Ecosyste.ms: Awesome

An open API service indexing awesome lists of open source software.

Awesome Lists | Featured Topics | Projects

https://github.com/mcraiha/csharp-nth_element

CSharp (C#) versions of std::nth_element (or almost)
https://github.com/mcraiha/csharp-nth_element

csharp nthelement

Last synced: 19 days ago
JSON representation

CSharp (C#) versions of std::nth_element (or almost)

Awesome Lists containing this project

README 

        
# CSharp-nth_element

CSharp (C#) versions of std::nth_element (or almost)



## Status

[![.NET](https://github.com/mcraiha/CSharp-nth_element/actions/workflows/dotnet-tests.yml/badge.svg)](https://github.com/mcraiha/CSharp-nth_element/actions/workflows/dotnet-tests.yml)



## Introduction to this project

Since C# doesn't have build-in replacement for std::nth_element, I created few versions from existing implementations. **WARNING** All error handling code has been stripped away, so YMMV



## Introduction to std::nth_element

[std::nth_element](http://www.cplusplus.com/reference/algorithm/nth_element/) does two things if you use it to **array** with index **K**:



1. item in index K is same as it would be if array was sorted

2. Items before index K are <= or >= and items after K are >= or <= based on your sorting method



e.g. we have input array of ints (5 4 8 9 1 6 3 2 7) and we want to find out the 4th smallest number from that array. With std::nth_element using 4 as K, the output array would be (2 1 3 **4** 9 6 8 5 7). Fourth element is 4, and elements before it are smaller than 4, elements after it are larger than 4. 



Since there are multiple ways to implement std::nth_element, the outputs can vary. e.g. for our earlier example (5 4 8 9 1 6 3 2 7) with K as 4, another valid output would be (1 2 3 **4** 9 7 8 5 6). Output can also be fully sorted (1 2 3 **4** 5 6 7 8 9), but that is **NEVER** guaranteed for non trival sized arrays.



There two main uses for std::nth_element:

* Find nth largest/smallest item from array (useful if you need e.g. find the median value from array)

* Give helping hand to sorting algorithms by splitting their work



## Differences between C++ and C# versions

C++ version uses iterators. C# versions presented in here use zero based indexes (e.g. [0] means first item in array).



## Implementations

First version (**nthelement-PD.cs**) comes from **Adam Horvath**. His [blog](http://blog.teamleadnet.com/2012/07/quick-select-algorithm-find-kth-element.html) has an Java example which uses Quick select algorithm.



Second one (**nthelement-GPLv2.cs**) follows the [C source code](http://sourcecodebrowser.com/cdo/1.4.0.1~dfsg/nth__element_8c_source.html) given in CDO project.



## Examples

You have following C++ line of code

```c++

std::nth_element(myvector.begin(), myvector.begin() + 4, myvector.end());

```

you would replace it with

```cs

PartialSort.nth_element(array: myArray, startIndex: 0, nthSmallest: 4, endIndex: myArray.Length - 1);

```



If you want to use custom comparer (like descending order), follow this example

```cs

PartialSort.nth_element(array: arr, startIndex: 0, nthToSeek: 10, endIndex: arr.Length - 1, (i1, i2) => i2.CompareTo(i1));

```



## Benchmarks



You can run benchmarks (which compares both implementations) by moving to **benchmarks** folder and running following command

```bash

dotnet run -c Release

```



there are three different input sizes (16 bytes, 64 bytes and 1024 bytes)



## Licenses

This text file (**README.md**) is licensed under Creative Commons Zero (CC0 1.0 Universal), see [LICENSE](https://github.com/mcraiha/CSharp-nth_element/blob/master/LICENSE) file



**nthelement-PD.cs** is released into the public domain. See [PUBLICDOMAIN](https://github.com/mcraiha/CSharp-nth_element/blob/master/PUBLICDOMAIN) file



**nthelement-GPLv2.cs** is licensed under GPLv2, see [COPYING](https://github.com/mcraiha/CSharp-nth_element/blob/master/COPYING) file