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Why cannot I modify individual list items? [duplicate]

This question already has answers here:
Changing the value of an element in a list of structs
(7 answers)
Closed 10 months ago.
In the following program, I am unable to modify individual list items:
public class Program
{
static void Main(string[] args)
{
List<Point2d> list = new List<Point2d>();
list.Add(new Point2d(0, 0));
list.Add(new Point2d(0, 1));
foreach (Point2d item in list)
{
item.Print();
}
Point2d p = list[0];
p.Set(-1, -1);
foreach (Point2d item in list)
{
item.Print();
}
Console.ReadKey();
}
}
Output:
(0,0) (0,1) (0,0) (0,1)
My expected output was:
(0,0) (0,1) (-1,-1) (0,1)
What am I doing incorrectly?
Relevant source code:
public struct Point2d : IEquatable<Point2d>
{
public double X { get; set; }
public double Y { get; set; }
#region constructor
public Point2d(double x, double y)
{
X = x;
Y = y;
}
#endregion
public void Print()
{
Console.Write("(");
Console.Write(X);
Console.Write(",");
Console.Write(Y);
Console.Write(") ");
}
public void Set(double x, double y)
{
X = x;
Y = y;
}
public double GetDistance(Point2d otherPoint)
{
return Math.Sqrt(GetSquaredDistance(otherPoint));
}
public double GetSquaredDistance(Point2d otherPoint)
{
return ((otherPoint.X - X) * (otherPoint.X - X))
+ ((otherPoint.Y - Y) * (otherPoint.Y - Y));
}
public Point2d GetTranslated(Point2d center)
{
return new Point2d(X + center.X, Y + center.Y);
}
#region override string ToString()
public override string ToString()
{
StringBuilder sb = new StringBuilder();
sb.Append("(" + X + " , " + Y + ")");
return sb.ToString();
}
#endregion
#region equality comparison implementations
public override bool Equals(object other)
{
if (!(other is Point2d)) return false;
return Equals((Point2d)other);
}
public bool Equals(Point2d other)
{
return X == other.X && Y == other.Y;
}
public override int GetHashCode()
{
return (int)Math.Round(Y * 31.0 + X, 0); // 31 = some prime number
}
public static bool operator ==(Point2d a1, Point2d a2)
{
return a1.Equals(a2);
}
public static bool operator !=(Point2d a1, Point2d a2)
{
return !a1.Equals(a2);
}
#endregion
}

Point2d is a struct so when you did Point2d p = list[0]; you made a totally separate copy of the object. Your set only changed the copy not the original, you either need to make Point2d a class or add a list[0] = p; after the set.
Bugs like this is why it is recommended to make structs immutable and have no Set methods.

How can i convert the date type of the object to another?

This is my code I wrote a comment under the mistake. I am not allowed to do it in another way it should be two classes and it should be done in this way. If someone can help me i would appreciate this
Thank u
using System;
using MathLibrary;
namespace MathLibraryApp
{
class Program
{
static void Main(string[] args)
{
Vector v = new Vector();
Vector v1 = new Vector(4, 8, 12);
Vector v2 = new Vector(8,16,24);
Vector[] vectors = { v1, v2 };
Console.WriteLine(v.Add(vectors));
}
}
}
using System;
namespace MathLibrary
{
public class PointVectorBase
{
public PointVectorBase(double x=0 , double y=0 , double z=0 )
{
this.X = x;this.Y = y;this.Z = z;
}
protected virtual PointVectorBase CalculateSum(params Vector[] addends)
{
for (int i = 0; i < addends.Length; i++)
{
this.X = this.X + addends[i].X;
this.Y = this.Y + addends[i].Y;
this.Z = this.Z + addends[i].Z;
}
return this;
}
}
public class Vector : PointVectorBase
{
public Vector(double x = 0, double y = 0, double z = 0) : base(x, y, z){ }
public Vector Add(params Vector[] addends)
{
return this.CalculateSum(addends) ;
//Cannot implicitly convert type MathLibrary.PointVectorBase to MathLibrary.Vector. An explicit conversion exists (are you missing a cast?)
}
}
}

