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C# Generic List<T> update items

I'm using a List<T> and I need to update the objects properties that the list has.
What would be the most efficient/faster way to do this? I know that scanning through the index of a List<T> would be slower as this list grows and that the List<T> is not the most efficient collection to do updates.
That sad, would be better to:
Remove the match object then add a new one?
Scan through the list indexes until you find the matching object and then update the object's properties?
If I have a collection, let's IEnumerable and I want to update that IEnumerable into the List, what would be best approach.
Stub code sample:
public class Product
{
public int ProductId { get; set; }
public string ProductName { get; set; }
public string Category { get; set; }
}
public class ProductRepository
{
List<Product> product = Product.GetProduct();
public void UpdateProducts(IEnumerable<Product> updatedProduct)
{
}
public void UpdateProduct(Product updatedProduct)
{
}
}

You could consider using Dictionary instead of List if you want fast lookups. In your case it would be the product Id (which I am assuming is unique). Dictionary MSDN
For example:
public class ProductRepository
{
Dictionary<int, Product> products = Product.GetProduct();
public void UpdateProducts(IEnumerable<Product> updatedProducts)
{
foreach(var productToUpdate in updatedProducts)
{
UpdateProduct(productToUpdate);
}
///update code here...
}
public void UpdateProduct(Product productToUpdate)
{
// get the product with ID 1234
if(products.ContainsKey(productToUpdate.ProductId))
{
var product = products[productToUpdate.ProductId];
///update code here...
product.ProductName = productToUpdate.ProductName;
}
else
{
//add code or throw exception if you want here.
products.Add(productToUpdate.ProductId, productToUpdate);
}
}
}

Your use case is updating a List<T>, which can contains millions of records, and updated records can be a sub-list or just a single record
Following is the Schema:
public class Product
{
public int ProductId { get; set; }
public string ProductName { get; set; }
public string Category { get; set; }
}
Does Product contains a primary key, which means every Product object can be uniquely identified and there are no duplicates and every update target a single unique record?
If Yes, then it is best to arrange the List<T> in the form of Dictionary<int,T>, which would mean for an IEnumerable<T> every update would be an O(1) time complexity and that would mean all the updates could be done depending on the size of the IEnumerable<T>, which i don't expect to be very big and though there would be extra memory allocation of different data structure required, but would be a very fast solution.#JamieLupton has already provided a solution on similar lines
In case Product is repeated, there's no primary key, then above solution is not valid, then ideal way to scan through the List<T> is Binary Search, whose time complexity is O(logN)
Now since size of IEnumerable<T> is comparatively small say M, so the overall time complexity would be O(M*logN), where M is much smaller than N and can be neglected.
List<T> support Binary Search API, which provides the element index, which can then be used to update the object at relevant index, check example here
Best Option as per me for such a high number of records would be parallel processing along with binary search
Now since, thread safety is an issue, what I normally do is divide a List<T> into List<T>[], since then each unit can be assigned to a separate thread, a simple way is use MoreLinq batch Api, where you can fetch the number of system processors as using Environment.ProcessorCount and then create IEnumerable<IEnumerable<T>> as follows:
var enumerableList = List<T>.Batch(Environment.ProcessorCount).ToList();
Another way is following custom code:
public static class MyExtensions
{
// data - List<T>
// dataCount - Calculate once and pass to avoid accessing the property everytime
// Size of Partition, which can be function of number of processors
public static List<T>[] SplitList<T>(this List<T> data, int dataCount, int partitionSize)
{
int remainderData;
var fullPartition = Math.DivRem(dataCount, partitionSize, out remainderData);
var listArray = new List<T>[fullPartition];
var beginIndex = 0;
for (var partitionCounter = 0; partitionCounter < fullPartition; partitionCounter++)
{
if (partitionCounter == fullPartition - 1)
listArray[partitionCounter] = data.GetRange(beginIndex, partitionSize + remainderData);
else
listArray[partitionCounter] = data.GetRange(beginIndex, partitionSize);
beginIndex += partitionSize;
}
return listArray;
}
}
Now you can create Task[], where each Task is assigned for every element List<T>, on the List<T>[] generated above, then Binary search for each sub partition. Though its repetitive but would be using the power of Parallel processing and Binary search. Each Task can be started and then we can wait using Task.WaitAll(taskArray) to wait for Task processing to finish
Over and above that, if you want to create a Dictionary<int,T>[] and thus use parallel processing then this would be fastest.
Final integration of List<T>[] to List<T> can be done using Linq Aggregation or SelectMany as follows:
List<T>[] splitListArray = Fetch splitListArray;
// Process splitListArray
var finalList = splitListArray.SelectMany(obj => obj).ToList()
Another option would be to use Parallel.ForEach along with a thread safe data structure like ConcurrentBag<T> or may be ConcurrentDictionary<int,T> in case you are replacing complete object, but if its property update then a simple List<T> would work. Parallel.ForEach internally use range partitioner similar to what I have suggested above
Solutions mentioned above ideally depends on your use case, you shall be able to use combination to achieve the best possible result. Let me know, in case you need specific example

