrete classes. In normal usage, the client software creates a concrete implementation of the abstract factory and then uses the generic interface of the factory to create the concrete objects that are part of the theme. The client doesn't know (or care) which concrete objects it gets from each of these internal factories, since it uses only the generic interfaces of their products. This pattern separates the details of implementation of a set of objects from their general usage and relies on object composition, as object creation is implemented in methods exposed in the factory interface.

See example | Read wiki

Builder pattern

The builder pattern is an object creation software design pattern. Unlike the abstract factory pattern and the factory method pattern whose intention is to enable polymorphism, the intention of the builder pattern is to find a solution to the telescoping constructor anti-pattern[citation needed]. The telescoping constructor anti-pattern occurs when the increase of object constructor parameter combination leads to an exponential list of constructors. Instead of using numerous constructors, the builder pattern uses another object, a builder, that receives each initialization parameter step by step and then returns the resulting constructed object at once.

See example | Read wiki

Factory pattern

In class-based programming, the factory method pattern is a creational pattern that uses factory methods to deal with the problem of creating objects without having to specify the exact class of the object that will be created. This is done by creating objects by calling a factory method—either specified in an interface and implemented by child classes, or implemented in a base class and optionally overridden by derived classes—rather than by calling a constructor.

See example | Read wiki

Prototype pattern

The prototype pattern is a creational design pattern in software development. It is used when the type of objects to create is determined by a prototypical instance, which is cloned to produce new objects. This pattern is used to:

  • avoid subclasses of an object creator in the client application, like the abstract factory pattern does.
  • avoid the inherent cost of creating a new object in the standard way (e.g., using the 'new' keyword) when it is prohibitively expensive for a given application.

See example | Read wiki

Singleton pattern

Ensure a class only has one instance, and provide a global point of access to it. This is useful when exactly one object is needed to coordinate actions across the system. The concept is sometimes generalized to systems that operate more efficiently when only one object exists, or that restrict the instantiation to a certain number of objects.

See example | Read wiki

Not covered patterns:

Structural pattern

In software engineering, structural design patterns are design patterns that ease the design by identifying a simple way to realize relationships between entities.

Read wiki

Adapter pattern

In software engineering, the adapter pattern is a software design pattern that allows the interface of an existing class to be used as another interface. It is often used to make existing classes work with others without modifying their source code.

See example | Read wiki

Composite pattern

The composite design pattern is a structural pattern used to represent objects that have a hierarchical tree structure. It allows for the uniform treatment of both individual leaf nodes and of branches composed of many nodes.

See example | Read wiki

Decorator pattern

In object-oriented programming, the decorator pattern (also known as Wrapper, an alternative naming shared with the Adapter pattern) is a design pattern that allows behavior to be added to an individual object, either statically or dynamically, without affecting the behavior of other objects from the same class. The decorator pattern is often useful for adhering to the Single Responsibility Principle, as it allows functionality to be divided between classes with unique areas of concern.

See example | Read wiki

Facade pattern

The Facade design pattern is often used when a system is very complex or difficult to understand because the system has a large number of interdependent classes or its source code is unavailable. This pattern hides the complexities of the larger system and provides a simpler interface to the client. It typically involves a single wrapper class which contains a set of members required by client. These members access the system on behalf of the facade client and hide the implementation details.

See example | Read wiki

Flyweight pattern

In computer programming, flyweight is a software design pattern. A flyweight is an object that minimizes memory use by sharing as much data as possible with other similar objects; it is a way to use objects in large numbers when a simple repeated representation would use an unacceptable amount of memory. Often some parts of the object state can be shared, and it is common practice to hold them in external data structures and pass them to the flyweight objects temporarily when they are used.

See example | Read wiki

Proxy pattern

A proxy, in its most general form, is a class functioning as an interface to something else. The proxy could interface to anything: a network connection, a large object in memory, a file, or some other resource that is expensive or impossible to duplicate. In short, a proxy is a wrapper or agent object that is being called by the client to access the real serving object behind the scenes. Use of the proxy can simply be forwarding to the real object, or can provide additional logic. In the proxy extra functionality can be provided, for example caching when operations on the real object are resource intensive, or checking preconditions before operations on the real object are invoked. For the client, usage of a proxy object is similar to using the real object, because both implement the same interface.