You can either cast the result like this:
public Vector Add(params Vector[] addends)
{
return this.CalculateSum(addends) As Vector;
}
This is dangerous though. Not all base vectors are vectors so you could have a null return. Same way as an animal is not always a cat in the public class cat: animal example.
Creating the implicit conversion is safer, though not always possible: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/user-defined-conversion-operators

Your method CalculateSum returns value type PointVectorBase. Method Add in Vector class should return Vector.
Due to inheritance you can cast result of a CalculateSum to a Vector so it would be return this.CalculateSum(addends) as Vector;

In this case I wouldn't go for inheritance. You are only extending the base class with methods.
The problem on your CalculateSum is that you're returning this as a result. Which is a strange pattern. Either go for a void method which alters the current instance or return a new instance (so leave the current instance unmodified). I would go for the latter.
If your question is about inheritance, this is not a good example you gave.
But if you want an other way:
In your example I would go for extension methods. Also this is a nice case to use structs. By writing extension methods, you can extend the Vector3 with extra methods..
using System;
namespace MathLibrary
{
public struct Vector3
{
public double X;
public double Y;
public double Z;
public Vector3(double x=0 , double y=0 , double z=0 )
{
this.X = x;
this.Y = y;
this.Z = z;
}
public Vector3 CalculateSum(params Vector3[] addends)
{
var result = new Vector3();
for (int i = 0; i < addends.Length; i++)
{
result.X = result.X + addends[i].X;
result.Y = result.Y + addends[i].Y;
result.Z = result.Z + addends[i].Z;
}
return result;
}
}
public static class VectorExtensions
{
public static Vector3 Add(this Vector3 vector, params Vector3[] addends)
{
return vector.CalculateSum(addends);
// the add should actually add to the current vector,
// which makes it less readable.. calculate sum and add is almost the same.
return vector.CalculateSum(
new Vector3 [] { vector }
.Concat(addends)
.ToArray() );
}
}
}
The more your code has a functional approach the less strange things will happen.