What exactly is efficiency?
Unless there are literally thousands of items doing a foreach, or for or any other type of looping operation will most likely only show differences in the milleseconds. Really? Hence you have wasted more time (in costs of a programmer at $XX per hour than an end user costs) trying to find that best.
So if you have literally thousands of records I would recommend that efficiency be found by parallel processing the list with the Parallel.Foreach method which can process more records to save time with the overhead of threading.
IMHO if the record count is greater than 100 it implies that there is a database being used. If a database is involved, write an update sproc and call it a day; I would be hard pressed to write a one-off program to do a specific update which could be done in an easier fashion in said database.

How should I remove elements from a generic list based on the list s object's inclusion of elementfrom another list in C# using predicate logic?

I am trying to learn C# by making a simple program that shows the user sushi rolls given their desired ingredients. i.e. a user wants a roll with crab, and the program will spit out a list of sushi rolls that contain crab.
I've created a Roll class
public class Roll
{
private string name;
private List<string> ingredients = new List<string>();
}
With some getters and setters and other various methods.
In the GUI, I have some checkboxes which each call an update() method from the Control class, which will then need to check a list of rolls against a list of ingredients given by the GUI checkboxes. What I have is this
class Controller
{
static List<Roll> Rolls = new List<Roll>();
static RollList RL = new RollList();
static List<String> ingredients = new List<String>();
static Roll roll = new Roll();
}
public void update
{
foreach(Roll roll in Rolls)
{
foreach (String ingredient in ingredients)
if (!roll.checkForIngredient(ingredient))
Rolls.Remove(roll);
}
}
But a System.InvalidOperationException is thrown saying that because the collection was modified, the operation can't execute. OK, that's fair, but then what's the best way to do this? Here on Stack Overflow there's a post about removing elements from a generic list while iterating over it.
This was good and pointed me in the right direction, but unfortunately, my predicate condition simply doesn't match the top answer's.
It would have to iterate over the ingredients list, and I'm not even sure that's possible...
list.RemoveAll(roll => !roll.containsIngredient(each string ingredient in ingredients) );
shudder
I've tried the for loop, but I can't seem to get the enumeration to work either, and I wonder if it's even necessary to enumerate the class for just this method.
So I come here to try and find an elegant, professional solution to my problem. Keep in mind that I'm new to C# and I'm not all too familiar with predicate logic or enumeration on classes.

To use RemoveAll you can rewrite your condition to this:
list.RemoveAll(roll => !ingredients.All(roll.checkForIngredient));
This exploits the fact that when the compiler sees this, it will effectively rewrite it to this:
list.RemoveAll(roll => !ingredients.All(i => roll.checkForIngredient(i)));
Which is what you want. If not all the ingredients are present, remove the roll.
Now, having said that, since you say you're a beginner, perhaps you feel more comfortable keeping your loop, if you could just make it work (ie. stop crashing due to modifying the loop). To do that, just make a copy of the collection and then loop through the copy, you can do this by just modifying the foreach statement to this:
foreach(Roll roll in Rolls.ToList())
This will create a list based copy of the Rolls collection, and then loop on that. The list will not be modified, even if Rolls is, it is a separate copy containing all the elements of Rolls when it was created.
As requested in the comments, I'll try to explain how this line of code works:
list.RemoveAll(roll => !ingredients.All(roll.checkForIngredient));
The RemoveAll method, which you can see the documentation for here takes a predicate, a Predicate<T>, which is basically a delegate, a reference to a method.
This can be a lambda, syntax that creates an anonymous method, using the => operator. An anonymous method is basically a method declared where you want to use it, without a name, hence the anonymous part. Let's rewrite the code to use an anonymous method instead of a lambda:
list.RemoveAll(delegate(Roll roll)
{
return !ingredients.All(roll.checkForIngredient);
});
This is the exact same compiled code as for the lambda version above, just using the bit more verbose syntax of an anonymous method.
So, how does the code inside the method work.
The All method is an extension method, found on the Enumerable class: Enumerable.All.
It will basically loop through all the elements of the collection it is extending, in this case the ingredients collection of a single roll, and call the predicate function. If for any of the elements the predicate returns false, the result of calling All will also be false. If all the calls return true, the result will also be true. Note that if the collection (ingredients) is empty, the result will also be true.
So let's try to rewrite our lambda code, which again looked like this:
list.RemoveAll(roll => !ingredients.All(roll.checkForIngredient));
Into a more verbose method, not using the All extension method:
list.RemoveAll(delegate(Roll roll)
{
bool all = true;
foreach (var ingredient in ingredients)
if (!roll.checkForIngredient(ingredient))
{
all = false;
break;
}
return !all;
});
This now starts to look like your original piece of code, except that we're using the RemoveAll method, which needs a predicate that returns whether to remove the item or not. Since if all is false, we need to remove the roll, we use the not operator ! to reverse that value.