See example | Read wiki

Not covered patterns:

Behavioral pattern

In software engineering, behavioral design patterns are design patterns that identify common communication patterns between objects and realize these patterns. By doing so, these patterns increase flexibility in carrying out this communication.

Read wiki

Chain of responsibility pattern

In object-oriented design, the chain-of-responsibility pattern is a design pattern consisting of a source of command objects and a series of processing objects. Each processing object contains logic that defines the types of command objects that it can handle; the rest are passed to the next processing object in the chain. A mechanism also exists for adding new processing objects to the end of this chain.

See example | Read wiki

Command pattern

Encapsulate a request as an object, thereby allowing for the parameterization of clients with different requests, and the queuing or logging of requests. It also allows for the support of undoable operations.

See example | Read wiki

Interpreter pattern

In computer programming, the interpreter pattern is a design pattern that specifies how to evaluate sentences in a language. The basic idea is to have a class for each symbol (terminal or nonterminal) in a specialized computer language. The syntax tree of a sentence in the language is an instance of the composite pattern and is used to evaluate (interpret) the sentence for a client.

See example | Read wiki

Iterator pattern

The iterator design pattern provides sequential access to elements within a container without exposing how the container actually represents the elements. The iterator can be thought of as a moveable pointer that allows access to elements encapsulated within a container.

  • External iterator: The iteration logic is contained in a separate class. The iteration class can be generalized to handle multiple object types as long as they allow indexing. It require the additional class to do the actual iterating, but they do allow for greater flexibility because you can control the iteration, which elements are iterated over and in what order.

  • Internal iterator: all the iterating logic occurs inside the aggregate object. Use a code block to pass your logic into the aggregate which then calls the block for each of it's elements.

See example | Read wiki

Mediator pattern

Usually a program is made up of a large number of classes. So the logic and computation is distributed among these classes. However, as more classes are developed in a program, especially during maintenance and/or refactoring, the problem of communication between these classes may become more complex. This makes the program harder to read and maintain. Furthermore, it can become difficult to change the program, since any change may affect code in several other classes. With the mediator pattern, communication between objects is encapsulated with a mediator object. Objects no longer communicate directly with each other, but instead communicate through the mediator. This reduces the dependencies between communicating objects, thereby lowering the coupling.

See example | Read wiki

Memento pattern

The memento pattern is implemented with three objects: the originator, a caretaker and a memento. The originator is some object that has an internal state. The caretaker is going to do something to the originator, but wants to be able to undo the change. The caretaker first asks the originator for a memento object. Then it does whatever operation (or sequence of operations) it was going to do. To roll back to the state before the operations, it returns the memento object to the originator. The memento object itself is an opaque object (one which the caretaker cannot, or should not, change). When using this pattern, care should be taken if the originator may change other objects or resources - the memento pattern operates on a single object.

See example | Read wiki

Observer pattern

The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. It is mainly used to implement distributed event handling systems. The Observer pattern is also a key part in the familiar model–view–controller (MVC) architectural pattern. The observer pattern is implemented in numerous programming libraries and systems, including almost all GUI toolkits.

See example | Read wiki

State pattern

The state pattern is a behavioral software design pattern that implements a state machine in an object-oriented way. With the state pattern, a state machine is implemented by implementing each individual state as a derived class of the state pattern interface, and implementing state transitions by invoking methods defined by the pattern's superclass.

See example | Read wiki

Strategy pattern

In computer programming, the strategy pattern (also known as the policy pattern) is a software design pattern that enables an algorithm's behavior to be selected at runtime. The strategy pattern

  • defines a family of algorithms,
  • encapsulates each algorithm, and
  • makes the algorithms interchangeable within that family.

Strategy lets the algorithm vary independently from clients that use it. Strategy is one of the patterns included in the influential book Design Patterns by Gamma et al. that popularized the concept of using patterns to describe software design.

For instance, a class that performs validation on incoming data may use a strategy pattern to select a validation algorithm based on the type of data, the source of the data, user choice, or other discriminating factors. These factors are not known for each case until run-time, and may require radically different validation to be performed. The validation strategies, encapsulated separately from the validating object, may be used by other validating objects in different areas of the system (or even different systems) without code duplication.

See example | Read wiki

Template method pattern

In object-oriented programming, first a class is created that provides the basic steps of an algorithm design. These steps are implemented using abstract methods. Later on, subclasses change the abstract methods to implement real actions. Thus the general algorithm is saved in one place but the concrete steps may be changed by the subclasses.

