Develop Reference

How to sort List<T> in c#

I've got a List<Card>, and I want to sort these cards
So, I'm looking for a method to sort them with different criterias, like their ID, their Name ...
public class Card : IComparer
{
public string ID;
public string Name;
public int CompareId(object firstCard, object secondCard)
{
Card c1 = (Card)firstCard;
Card c2 = (Card)secondCard;
return c1.Id.CompareTo(c2.Id);
}
}
But then, visual studio sent me an error :
'Card' does not implement interface member 'IComparer<Card>.Compare(Card, Card)'

You, probably, want to have your class Comparable not a Comparator
public class Card : IComparable<Card>
{
public string ID;
public string Name;
public int CompareTo(Card other)
{
if (null == other)
return 1;
// string.Compare is safe when Id is null
return string.Compare(this.Id, other.Id);
}
}
then
List<Card> myList = ...
myList.Sort();
Edit: If you want to have several criteria to choose from, you have to implement several Comparers as separated classes, e.g.
public sealed class CardByIdComparer : IComparer<Card>
{
public int Compare(Card x, Card y)
{
if (object.ReferenceEquals(x, y))
return 0;
else if (null == x)
return -1;
else if (null == y)
return 1;
else
return string.Compare(x.Id, y.Id);
}
}
and when sorting provide the required:
List<Card> myList = ...
myList.Sort(new CardByIdComparer());
Edit 2: (inspired by spender's library). If you want to combine several comparers into one (i.e. use comparer1, on tie - comparer2 etc.)
public sealed class ComparerCombined<T> : IComparer<T> {
private IComparer<T>[] m_Comparers;
public ComparerCombined(params IComparer<T>[] comparers) {
if (null == comparers)
throw new ArgumentNullException(nameof(comparers));
m_Comparers = comparers
.Select(item => item == null ? Comparer<T>.Default : item)
.Where(item => item != null)
.Distinct()
.ToArray();
}
public int Compare(T x, T y) {
if (object.ReferenceEquals(x, y))
return 0;
else if (null == x)
return -1;
else if (null == y)
return 1;
foreach (var comparer in m_Comparers) {
int result = comparer.Compare(x, y);
if (result != 0)
return result;
}
return 0;
}
}
usage:
myList.Sort(new ComparerCombined(
new CardByIdComparer(), // Sort By Id
new CardByNameComparer() // On tie (equal Id's) sort by name
));

The easiest way You can use Linq:
List<Card> objSortedList = objListObject.OrderBy(o=>o.ID).ToList();
or
List<Card> objSortedList = objListObject.OrderByDescending(o=>o.ID).ToList();

Good examples for demonstrate the concept of
List<T>.Sort(IComparer <T>) method check the link please.
IComparer<T> in this example compare method used for strings IComparer<T>
but you can use this for ID(int) too.
using System;
using System.Collections.Generic;
class GFG : IComparer<string>
{
public int Compare(string x, string y)
{
if (x == null || y == null)
{
return 0;
}
// "CompareTo()" method
return x.CompareTo(y);
}
}
public class geek
{
public static void Main()
{
List<string> list1 = new List<string>();
// list elements
list1.Add("C++");
list1.Add("Java");
list1.Add("C");
list1.Add("Python");
list1.Add("HTML");
list1.Add("CSS");
list1.Add("Scala");
list1.Add("Ruby");
list1.Add("Perl");
int range = 4;
GFG gg = new GFG();
Console.WriteLine("\nSort a range with comparer:");
// sort the list within a
// range of index 1 to 4
// where range = 4
list1.Sort(1, range, gg);
Console.WriteLine("\nBinarySearch and Insert Dart");
// Binary Search and storing
// index value to "index"
int index = list1.BinarySearch(0, range,
"Dart", gg);
if (index < 0)
{
list1.Insert(~index, "Dart");
range++;
}
}
}

You need to implement IComparer
public int Compare(Card card1, Card card2)
{
if (card1.ID > card2.ID)
return 1; //move card1 up
if (card2.ID < card1.ID)
return -1; //move card2 up
return 0; //do nothing
}