Since you are both new to C# but also asked for an elegant solution, I will give you an example of how to solve this using a more object-oriented approach.
First of all, any "thing" of significance should be modeled as a class, even if it has just one property. This makes it easier to extend the behavior later on. You already defined a class for Roll. I would also add a class for Ingredient:
public class Ingredient
{
private string _name;
public string Name
{
get { return _name; }
}
public Ingredient(string name)
{
_name = name;
}
}
Note the Name property which only has a getter, and the constructor which accepts a string name. This might look like unnecessary complexity at first but will make your code more straightforward to consume further down the road.
Next, we'll modify your Roll class according to this guideline and give it some helper methods that make it easier for us to check if a roll contains a certain (list of) ingredients:
public class Roll
{
private string _name;
private List<Ingredient> _ingredients = new List<Ingredient>();
public string Name
{
// By only exposing the property through a getter, you are preventing the name
// from being changed after the roll has been created
get { return _name; }
}
public List<Ingredient> Ingredients
{
// Similarly here, you are forcing the consumer to use the AddIngredient method
// where you can do any necessary checks before actually adding the ingredient
get { return _ingredients; }
}
public Roll(string name)
{
_name = name;
}
public bool AddIngredient(Ingredient ingredient)
{
// Returning a boolean value to indicate whether the ingredient was already present,
// gives the consumer of this class a way to present feedback to the end user
bool alreadyHasIngredient = _ingredients.Any(i => i.Name == ingredient.Name);
if (!alreadyHasIngredient)
{
_ingredients.Add(ingredient);
return true;
}
return false;
}
public bool ContainsIngredients(IEnumerable<Ingredient> ingredients)
{
// We use a method group to check for all of the supplied ingredients
// whether or not they exist
return ingredients.All(ContainsIngredient);
// Could be rewritten as: ingredients.All(i => ContainsIngredient(i));
}
public bool ContainsIngredient(Ingredient ingredient)
{
// We simply check if an ingredient is present by comparing their names
return _ingredients.Any(i => i.Name == ingredient.Name);
}
}
Pay attention to the ContainsIngredient and ContainsIngredients methods here. Now you can do stuff like if (roll.ContainsIngredient(ingredient)), which will make your code more expressive and more readable. You'll see this in action in the next class that I'm going to add, RollCollection.
You are modeling collections of food to pick from, presumably in the context of a restaurant menu or some similar domain. You might as well go ahead and model just that: a RollCollection. This will allow you to encapsulate some meaningful logic inside of the collection.
Again, this sort of thing tends to require some boilerplate code and may look overly complex at first, but it will make your classes easier to consume. So let's add a RollCollection:
public class RollCollection : IEnumerable<Roll>
{
private List<Roll> _rolls = new List<Roll>();
public RollCollection()
{
// We need to provide a default constructor if we want to be able
// to instantiate an empty RollCollection and then add rolls later on
}
public RollCollection(IEnumerable<Roll> rolls)
{
// By providing a constructor overload which accepts an IEnumerable<Roll>,
// we have the opportunity to create a new RollCollection based on a filtered existing collection of rolls
_rolls = rolls.ToList();
}
public RollCollection WhichContainIngredients(IEnumerable<Ingredient> ingredients)
{
IEnumerable<Roll> filteredRolls = _rolls
.Where(r => r.ContainsIngredients(ingredients));
return new RollCollection(filteredRolls);
}
public bool AddRoll(Roll roll)
{
// Similar to AddIngredient
bool alreadyContainsRoll = _rolls.Any(r => r.Name == roll.Name);
if (!alreadyContainsRoll)
{
_rolls.Add(roll);
return true;
}
return false;
}
#region IEnumerable implementation
public IEnumerator<Roll> GetEnumerator()
{
foreach (Roll roll in _rolls)
{
yield return roll;
}
}
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
#endregion
}
WhichContainIngredients is the thing we were really looking for, as it allows you to do something like this:
// I have omitted the (proper) instantiation of Rolls and ChosenIngredients for brevity here
public RollCollection Rolls { get; set; }
public List<Ingredient> ChosenIngredients { get; set; }
public void Update()
{
Rolls = Rolls.WhichContainIngredients(ChosenIngredients);
}
This is simple and clean, just the sort of thing you want to be doing in your presentation layer. The logic to accomplish your requirement is now nicely encapsulated in the RollCollection class.
EDIT: a more complete (but still simplified) example of how your Controller class might end up looking like:
public class Controller
{
private RollCollection _availableRolls = new RollCollection();
private List<Ingredient> _availableIngredients = new List<Ingredient>();
public RollCollection AvailableRolls
{
get { return _availableRolls; }
}
public List<Ingredient> AvailableIngredients
{
get { return _availableIngredients; }
}
public RollCollection RollsFilteredByIngredients
{
get { return AvailableRolls.WhichContainIngredients(ChosenIngredients); }
}
public List<Ingredient> ChosenIngredients { get; set; }
public Controller()
{
ChosenIngredients = new List<Ingredient>();
InitializeTestData();
}
private void InitializeTestData()
{
Ingredient ingredient1 = new Ingredient("Ingredient1");
Ingredient ingredient2 = new Ingredient("Ingredient2");
Ingredient ingredient3 = new Ingredient("Ingredient3");
_availableIngredients.Add(ingredient1);
_availableIngredients.Add(ingredient2);
_availableIngredients.Add(ingredient3);
Roll roll1 = new Roll("Roll1");
roll1.AddIngredient(ingredient1);
roll1.AddIngredient(ingredient2);
Roll roll2 = new Roll("Roll2");
roll2.AddIngredient(ingredient3);
_availableRolls.AddRoll(roll1);
_availableRolls.AddRoll(roll2);
}
}