See example | Read wiki

Visitor pattern

n object-oriented programming and software engineering, the visitor design pattern is a way of separating an algorithm from an object structure on which it operates. A practical result of this separation is the ability to add new operations to existing object structures without modifying those structures. It is one way to follow the open/closed principle.

In essence, the visitor allows one to add new virtual functions to a family of classes without modifying the classes themselves; instead, one creates a visitor class that implements all of the appropriate specializations of the virtual function. The visitor takes the instance reference as input, and implements the goal through double dispatch.

See example | Read wiki

Not covered patterns:
Code and articles were taken from resources:

Data structures

In computer science, big O notation is used to classify algorithms by how they respond to changes in input size, such as how the processing time of an algorithm changes as the problem size becomes extremely large. In analytic number theory it is used to estimate the "error committed" while replacing the asymptotic size of an arithmetical function by the value it takes at a large finite argument. A famous example is the problem of estimating the remainder term in the prime number theorem.

Basic axioms of data structures

The running time performance of the common language runtime is given by a set of axioms which we shall now postulate.

See description with examples

Big O notation



The stack is the sibling of the queue. It mimicks a real-life stack (e.g. of paper). It is FILO (first-in-last-out), so that when items are retrieved from the stack, they are returned in the reverse of the order in which they were added. Again, Ruby Arrays provide a perfect container. As with the Queue, it could also be implemented using a linked list.

Structure Average Access Average Search Average Insertion Average Deletion Worst Access Worst Search Worst Insertion Worst Deletion
Stack Θ(n) Θ(n) Θ(1) Θ(1) O(n) O(n) O(1) O(1)

See stack as array example

See stack as linked list example


A queue is a simple container-based structure that mimics a real-life queue (e.g. waiting in line at the bank). It is FIFO (first-in-first-out), meaning that when you retrieve items from the queue, they are returned in the order in which they entered. Ruby Arrays provide methods that make Queue implementation trivially easy, but having them named appropriately and contained in a convenience class is worth it to see how they are implemented, and because other structures will inherit from this one. An alternate implementation could be done using a linked list.

Structure Average Access Average Search Average Insertion Average Deletion Worst Access Worst Search Worst Insertion Worst Deletion
Queue Θ(n) Θ(n) Θ(1) Θ(1) O(n) O(n) O(1) O(1)

See queue as array example

See queue as linked list example


A Deque is a queue which allows adding and removing items at both ends.

Structure Average Access Average Search Average Insertion Average Deletion Worst Access Worst Search Worst Insertion Worst Deletion
Deque Θ(n) Θ(n) Θ(1) Θ(1) O(n) O(n) O(1) O(1)

See queue as array example

See queue as linked list example

Singly Linked List

Singly linked lists contain nodes which have a data field as well as a 'next' field, which points to the next node in line of nodes. Operations that can be performed on singly linked lists include insertion, deletion and traversal.

Structure Average Access Average Search Average Insertion Average Deletion Worst Access Worst Search Worst Insertion Worst Deletion
Singly-Linked List Θ(n) Θ(n) Θ(1) Θ(1) O(n) O(n) O(1) O(1)

See example | Read wiki

Double linked list

In a doubly-linked list , each list element contains two references--one to its successor and one to its predecessor.

Structure Average Access Average Search Average Insertion Average Deletion Worst Access Worst Search Worst Insertion Worst Deletion
Doubly-Linked List Θ(n) Θ(n) Θ(1) Θ(1) O(n) O(n) O(1) O(1)

See example

Ordered list

An ordered list is a list in which the order of the items is significant. However, the items in an ordered lists are not necessarily sorted. Consequently, it is possible to change the order of items and still have a valid ordered list.

Structure Average Access Average Search Average Insertion Average Deletion Worst Access Worst Search Worst Insertion Worst Deletion
OrderedListAsArray Θ(1) Θ(n) Θ(1) Θ(1) O(1) O(n) O(1) O(1)
OrderedListAsLinkedList Θ(n) Θ(n) Θ(1) Θ(1) O(n) O(n) O(1) O(1)

See ordered list as array example

Hash Table

A hash table is a searchable container. As such, it provides methods for putting an object into the container, finding an object in the container, and removing an object from the container.