How to have multiple argument's in a C# if statement

[That title may be wrong for the question, please inform me if so]
I'm coding a little maths quiz in C#, and I was wondering how to make an if statement that says something similiar to:
"if the user responds with 'this' or ' this'
{
do blahblahblah
}
But I don't know how to say the OR bit in C#, I looked through the C# operators page, but kind of got lost in the technical jargon (I'm a rookie).
This is what I have so far:
Console.WriteLine("What is 200 / 5?");
string sFirstAnswer = Console.ReadLine();
if (sFirstAnswer == "40" || " 40")
{
sUser1Score++;
Console.WriteLine("\n Correct, 200 / 5 = 40. You have been awarded 1 point.");
Console.ReadLine();
}

Write
if (sFirstAnswer == "40" || sFirstAnswer == " 40")
or better yet, trim the answer:
if (sFirstAnswer.Trim() == "40")

if (sFirstAnswer == "40" || sFirstAnswer == "40")

You can create a list of allowed answers and then check it's in the list.
var correctFirstAnswers = new List<string>{"40", " 40"};
if (correctFirstAnswers.Contains(sFirstAnswer))
this is more readable than || when there are multiple possible answers.

I thought I might give an (over-the-top) example of what I meant to make it a bit more Dynamic
A few classes now help to ask you the questions, and with a few functions built around it, you can easily show your questions in a menu format, and then ask the question, with random nr's (only whole number division was a bit more annoying :))
You could make it easier that the Generate method limits the range a bit more, but I just thought I wanted to give you an idea of how it could look like
using System;
using System.Collections.Generic;
namespace MathQuiz
{
class Program
{
interface IExercise
{
string Title { get; }
void Generate();
}
abstract class Exercise<TResult> : IExercise
{
public virtual string Title
{
get
{
return "Exercise";
}
}
public abstract bool isCorrect(TResult reply);
public abstract TResult Solve();
public abstract bool TryParse(string value, out TResult result);
public abstract void Generate();
}
abstract class ExerciseWith2Items<TSource, TResult> : Exercise<TResult>
{
public virtual TSource Item1 { get; set; }
public virtual TSource Item2 { get; set; }
public abstract string Operator { get; }
public override string ToString()
{
return string.Format("{0} {1} {2}", Item1, Operator, Item2);
}
}
abstract class WholeNumberExercise : ExerciseWith2Items<int, int>
{
public override void Generate()
{
Random next = new Random();
Item1 = next.Next(100) + 15;
Item2 = next.Next(100) + 15;
}
public override bool TryParse(string value, out int result)
{
return int.TryParse(value, out result);
}
}
class Division : WholeNumberExercise
{
protected bool IsPrime(int nr)
{
int max = (int)Math.Sqrt(nr);
if (nr <= 2)
{
return true;
}
for (int i = 2; i < max; i++)
{
if (nr % i == 0)
{
return false;
}
}
return true;
}
public override int Item1
{
get
{
return base.Item1;
}
set
{
// primes cannot be divived, so increase the value until we don't have a prime
while (IsPrime(value))
{
value++;
}
base.Item1 = value;
}
}
public override int Item2
{
get
{
return base.Item2;
}
set
{
if (value <= 0)
{
// minimum 2
value = 2;
}
// small override: we only want whole number division, so change the nr to the closest nr that has no rest after division
int closest = 0;
while ((value - closest > 1 && Item1 % (value - closest) != 0) ||
(value + closest < Item1 && Item1 % (value + closest) != 0))
{
closest++;
}
// in case closest == 0, it doesn't really change anything
if (Item1 % (value - closest) == 0)
{
value -= closest;
}
else
{
value += closest;
}
base.Item2 = value;
}
}
public override string Operator
{
get { return "/"; }
}
public override bool isCorrect(int reply)
{
return reply == (Item1 / Item2);
}
public override void Generate()
{
Random r = new Random();
Item1 = r.Next(500) + 100;
Item2 = r.Next(50) + 2;
}
public override int Solve()
{
return (Item1 / Item2);