I am trying to learn C# by making a simple program that shows the user
sushi rolls given their desired ingredients. i.e. a user wants a roll
with crab, and the program will spit out a list of sushi rolls that
contain crab.
Here's my solution to the given problem:
public class Roll
{
public string Name { get; set; }
private List<string> ingredients = new List<string>();
public IList<string> Ingredients { get { return ingredients; } }
public bool Contains(string ingredient)
{
return Ingredients.Any(i => i.Equals(ingredient));
}
}
You can use the LINQ extension method .Where to filter your collection of Rolls
public class Program
{
static void Main()
{
var allRolls = new List<Roll>
{
new Roll
{
Name = "Roll 1",
Ingredients = { "IngredientA", "Crab", "IngredientC" }
},
new Roll
{
Name = "Roll 2",
Ingredients = { "IngredientB", "IngredientC" }
},
new Roll
{
Name = "Roll 3",
Ingredients = { "Crab", "IngredientA" }
}
};
var rollsWithCrab = allRolls.Where(roll => roll.Contains("Crab"));
foreach (Roll roll in rollsWithCrab)
{
Console.WriteLine(roll.Name);
}
}
}
From what I see you're trying to remove all rolls that don't contain crab from your list of rolls. A better approach is to filter out those rolls that don't contain crab (using .Where), you can then use .ToList() if you need to manipulate the whole list directly rather than iterating through the collection (fetching one item at a time).
You should read up on Delegates, Iterators, Extension Methods and LINQ to better understand what's going on under the covers.