Overloading operators (like +,-,*,/) in abstract classes in C#

I am trying to implement the class Vector as an abstract one and have stuff like Vector2D and vector3D inherit from it. However, I ran into a problem with the overriding of operators, because they demand a static object reference be returned and I can't create one since I'm not allowed to call the abstract class' constructor there.
Here's the override I came up with:
public static Vector operator +(Vector v1, Vector v2)
{
if (v1.Dimension != v2.Dimension)
{
throw new Exception("Vector Dimensions Must Be Equal");
}
else
{
double[] newMatrix = new double[v1.Dimension]; //the matrix for the new (resulting) vector
for (int i = 0; i < v1.Dimension; i++)
{
newMatrix[i] = v1.Matrix[i] + v2.Matrix[i];
}
return new Vector(newMatrix);
//the abstract class has a constructor that takes a matrix
//and constructs a vector based on it, but it can't be called here
}
}
I tried both using the matrix constructor and generating a new empty instance (I have either of those constructors). Both boil down to the "can't instantiate abstract class" error.
I am aware that I could implement the operations in each of the classes that will derive from Vector, but I hoped there is some workaround/technique/design pattern which solves the problem in a more elegant way, but I don't seem to find any article out there talking about overriding operators in abstract classes (maybe that approach makes no sense..?). Anyway, any help is highly appreciated. Here is the whole class, if needed:
using System;
using System.Text;
namespace PhysicsProblems
{
public abstract class Vector
{
private static int IdCounter;
protected readonly int ID;
protected double[] Matrix;
protected int Dimension
{
get
{
return Matrix.Length;
}
}
public double Magnitude
{
get
{
double sum = 0;
foreach (var value in Matrix)
{
sum += value * value;
}
return Math.Sqrt(sum);
}
}
public Vector(int dimension)
{
ID = IdCounter++;
for (int i = 0; i < dimesion; i++)
{
Matrix[i] = 0;
}
}
public Vector(double[] matrix)
{
ID = IdCounter++;
matrix.CopyTo(Matrix, 0);
}
public static Vector operator +(Vector v1)
{
return v1;//to fix
}
public static Vector operator -(Vector v1)
{
for (int i = 0; i < v1.Matrix.Length; i++)
{
v1.Matrix[i] = -v1.Matrix[i];
}
return v1;//to fix
}
public static Vector operator +(Vector v1, Vector v2)
{
if (v1.Dimension != v2.Dimension)
{
throw new Exception("Vector Dimensions Must Be Equal");
}
else
{
double[] newMatrix = new double[v1.Dimension]; //the matrix for the new (resulting) vector
for (int i = 0; i < v1.Dimension; i++)
{
newMatrix[i] = v1.Matrix[i] + v2.Matrix[i];
}
return new Vector(newMatrix);
//the abstract class has a constructor that takes a matrix
//and constructs a vector based on it, but it can't be called here
}
}
public bool Equals(Vector vector)
{
if (vector.Dimension != Dimension) return false;
bool check = true;
for (int i = 0; i < Matrix.Length; i++)
{
if (vector.Matrix[i] != Matrix[i]) check = false;
}
return check;
}
public override bool Equals(object obj)
{
return Equals(obj as Vector);
}
public override int GetHashCode()
{
unchecked
{
int hash = 19;
hash = (hash * 486187739) + ID.GetHashCode();
hash = (hash * 486187739) + Dimension.GetHashCode();
return hash;
}
}
public override string ToString()
{
if (Dimension < 4)
{
var stringBuilder = new StringBuilder();
for (int i = 0; i < Dimension; i++)
{
stringBuilder
.Append(Constants.AxisLetters[i])
.Append(": ")
.Append(Matrix[i]);
if (i != Dimension - 1) stringBuilder.Append(", ");
}
return stringBuilder.ToString();
}
else
{
throw new NotImplementedException();
}
}
}
}
And the inheriting class Vector2D:
using System;
using System.Text;
namespace PhysicsProblems
{
public class Vector2D : Vector
{
public double X
{
get { return Matrix[0]; }
set { Matrix[0] = value; }
}
public double Y
{
get { return Matrix[1]; }
set { Matrix[1] = value; }
}
public Vector2D(int dimension) : base(dimension)
{ }
public Vector2D(double[] matrix) : base(matrix)
{ }
}
}
A little disclaimer I consider important: I'm fairly OK with OOP and have some experience, however, I took up C# a week or two ago. Therefore, rookie mistakes are to be expected

How cna I implement a priority queue for edges in a graph?

I want to implement my own Priority Queue in C# using the naive approach. I need to use PQ for edges in a graph to implement Prims algorithm. Here is what I've done so far:
public class Edge
{
private int v; // one vertex
private int w; // the other vertex
private int weight; // edge weight
public Edge(int v0, int w0, int weight0)
{
v = v0;
w = w0;
weight = weight0;
}
public Edge()
{
v = 0;
w = 0;
weight = 0;
}
public int get_weight()
{
return weight;
}
public int either()
{
return v;
}
public int the_other(int vertex)
{
if (vertex == v) return w;
else return v;
}
public int compareTo(Edge that)
{
if (this.get_weight() < that.get_weight()) return -1;
if (this.get_weight() < that.get_weight()) return 1;
else return 0;
}
public string to_str()
{
string s = v.ToString() + " " + weight.ToString() + " " + w.ToString() + '\n';
return s;
}
}
public class MyPriorityQueue_Edge
{
private List<Edge> q = new List<Edge>();
int size;
public MyPriorityQueue_Edge()
{
size = 0;
}
public void insert(Edge e)
{
q.Add(e);
size++;
}
Edge Min()
{
int min_weight = 1000;
Edge min_edge = new Edge();
foreach(Edge e in q)
if(e.get_weight()< min_weight)
min_edge = e;
return min_edge;
}
public Edge delmin()
{
Edge e = q.Min();
q.Remove(e);
size--;
return e;
}
public bool is_empty()
{
if (size == 0) return true;
else return false;
}
}
When I compile it the compiler points to this line:
Edge e = q.Min();
And says that I did not work on the exception "System.Argument.Exception". How can I fix this?