}
}
class Multiplication : WholeNumberExercise
{
public override string Operator
{
get { return "*"; }
}
public override bool isCorrect(int reply)
{
return reply == (Item1 * Item2);
}
public override int Solve()
{
return (Item1 * Item2);
}
}
class Addition : WholeNumberExercise
{
public override string Operator
{
get { return "+"; }
}
public override bool isCorrect(int reply)
{
return reply == (Item1 + Item2);
}
public override int Solve()
{
return (Item1 + Item2);
}
}
class Subtraction : WholeNumberExercise
{
public override string Operator
{
get { return "-"; }
}
public override bool isCorrect(int reply)
{
return reply == (Item1 - Item2);
}
public override int Solve()
{
return (Item1 - Item2);
}
}
static IExercise ShowMenu(IList<IExercise> exercises)
{
int menu;
do
{
Console.Clear();
Console.WriteLine("Test your match skills :)\r\n");
for (int i = 0; i < exercises.Count; i++)
{
Console.WriteLine("\t{0}\t{1}", i, exercises[i].GetType().Name);
}
Console.WriteLine("\r\n\t99\tExit\r\n");
Console.Write("Please enter your choice: ");
if (!int.TryParse(Console.ReadLine(), out menu))
{
// wrong input
menu = -1;
}
if (menu != 99)
{
if (menu >= exercises.Count)
{
menu = -1;
}
}
} while (menu < 0);
IExercise result = null;
if (menu != 99)
{
result = exercises[menu];
}
return result;
}
static void Solve(IExercise exercise)
{
if (exercise == null)
{
return;
}
if (!(exercise is WholeNumberExercise))
{
Console.WriteLine("Don't know how to solve this exercise, please contact developer :)");
Console.ReadLine();
return;
}
var solvable = exercise as WholeNumberExercise;
solvable.Generate();
Console.Write("{0}: '{1}' = ", solvable.GetType().Name, exercise);
int reply;
bool validAnswerGiven;
do
{
validAnswerGiven = solvable.TryParse(Console.ReadLine(), out reply);
if (validAnswerGiven)
{
if (solvable.isCorrect(reply))
{
Console.WriteLine("Correct!");
}
else
{
Console.WriteLine("Incorrect, the correct result is {0}", solvable.Solve());
}
}
else
{
Console.WriteLine("Please enter valid value (whole number)!");
}
} while (!validAnswerGiven);
Console.ReadLine();
}
static void Main(string[] args)
{
IList<IExercise> potentialExercises = new List<IExercise>()
{
new Addition(),
new Subtraction(),
new Division(),
new Multiplication()
};
IExercise selectedExercise;
do
{
selectedExercise = ShowMenu(potentialExercises);
Solve(selectedExercise);
} while (selectedExercise != null);
Console.WriteLine("Program completed!");
}
}
}
it is runnable code, so copy and paste in visual studio console project should do the trick ;)

In addition to the above you could also try to convert the string to an integer
int number = 0
bool result = Int32.TryParse(Console.ReadLine(), out number);
if (number== 40){
...
}
number will be 0 if conversion fails to int and result false but for your case you dont care its not 40...
MSDN Int TryParse

Since you're validating an answer from an equation, you want to Parse it and get it into a numeric form pronto.
var answer = Convert.ToInt32(sFirstAnswer.Trim());
var expected = 40;
if(answer == expected){
//gold ribbon
}

Distinct values from List of objects

I need your help. I am trying to get distinct values from List of objects.
My class looks like this:
class Chromosome
{
public bool[][] body { get; set; }
public double fitness { get; set; }
}
Now I have List<Chromosome> population. And now what I need is a way, how I can get new list: List<Chromosome> newGeneration. This new list will contain only unique chromosomes from original list - population.Chromosome is unique, when his whole body (which in this case is 2D bool array) is unique in comparison to the other chromosomes.
I know, that there is something like MoreLINQ, but I am not sure, whether I should use 3rd party code and I know that I should overwrite some methods, but I am kind of lost. So I would really appreciate some nice step by step description, that even idiot could accomplish :)
THX