StackOverflowException on nested query, small item count

I have found some threads with similar titles, but none seem to have a fitting answer.
One mentioned a bug in .NET versions prior to 4.0 - I use 4.0, so this should not be it.
Consider the example:
I am attempting to create a collection of instances of class Part, which do not belong to any instance of class PartGroup.
Func<Part,bool> hasGroup = P => partColl.Groups.Any( G => G.Parts.Contains(P) );
var groupless = partColl.Parts.Where( P => ! hasGroup(P) );
partColl is an instance of a class implementing the properties Groups and Parts, each IEnumerable<T> where T is PartGroup or Part respectively, internally implemented as List<T>.
partColl.Parts contains all parts in existence.
class Group has property IEnumerable<Part> Parts, listing references to parts belonging to the group.
In my current program, there are 27 parts and 5 groups with no overlap in elements.
Nothing that should trouble the stack, despite quadratic complexity, if something wasn't foul.
When I run this, it will crash with said exception on hasGroup.
What am I missing?
EDIT:
A little detail slipped my abstraction presented here:
IEnumerable PartGroup.Parts was, unlike the two properties of PartCollection, not backed by a List, it was an auto-property with private set, initialized in the c'tor with a passed-in IEenumerable. The instance behind this IEnumerable is also a list, and an own one for each group, so I'm not sure yet what is going on exactly.
BUT, the exception is gone by: also backing that property with a variable of type List and in the constructor, assigning to it: _list = parts.ToList(), where parts is IEnumerable<Parts>, passed as a param to the ctor. I did ToList just to make sure it will really be a list, not some half baked query, but it should, since at the place where I am building the groups, one new List for each group is allocated before passing it in...
The remaining question is still interesting: What was going on, why did the auto-property 'cause' the exception?
I'll post more details on request, but gotta go for a while now.

This small sample does not reproduce the issue.
using System;
using System.Linq;
using System.Collections.Generic;
class P
{
class PartGroup
{
public List<Part> Parts { get; private set; }
public PartGroup()
{
Parts = new List<Part>();
}
}
class Part
{
}
class PartCollection
{
public List<Part> Parts { get; set; }
public List<PartGroup> Groups { get; set; }
public PartCollection()
{
Parts = new List<Part>();
Groups = new List<PartGroup>();
}
}
static void Main()
{
var groups = new List<PartGroup> { new PartGroup(), new PartGroup(), new PartGroup(), new PartGroup(), new PartGroup() };
var partColl = new PartCollection();
partColl.Parts.Add(new Part());
partColl.Groups.AddRange(groups);
for (int i = 0; i < 27; i++)
{
var part = new Part();
groups[i % groups.Count].Parts.Add(part);
partColl.Parts.Add(part);
}
partColl.Parts.Add(new Part());
Func<Part, bool> hasGroup = P => partColl.Groups.Any(G => G.Parts.Contains(P));
var groupless = partColl.Parts.Where(P => !hasGroup(P)).ToList();
}
}

How to use Comparer for a HashSet

As a result of another question I asked here I want to use a HashSet for my objects
I will create objects containing a string and a reference to its owner.
public class Synonym
{
private string name;
private Stock owner;
public Stock(string NameSynonym, Stock stock)
{
name=NameSynonym;
owner=stock
}
// [+ 'get' for 'name' and 'owner']
}
I understand I need a comparer , but never used it before. Should I create a separate class? like:
public class SynonymComparer : IComparer<Synonym>
{
public int Compare(Synonym One, Synonym Two)
{ // Should I test if 'One == null' or 'Two == null' ????
return String.Compare(One.Name, Two.Name, true); // Caseinsesitive
}
}
I prefer to have a function (or nested class [maybe a singleton?] if required) being PART of class Synonym instead of another (independent) class. Is this possible?
About usage:
As i never used this kind of thing before I suppose I must write a Find(string NameSynonym) function inside class Synonym, but how should I do that?
public class SynonymManager
{
private HashSet<SynonymComparer<Synonym>> ListOfSynonyms;
public SynonymManager()
{
ListOfSymnonyms = new HashSet<SynonymComparer<Synonym>>();
}
public void SomeFunction()
{ // Just a function to add 2 sysnonyms to 1 stock
Stock stock = GetStock("General Motors");
Synonym otherName = new Synonym("GM", stock);
ListOfSynonyms.Add(otherName);
Synonym otherName = new Synonym("Gen. Motors", stock);
ListOfSynonyms.Add(otherName);
}
public Synonym Find(string NameSynomym)
{
return ListOfSynonyms.??????(NameSynonym);
}
}
In the code above I don't know how to implement the 'Find' method. How should i do that?
Any help will be appreciated
(PS If my ideas about how it should be implemented are completely wrong let me know and tell me how to implement)