Structure Average Access Average Search Average Insertion Average Deletion Worst Access Worst Search Worst Insertion Worst Deletion
Hash Table Θ(n) Θ(1) Θ(1) Θ(1) Θ(n) O(n) O(n) O(n)

See example

Binary Tree

A binary tree is a tree in which each node can have a maximum of two children. The children are designated left and right.

Structure Average Access Average Search Average Insertion Average Deletion Worst Access Worst Search Worst Insertion Worst Deletion
Binary Tree Θ(log(n)) Θ(log(n)) Θ(log(n)) Θ(log(n)) O(n) O(n) O(n) O(n)

See example

Binary Search Tree

In computer science, binary search trees (BST), sometimes called ordered or sorted binary trees, are a particular type of containers: data structures that store "items" (such as numbers, names etc.) in memory. They allow fast lookup, addition and removal of items, and can be used to implement either dynamic sets of items, or lookup tables that allow finding an item by its key (e.g., finding the phone number of a person by name)

Structure Average Access Average Search Average Insertion Average Deletion Worst Access Worst Search Worst Insertion Worst Deletion
Binary Search Tree Θ(log(n)) Θ(log(n)) Θ(log(n)) Θ(log(n)) O(n) O(n) O(n) O(n)

See example

B Tree

In computer science, a B-tree is a self-balancing tree data structure that keeps data sorted and allows searches, sequential access, insertions, and deletions in logarithmic time. The B-tree is a generalization of a binary search tree in that a node can have more than two children (Comer . Unlike self-balancing binary search trees, the B-tree is optimized for systems that read and write large blocks of data. B-trees are a good example of a data structure for external memory. It is commonly used in databases and filesystems.

Structure Average Access Average Search Average Insertion Average Deletion Worst Access Worst Search Worst Insertion Worst Deletion
B-Tree Θ(log(n)) Θ(log(n)) Θ(log(n)) O(log(n)) O(log(n)) O(log(n)) O(log(n)) O(log(n))

See example

Binary Heap

A binary heap is a heap-ordered complete binary tree which is implemented using an array. In a heap the smallest key is found at the root and since the root is always found in the first position of the array, finding the smallest key is a trivial operation in a binary heap.

Structure Average Access Average Search Average Insertion Average Deletion Worst Access Worst Search Worst Insertion Worst Deletion
Binary Heap Θ(n) Θ(n) Θ(1) O(log(n)) O(n) O(n)) O(log(n)) O(log(n))

See example

Code and articles were taken from resources:



A sorting algorithm is an algorithm that puts elements of a list in a certain order. The most-used orders are numerical order and lexicographical order. Efficient sorting is important for optimizing the use of other algorithms (such as search and merge algorithms) which require input data to be in sorted lists; it is also often useful for canonicalizing data and for producing human-readable output.

Bubble sort

Bubble sort has many of the same properties as insertion sort, but has slightly higher overhead. In the case of nearly sorted data, bubble sort takes O(n) time, but requires at least 2 passes through the data (whereas insertion sort requires something more like 1 pass).

Best Average Worst
Ω(n) Θ(n^2) O(n^2)

See example | Read wiki

Insertion sort

Although it is one of the elementary sorting algorithms with O(n2) worst-case time, insertion sort is the algorithm of choice either when the data is nearly sorted (because it is adaptive) or when the problem size is small (because it has low overhead). For these reasons, and because it is also stable, insertion sort is often used as the recursive base case (when the problem size is small) for higher overhead divide-and-conquer sorting algorithms, such as merge sort or quick sort

Best Average Worst
Ω(n) Θ(n^2) O(n^2)

See example | Read wiki

Selection sort

From the comparions presented here, one might conclude that selection sort should never be used. It does not adapt to the data in any way (notice that the four animations above run in lock step), so its runtime is always quadratic. However, selection sort has the property of minimizing the number of swaps. In applications where the cost of swapping items is high, selection sort very well may be the algorithm of choice.