First, implement the equality operator (this goes into class Chromosome):
public class Chromosome : IEquatable<Chromosome>
{
public bool[][] body { get; set; }
public double fitness { get; set; }
bool IEquatable<Chromosome>.Equals(Chromosome other)
{
// Compare fitness
if(fitness != other.fitness) return false;
// Make sure we don't get IndexOutOfBounds on one of them
if(body.Length != other.body.Length) return false;
for(var x = 0; x < body.Length; x++)
{
// IndexOutOfBounds on inner arrays
if(body[x].Length != other.body[x].Length) return false;
for(var y = 0; y < body[x].Length; y++)
// Compare bodies
if(body[x][y] != other.body[x][y]) return false;
}
// No difference found
return true;
}
// ReSharper's suggestion for equality members
public override bool Equals(object obj)
{
if (ReferenceEquals(null, obj))
{
return false;
}
if (ReferenceEquals(this, obj))
{
return true;
}
if (obj.GetType() != this.GetType())
{
return false;
}
return this.Equals((Chromosome)obj);
}
public override int GetHashCode()
{
unchecked
{
return ((this.body != null ? this.body.GetHashCode() : 0) * 397) ^ this.fitness.GetHashCode();
}
}
}
Then, use Distinct:
var newGeneration = population.Distinct().ToList();

public class ChromosomeBodyComparer : IEqualityComparer<Chromosome>
{
private bool EqualValues(bool[][] left, bool[][] right)
{
if (left.Length != right.Length)
{
return false;
}
return left.Zip(right, (x, y) => x.SequenceEquals(y)).All();
}
public bool Equals(Chromosome left, Chromosome right)
{
return EqualValues(left.body, right.body)
}
//implementing GetHashCode is hard.
// here is a rubbish implementation.
public int GetHashCode(Chromosome c)
{
int numberOfBools = c.body.SelectMany(x => x).Count();
int numberOfTrues = c.body.SelectMany(x => x).Where(b => b).Count();
return (17 * numberOfBools) + (23 * numberOfTrues);
}
}
Called by:
List<Chromosome> nextGeneration = population
.Distinct(new ChromosomeBodyComparer())
.ToList();

Implementing IComparer combining multiple Linq OrderBy's

My problem is that I always want to order a collection of objects in a certain fashion.
For example:
class foo{
public string name {get;set;}
public DateTime date {get;set;}
public int counter {get;set;}
}
...
IEnumerable<foo> dosomething(foo[] bar){
return bar.OrderBy(a=>a.name).ThenBy(a=>a.date).ThenBy(a=>a.counter);
}
The issue I have is its quite longwinded tacking-on the sort order all the time. A neat solution appears to just create a class that implements IComparer<foo>, meaning I can do:
IEnumerable<foo> dosomething(foo[] bar){
return bar.OrderBy(a=>a, new fooIComparer())
}
.
The problem is, the order method this implements is as follows
...
public int Compare(foo x, foo y){ }
Meaning it compares on a very granular basis.
The currently implementation (which will probably work, although im writing pseudocode)
public int Compare(foo x, foo y){
if (x==y)
return 0;
var order = new []{x,y}.OrderBy(a=>a.name).ThenBy(a=>a.date).ThenBy(a=>a.counter);
return (order[0] == x) ? -1 : -1;//if x is first in array it is less than y, else it is greater
}
This is not exactly efficient, can another offer a neater solution? Ideally without a Compare(x,y) method altogether?