A HashSet doesn't need a IComparer<T> - it needs an IEqualityComparer<T>, such as
public class SynonymComparer : IEqualityComparer<Synonym>
{
public bool Equals(Synonym one, Synonym two)
{
// Adjust according to requirements.
return StringComparer.InvariantCultureIgnoreCase
.Equals(one.Name, two.Name);
}
public int GetHashCode(Synonym item)
{
return StringComparer.InvariantCultureIgnoreCase
.GetHashCode(item.Name);
}
}
However, your current code only compiles because you're creating a set of comparers rather than a set of synonyms.
Furthermore, I don't think you really want a set at all. It seems to me that you want a dictionary or a lookup so that you can find the synonyms for a given name:
public class SynonymManager
{
private readonly IDictionary<string, Synonym> synonyms = new
Dictionary<string, Synonym>();
private void Add(Synonym synonym)
{
// This will overwrite any existing synonym with the same name.
synonyms[synonym.Name] = synonym;
}
public void SomeFunction()
{
// Just a function to add 2 synonyms to 1 stock.
Stock stock = GetStock("General Motors");
Synonym otherName = new Synonym("GM", stock);
Add(otherName);
ListOfSynonyms.Add(otherName);
otherName = new Synonym("Gen. Motors", stock);
Add(otherName);
}
public Synonym Find(string nameSynonym)
{
// This will throw an exception if you don't have
// a synonym of the right name. Do you want that?
return synonyms[nameSynonym];
}
}
Note that there are some questions in the code above, about how you want it to behave in various cases. You need to work out exactly what you want it to do.
EDIT: If you want to be able to store multiple stocks for a single synonym, you effectively want a Lookup<string, Stock> - but that's immutable. You're probably best storing a Dictionary<string, List<Stock>>; a list of stocks for each string.
In terms of not throwing an error from Find, you should look at Dictionary.TryGetValue which doesn't throw an exception if the key isn't found (and also returns whether or not the key was found); the mapped value is "returned" in an out parameter.

Wouldn't it be more reasonable to scrap the Synonym class entirely and have list of synonyms to be a Dictonary (or, if there is such a thing, HashDictionary) of strings?
(I'm not very familiar with C# types, but I hope this conveys general idea)
The answer I recommend (edited, now respects the case):
IDictionary<string, Stock>> ListOfSynonyms = new Dictionary<string,Stock>>();
IDictionary<string, string>> ListOfSynForms = new Dictionary<string,string>>();
class Stock
{
...
Stock addSynonym(String syn)
{
ListOfSynForms[syn.ToUpper()] = syn;
return ListOfSynonyms[syn.ToUpper()] = this;
}
Array findSynonyms()
{
return ListOfSynonyms.findKeysFromValue(this).map(x => ListOfSynForms[x]);
}
}
...
GetStock("General Motors").addSynonym('GM').addSynonym('Gen. Motors');
...
try
{
... ListOfSynonyms[synonym].name ...
}
catch (OutOfBounds e)
{
...
}
...
// output everything that is synonymous to GM. This is mix of C# and Python
... GetStock('General Motors').findSynonyms()
// test if there is a synonym
if (input in ListOfSynonyms)
{
...
}

You can always use LINQ to do the lookup:
public Synonym Find(string NameSynomym)
{
return ListOfSynonyms.SingleOrDefault(x => x.Name == NameSynomym);
}
But, have you considered using a Dictionary instead, I believe it is better suited for extracting single members, and you can still guarantee that there are no duplicates based on the key you choose.
I am not sure that lookup time is of SingleOrDefault, but I am pretty sure it is linear (O(n)), so if lookup time is important to you, a Dictionary will provide you with O(1) lookup time.

Am I done with this Linked List code?

Hi I'm trying to get some practice with Linked Lists.
I Defined an Object class called Student:
public class Student
{
protected string Student_Name;
protected int Student_ID;
protected int Student_Mark;
protected char Student_Grade;
public Student() // default Constructor
{
Student_Name = " ";
Student_ID = 0;
Student_Mark = 0;
Student_Grade = ' ';
}
public Student(string Sname, int Sid, int Smark, char Sgrade) // Constructor
{
int len = sname.Length;
Student_Name = sname.Substring(0, len);
//Student_Name = sname.Substring(0, sname.Length);
Student_ID = Sid;
Student_Mark = Smark;
Student_Grade = Sgrade;
}
}
and then a Node class:
public class S_Node
{
public Student Element;
public S_Node Link;
public S_Node()
{
Element = new Student();
Link = null;
}
public Node(Student theElement)
{
Element = theElement;
Link = null;
}
}
and the LinkedList:
public class S_LinkedList
{
protected S_Node header;
protected S_Node tail;
public S_LinkedList()
{
header = new S_Node();
Tail = new S_Node();
header.Link = Tail;
}
// METHODS which i don't know how to do it (never use linkedlist before)
}
I need to organize this data using a “linkedlist data structure type”.
Contain all methods of linkedlist as Adding nodes to the list as I've learned -->(Insert),Deleting nodes from the list,as I've learned -->((Remove),Traversing the lists I've learned -->((PrintList),Finding a node in the list as I've learned -->((Find , FindPrevious) the problem I'm selflearning and I've tried to search the net and read more from the stupid C# that was a disaster.
I've done too much that I'm so sad that i don't know how to complete it .
I'm trying hard to Use this classes to write an executable program and to Test it .
If you don't want to help in completing this program (hope not) at least show me some real examples or ideas , after all I'm a selflearner geek :-)