Best Average Worst
Ω(n^2) Θ(n^2) O(n^2)

See example | Read wiki

Shell sort

The worse-case time complexity of shell sort depends on the increment sequence. For the increments 1 4 13 40 121…, which is what is used here, the time complexity is O(n3/2). For other increments, time complexity is known to be O(n4/3) and even O(n·lg2(n)). Neither tight upper bounds on time complexity nor the best increment sequence are known. Because shell sort is based on insertion sort, shell sort inherits insertion sort’s adaptive properties. The adapation is not as dramatic because shell sort requires one pass through the data for each increment, but it is significant. For the increment sequence shown above, there are log3(n) increments, so the time complexity for nearly sorted data is O(n·log3(n)). Because of its low overhead, relatively simple implementation, adaptive properties, and sub-quadratic time complexity, shell sort may be a viable alternative to the O(n·lg(n)) sorting algorithms for some applications when the data to be sorted is not very large.

Best Average Worst
Ω(n log(n)) Θ(n(log(n))^2) O(n(log(n))^2)

See example | Read wiki

Heap sort

Heap sort is simple to implement, performs an O(n·lg(n)) in-place sort, but is not stable. The first loop, the Θ(n) “heapify” phase, puts the array into heap order. The second loop, the O(n·lg(n)) “sortdown” phase, repeatedly extracts the maximum and restores heap order. The sink function is written recursively for clarity. Thus, as shown, the code requires Θ(lg(n)) space for the recursive call stack. However, the tail recursion in sink() is easily converted to iteration, which yields the O(1) space bound. Both phases are slightly adaptive, though not in any particularly useful manner. In the nearly sorted case, the heapify phase destroys the original order. In the reversed case, the heapify phase is as fast as possible since the array starts in heap order, but then the sortdown phase is typical. In the few unique keys case, there is some speedup but not as much as in shell sort or 3-way quicksort.

Best Average Worst
Ω(n log(n)) Θ(n log(n)) O(n log(n))

See example | Read wiki

Merge sort

Merge sort is very predictable. It makes between 0.5lg(n) and lg(n) comparisons per element, and between lg(n) and 1.5lg(n) swaps per element. The minima are achieved for already sorted data; the maxima are achieved, on average, for random data. If using Θ(n) extra space is of no concern, then merge sort is an excellent choice: It is simple to implement, and it is the only stable O(n·lg(n)) sorting algorithm. Note that when sorting linked lists, merge sort requires only Θ(lg(n)) extra space (for recursion). Merge sort is the algorithm of choice for a variety of situations: when stability is required, when sorting linked lists, and when random access is much more expensive than sequential access (for example, external sorting on tape). There do exist linear time in-place merge algorithms for the last step of the algorithm, but they are both expensive and complex. The complexity is justified for applications such as external sorting when Θ(n) extra space is not available.

Best Average Worst
Ω(n log(n)) Θ(n log(n)) O(n log(n))

See example | Read wiki

Quick sort

When carefully implemented, quick sort is robust and has low overhead. When a stable sort is not needed, quick sort is an excellent general-purpose sort – although the 3-way partitioning version should always be used instead. The 2-way partitioning code shown above is written for clarity rather than optimal performance; it exhibits poor locality, and, critically, exhibits O(n2) time when there are few unique keys. A more efficient and robust 2-way partitioning method is given in Quicksort is Optimal by Robert Sedgewick and Jon Bentley. The robust partitioning produces balanced recursion when there are many values equal to the pivot, yielding probabilistic guarantees of O(n·lg(n)) time and O(lg(n)) space for all inputs. With both sub-sorts performed recursively, quick sort requires O(n) extra space for the recursion stack in the worst case when recursion is not balanced. This is exceedingly unlikely to occur, but it can be avoided by sorting the smaller sub-array recursively first; the second sub-array sort is a tail recursive call, which may be done with iteration instead. With this optimization, the algorithm uses O(lg(n)) extra space in the worst case.

Best Average Worst
Ω(n log(n)) Θ(n log(n)) O(n^2)

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Other sorting algorithms
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Binary search

In computer science, binary search, also known as half-interval search or logarithmic search, is a search algorithm that finds the position of a target value within a sorted array. It compares the target value to the middle element of the array; if they are unequal, the half in which the target cannot lie is eliminated and the search continues on the remaining half until it is successful.

Best Average Worst
Θ(1) Θ(n log(n)) O(log(n))

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Knuth-Morris-Pratt search

In computer science, the Knuth–Morris–Pratt string searching algorithm (or KMP algorithm) searches for occurrences of a "word" W within a main "text string" S by employing the observation that when a mismatch occurs, the word itself embodies sufficient information to determine where the next match could begin, thus bypassing re-examination of previously matched characters.

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Other search algorithms
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