Option 1 - The Comparer
As you're ordering by multiple conditions, you'll to check them individually within each case; for example, if x.name and y.name are equal, then you would check x.date and y.date, and so on.
public class FooComparer : IComparer<Foo>
{
public int Compare(Foo x, Foo y)
{
// nasty null checks!
if (x == null || y == null)
{
return x == y ? 0
: x == null ? -1
: 1;
}
// if the names are different, compare by name
if (!string.Equals(x.Name, y.Name))
{
return string.Compare(x.Name, y.Name);
}
// if the dates are different, compare by date
if (!DateTime.Equals(x.Date, y.Date))
{
return DateTime.Compare(x.Date, y.Date);
}
// finally compare by the counter
return x.Counter.CompareTo(y.Counter);
}
}
Option 2 - The extension method
An alternative, not so appealing approach, could be an extension method. Sadly as the TKey for each ThenBy can be different, we lose the power of generics, but can safely replace it with the type object in this case.
public static IOrderedEnumerable<T> OrderByThen<T>(this IEnumerable<T> source, Func<T, object> selector, params Func<T, object>[] thenBySelectors)
{
IOrderedEnumerable<T> ordered = source.OrderBy(selector);
foreach (Func<T, object> thenBy in thenBySelectors)
{
ordered = ordered.ThenBy(thenBy);
}
return ordered;
}

You have to implement IComparable<foo> and compare all properties:
class foo: IComparable<foo>, IComparer<foo>
{
public string name { get; set; }
public DateTime date { get; set; }
public int counter { get; set; }
public int Compare(foo x, foo y)
{
if (x == null || y == null) return int.MinValue;
if (x.name != y.name)
return StringComparer.CurrentCulture.Compare(x.name, y.name);
else if (x.date != y.date)
return x.date.CompareTo(y.date);
else if (x.counter != y.counter)
return x.counter.CompareTo(y.counter);
else
return 0;
}
public int CompareTo(foo other)
{
return Compare(this, other);
}
}
Then you can use OrderBy in this way:
var ordered = foos.OrderBy(f => f).ToList();

what's wrong with an extension method?

Why wont you simply compare your values:
int Compare(foo x, foo y)
{
if (x== null && y == null)
return 0;
else if (x == null)
return -1;
else if (y == null)
return 1;
var nameComparision = string.Compare(x.name,y.name);
if (nameComparision != 0)
return nameComparision;
var dateComparision = x.date.CompareTo(y.date);
if (dateComparision != 0)
return dateComparision;
var counterComparision = x.counter.CompareTo(y.counter);
return counterComparision;
}

How to specify to validate this situation with generics?

I'm trying to validate this class: min >= max. I realized using generics I can't use the comparators.
This is my generic class.
public class Range<T>
{
public T MinValue { get; set; }
public T MaxValue { get; set; }
public Range() { }
public Range(T min, T max)
{
this.MinValue = min;
this.MaxValue = max;
}
public override bool Equals(object obj)
{
if (obj == null) return false;
var other = obj as Range<T>;
return this.MinValue.Equals(other.MinValue) &&
this.MaxValue.Equals(other.MaxValue);
}
public override string ToString()
{
return string.Format("{0},{1}", this.MinValue, this.MaxValue);
}
}
T datatype can be only numbers, is there a way to accept just numbers and accept the <=?

No, you can't constrain generics to numbers, but you can constrain T to IComparable<T> and then use CompareTo()
public class Range<T> where T : IComparable<T>
{
....
}
Then you can say:
if (min.CompareTo(max) >= 0)...
And throw a validation exception or whatever validation you'd like. You can use the same thing to make sure value is >= min and <= max.
if (value.CompareTo(min) >= 0 && value.CompareTo(max) <= 0)...

public class Range<T> where T : IComparable<T>
{
...
public bool Check(T value)
{
return value.CompareTo(min) >= 0 && value.CompareTo(max) <= 0;
}
}
If you range between 0 and 10, and want 0, 10 to fail (exclude min and max) - simply replace ">=" with ">" and "<=" with "<".
I would also recommend changing in the Equals override from:
return this.MinValue.Equals(other.MinValue) &&
this.MaxValue.Equals(other.MaxValue);
to this:
return this.MinValue.CompareTo(other.MinValue) == 0 &&
this.MaxValue.CompareTo(other.MaxValue) == 0;