the head of the list.
( item1
Element: student1
Next ------------> ( item2
) Element: student2
Next ------------> ( item3
) Element: student3
Next: null
)
the tail of the list.
First of all, for you to be able to write the StudentList class, you need to write the client code first. Client code is the code that uses your student list. Also, don't just write one thing at a time and throw it away. Instead write a whole bunch of [test] cases that exercise the different ways you need to interact with the StudentList. Write exceptional cases too. But don't be tempted to write a swiss-army knife of a class that does everything just because it can. Write the minimal amount of code that gets the job done.
How you need to use the class will heavily dictate how the class is constructed. This is the essence of TDD or Test Driven Design.
Your biggest problem that I can see is you have no idea how you want to use the class. So lets do that first.
// create a list of students and print them back out.
StudentList list = new StudentList();
list.Add( new Student("Bob", 1234, 2, 'A') );
list.Add( new Student("Mary", 2345, 4, 'C') );
foreach( Student student in list)
{
Console.WriteLine(student.Name);
}
I add the students to the list, and then I print them out.
I have no need for my client code to see inside the StudentList. Therefore StudentList hides how it implements the linked list. Let's write the basics of the StudentList.
public class StudentList
{
private ListNode _firstElement; // always need to keep track of the head.
private class ListNode
{
public Student Element { get; set; }
public ListNode Next { get; set; }
}
public void Add(Student student) { /* TODO */ }
}
StudentList is pretty basic. Internally it keeps track of the first or head nodes. Keeping track of the first node is obviously always required.
You also might wonder why ListNode is declared inside of StudentList. What happens is the ListNode class is only accessible to the StudentList class. This is done because StudentList doesn't want to give out the details to it's internal implementation because it is controlling all access to the list. StudentList never reveals how the list is implemented. Implementation hiding is an important OO concept.
If we did allow client code to directly manipulate the list, there'd be no point having StudentList is the first place.
Let's go ahead and implement the Add() operation.
public void Add(Student student)
{
if (student == null)
throw new ArgumentNullException("student");
// create the new element
ListNode insert = new ListNode() { Element = student };
if( _firstElement == null )
{
_firstElement = insert;
return;
}
ListNode current = _firstElement;
while (current.Next != null)
{
current = current.Next;
}
current.Next = insert;
}
The Add operation has to find the last item in the list and then puts the new ListNode at the end. Not terribly efficient though. It's currently O(N) and Adding will get slower as the list gets longer.
Lets optimize this a little for inserts and rewrite the Add method. To make Add faster all we need to do is have StudentList keep track of the last element in the list.
private ListNode _lastElement; // keep track of the last element: Adding is O(1) instead of O(n)
public void Add(Student student)
{
if( student == null )
throw new ArgumentNullException("student");
// create the new element
ListNode insert = new ListNode() { Element = student };
if (_firstElement == null)
{
_firstElement = insert;
_lastElement = insert;
return;
}
// fix up Next reference
ListNode last = _lastElement;
last.Next = insert;
_lastElement = insert;
}
Now, when we add, we don't iterate. We just need to keep track of the head and tail references.
Next up: the foreach loop. StudentList is a collection, and being a collection we want to enumerate over it and use C#'s foreach. The C# compiler can't iterate magically. In order to use the foreach loop We need to provide the compiler with an enumerator to use even if the code we write doesn't explicitly appear to use the enumerator.
First though, lets re-visit how we iterate over a linked list.
// don't add this to StudentList
void IterateOverList( ListNode current )
{
while (current != null)
{
current = current.Next;
}
}
Okay. so let's hook into C#'s foreach loop and return an enumerator. To do that we alter StudentList to implement IEnumerable. This is getting a little bit advanced, but you should be able to figure out what's going on.
// StudentList now implements IEnumerable<Student>
public class StudentList : IEnumerable<Student>
{
// previous code omitted
#region IEnumerable<Student> Members
public IEnumerator<Student> GetEnumerator()
{
ListNode current = _firstElement;
while (current != null)
{
yield return current.Element;
current = current.Next;
}
}
#endregion
#region IEnumerable Members
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
#endregion
}
You should be able to spot the linked list iteration in there. Don't get thrown by the yield keyword. All yield is doing is returning the current student back to the foreach loop. The enumarator stops returning students when it gets to the end of the linked list.
And that's it! The code works the way we want it to.
* This is by no means the only way to implement the list. I've opted to put the list logic in the StudentList and keep ListNode very basic. But the code does only what my very first unit test needs and nothing more. There are more optimizations you could make, and there are other ways of constructing the list.
Going forward: What you need to do is first create [unit] tests for what your code needs to do, then add the implementation you require.
* fyi I also rewrote the Student class. Bad naming and strange casing from a C# persepctive, not to mention the code you provided doesn't compile. I prefer the _ as a leader to private member variables. Some people don't like that, however you're new to this so I'll leave them in because they're easy to spot.
public class Student
{
private string _name;
private int _id;
private int _mark;
private char _letterGrade;
private Student() // hide default Constructor
{ }
public Student(string name, int id, int mark, char letterGrade) // Constructor
{
if( string.IsNullOrEmpty(name) )
throw new ArgumentNullException("name");
if( id <= 0 )
throw new ArgumentOutOfRangeException("id");
_name = name;
_id = id;
_mark = mark;
_letterGrade = letterGrade;
}
// read-only properties - compressed to 1 line for SO answer.
public string Name { get { return _name; } }
public int Id { get { return _id; } }
public int Mark { get { return _mark; } }
public char LetterGrade { get { return _letterGrade; } }
}
check parameters
pay attention to the different casing of properties, classes, and variables.
hide the default constructor. Why do I want to create students without real data?
provide some read-only properties.
This class is immutable as written (i.e. once you create a student, you can't change it).

