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C# Mask Optimization
Use a bit mask to optimize checking for duplicate lowercase letters.Mask optimization. An int has 32 bits. Each bit can be set separately. Because of this, it can be used as a simple array of boolean values. In certain string processing tasks, this mask optimization yields good results.
Note: With this optimization, we can avoid handling duplicate letters by keeping track of ones already encountered.
OptimizationExample. The goal of this program is to detect duplicate letters in a string of lowercase chars. To do this, it uses a mask integer. The lowercase letters are converted to integers 0 to 25 by subtracting 97 from their ASCII representation.
Then: As each letter is encountered, its bit is tested. If it was already encountered, its bit will be set to 1.
Otherwise: We record the letter in the mask. The next time it is encountered we will recognize it.
C# program that checks duplicate letters
using System;
class Program
{
static void Main()
{
Console.WriteLine("Codex");
ReportDuplicateCharacters("Codex");
Console.WriteLine("massive");
ReportDuplicateCharacters("massive");
Console.WriteLine("mississippi");
ReportDuplicateCharacters("mississippi");
}
static void ReportDuplicateCharacters(string value)
{
int mask = 0;
for (int i = 0; i < value.Length; i++)
{
int index = value[i] - 97;
if ((mask & (1 << index)) != 0)
{
Console.WriteLine("Duplicate: {0}", value[i]);
}
else
{
mask |= (1 << index);
}
}
// To zero a bit: mask &= ~(1 << index);
}
}
Output
Codex
massive
Duplicate: s
mississippi
Duplicate: s
Duplicate: i
Duplicate: s
Duplicate: s
Duplicate: i
Duplicate: p
Duplicate: i
Discussion. My algorithm only requires one pass through the string. In testing I found that an array of bools is about as fast. It is possible to re-use the memory of an array of bools and get similar performance to this bitmask approach.
But: The mask approach here is more memory-efficient should you need to store this data somehow.
Summary. This style of bit-testing algorithm has a limited range of use in programming. Sometimes it is effective. Usually it is not worthwhile. I have rewritten this particular algorithm repeatedly—which means it's probably worth documenting.
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- C# Stopwatch Examples
- C# Stream
- C# StreamReader ReadToEnd Example (Read Entire File)
- C# StreamReader ReadToEndAsync Example (Performance)
- C# StreamReader Examples
- C# StreamWriter Examples
- C# String Append (Add Strings)
- C# String Compare and CompareTo Methods
- C# String Constructor (new string)
- C# string.Copy Method
- C# CopyTo String Method: Put Chars in Array
- C# Empty String Examples
- C# String Equals Examples
- C# String For Loop, Count Spaces in String
- C# string.Intern and IsInterned
- C# String IsUpper, IsLower
- C# String Length Property: Get Character Count
- C# String Literal: Newline and Quote Examples
- C# StringBuilder Capacity
- C# StringBuilder Clear (Set Length to Zero)
- C# StringBuilder Data Types
- C# StringBuilder Performance
- C# StringBuilder Equals (If Chars Are Equal)
- C# StringBuilder Memory
- C# StringBuilder ToString: Get String From StringBuilder
- C# StringWriter Class
- C# Sum Method: Add up All Numbers
- C# Switch Char, Test Chars With Cases
- C# Switch Enum
- C# System (using System namespace)
- C# Tag Property: Windows Forms
- C# TextInfo Examples
- C# TextReader, Returned by File.OpenText
- C# TextWriter, Returned by File.CreateText
- C# this Keyword
- C# ThreadPool
- C# Thread Join Method (Join Array of Threads)
- C# ThreadPool.SetMinThreads Method
- C# TimeZone Examples
- C# Get Title From HTML With Regex
- C# ToArray Extension Method
- C# ToCharArray: Convert String to Array
- C# ToDictionary Method
- C# Token
- C# ToList Extension Method
- C# ToLookup Method (Get ILookup)
- C# ToLower and ToUpper: Uppercase and Lowercase Strings
- ToolStripContainer Control: Dock, Properties
- C# ToolTip: Windows Forms
- C# ToString Integer Optimization
- C# ToString: Get String From Object
- C# ToTitleCase Method
- C# TrackBar: Windows Forms
- C# Tree and Nodes Example: Directed Acyclic Word Graph
- C# TreeView Tutorial
- C# Trim Strings
- C# Thread Methods
- C# History
- C# Features
- C# Variables
- C# Data Types
- C# Operators
- C# Keywords
- C# New Features | C# Version Features
- C# Programs
- C# Program to swap numbers without third variable
- C# Program to convert Decimal to Binary
- C# Program to Convert Number in Characters
- C# Program to Print Alphabet Triangle
- C# Program to print Number Triangle
- C# Program to generate Fibonacci Triangle
- C# String Compare() method
- C# String CompareOrdinal() method
- C# String CompareTo() method
- C# String Concat() method
- C# String Contains() method
- C# String CopyTo() method
- C# String EndsWith() method
- C# String Equals() method
- C# String Format() method
- C# String IndexOf() method
- C# String Insert() method
- C# String Intern(String str) method
- C# String IsInterned() method
- C# String Normalize() method
- C# String IsNullOrEmpty() method
- C# String IsNullOrWhiteSpace() method
- C# String Join() method
- C# String LastIndexOf() method
- C# String LastIndexOfAny() method
- C# String PadLeft() method
- C# String PadRight() method
- C# Nullable
- C# String Remove() method
- C# String Replace() method
- C# String Split() method
- C# String StartsWith() method
- C# String SubString() method
- C# Partial Types
- C# Iterators
- C# Delegate Covariance
- C# Delegate Inference
- C# Anonymous Types
- C# Extension Methods
- C# Query Expression
- C# Partial Method
- C# Implicitly Typed Local Variable
- C# Object and Collection Initializer
- C# Auto Implemented Properties
- C# Dynamic Binding
- C# Named and Optional Arguments
- C# Asynchronous Methods
- C# Caller Info Attributes
- C# Using Static Directive
- C# Exception Filters
- C# Await in Catch Finally Blocks
- C# Default Values for Getter Only Properties
- C# Expression Bodied Members
- C# Null Propagator
- C# String Interpolation
- C# nameof operator
- C# Dictionary Initializer
- C# Pattern Matching
- C# Tuples
- C# Deconstruction
- C# Local Functions
- C# Binary Literals
- C# Ref Returns and Locals
- C# Expression Bodied Constructors and Finalizers
- C# Expression Bodied Getters and Setters
- C# Async Main
- C# Default Expression