I'll do one for you! You need to draw diagrams with each node as a box, and work out what code you need to use to change the list for each operation. See this for some inspiration:
http://en.wikipedia.org/wiki/Linked_list
The diagrams there don't show the main list class as a box, which you should have, with two arrows coming out of it for the header and tail.
Draw yourself some diagrams for the two cases in the Insert method to work out what's going on. One diagram for when there's nothing in the list and header is null, and another diagram for when there's something already in the list. Then from there work out the other operations.
public class S_LinkedList {
protected S_Node header = null;
protected S_Node tail = null;
public S_LinkedList()
{
}
// METHODS which i don't know how to do it (never use linkedlist before)
void Insert(Student s)
{
if( header == null )
{
header = new S_Node(s);
tail = header;
}
else
{
tail.Link = new S_Node(s);
tail = tail.Link;
}
}
}

The operations you are missing are:
Add:
set the link of the tail node to be the added node and set the tail to be the new node.
Remove/Delete:
is a bit tricky if you don't have a doubly linked list but with a singly linked list go throught he list from the head till you find the node you require keeping the previous node in a separate variable. When you find the node you are removing set the Link of the previous node to that nodes link. An optimization might be to check that it's not the link you are looking for.
Alternatively make it a doubly linked list and you don't need to keep track of the previous node.
Find:
Traverse the list from node to node till you find the one you are looking for.
read this wikipedia article for more info.

Look at this...
Although if you really want to learn how to do this you should write it in c or c++ at least then you'd be doing something useful...

Actually I don't see a reason to write your own linked list in C# (other then learning how it works) since the .NET already contains LinkedList generic class.

The concept of linked lists is not very difficult to understand. Implementation on the other hand ... can get a bit tricky.
I can also understand your frustration of trying to find information on the web about it. I have been in your boat before and everything varies from site to site. You really might want to invest in a data structures book as you I think the information you find will be much more clear and helpful than most information you find in the wild.
Implementing a linked list in Java/C# will be much easier if you have never used ll's before. However, once you get a better feel for it you will get a much better understanding for the ll's by creating them in C/C++.
From your code above, you will be better off thinking of each S_Node as just regular node that contains a Student object instead of thinking of it as a Student node (hope that makes sense). Same rules apply for your S_LinkedList class. A linked list is a list mode up of nodes. These nodes contain Student objects.
Hope this helps.

Try this as your Student class.
public class Student
{
protected string Name;
protected int ID;
protected int Mark;
protected char Grade;
public Student() // default Constructor
{
Name = "";
ID = 0;
Mark = 0;
Grade = '';
}
public Student(string Name, int ID, int Mark, char Grade) // Constructor
{
this.Name = Name;
this.ID = ID;
this.Mark = Mark;
this.Grade = Grade;
}
}

Your question, as I read it, is too vague. I would start by googling 'linked lists' or picking up a book on 'data structures'. When you run into a specific problem, ask it on here, and I'm sure someone will help you.