From MSDN Enumerable.Min<TSource> Method (IEnumerable<TSource>)
If type TSource implements IComparable<T>, this method uses that implementation to compare values. Otherwise, if type TSource implements IComparable, that implementation is used to compare values.
You need to implement IComparable interface in your Edge class.
public class Edge : IComparable
Then add a CompareTo() method. Something like
public int CompareTo(object obj)
{
if (obj is Edge)
{
Edge other = (Edge)obj;
return this.compareTo(other); // you already implemented this in your Edge class
}
else
{
throw new InvalidOperationException();
}
}

Casting Type array to Generic array?

The short version of the question - why can't I do this? I'm restricted to .NET 3.5.
T[] genericArray;
// Obviously T should be float!
genericArray = new T[3]{ 1.0f, 2.0f, 0.0f };
// Can't do this either, why the hell not
genericArray = new float[3]{ 1.0f, 2.0f, 0.0f };
Longer version -
I'm working with the Unity engine here, although that's not important. What is - I'm trying to throw conversion between its fixed Vector2 (2 floats) and Vector3 (3 floats) and my generic Vector<> class. I can't cast types directly to a generic array.
using UnityEngine;
public struct Vector<T>
{
private readonly T[] _axes;
#region Constructors
public Vector(int axisCount)
{
this._axes = new T[axisCount];
}
public Vector(T x, T y)
{
this._axes = new T[2] { x, y };
}
public Vector(T x, T y, T z)
{
this._axes = new T[3]{x, y, z};
}
public Vector(Vector2 vector2)
{
// This doesn't work
this._axes = new T[2] { vector2.x, vector2.y };
}
public Vector(Vector3 vector3)
{
// Nor does this
this._axes = new T[3] { vector3.x, vector3.y, vector3.z };
}
#endregion
#region Properties
public T this[int i]
{
get { return _axes[i]; }
set { _axes[i] = value; }
}
public T X
{
get { return _axes[0];}
set { _axes[0] = value; }
}
public T Y
{
get { return _axes[1]; }
set { _axes[1] = value; }
}
public T Z
{
get
{
return this._axes.Length < 2 ? default(T) : _axes[2];
}
set
{
if (this._axes.Length < 2)
return;
_axes[2] = value;
}
}
#endregion
#region Type Converters
public static explicit operator Vector<T>(Vector2 vector2)
{
Vector<T> vector = new Vector<T>(vector2);
return vector;
}
public static explicit operator Vector<T>(Vector3 vector3)
{
Vector<T> vector = new Vector<T>(vector3);
return vector;
}
#endregion
}

"Generic" means "works with any type".
Your example code is not generic, because it only works if and only if T is float.
While you can't convert a Vector2D to a Vector<T&gt, you can, of course, convert a Vector2D to a Vector<float>. Add a Convert method to Vector2D or provide a set of extension methods like this:
public static class VectorExtensions
{
public static Vector<float> ToGenericVector(this Vector2D vector)
{
return new Vector<float>(vector.X, vector.Y);
}
public static Vector2D ToVector2D(this Vector<float> vector)
{
return new Vector2D(vector.X, vector.Y);
}
}
Usage:
Vector<float> v = new Vector<float>(3, 5);
Vector2D v2 = v.ToVector2D();

if T is defined as float, via Vector<T> as Vector<float> then this will work (on a restricted T), but if you just want a local conversion:
var genericArray = new float[3]{ 1.0f, 2.0f, 0.0f };
Of course, this restricts T to being a float anyway (the compiler can't convert just anything to T and knows this), it looks like you should replace T with float in the whole class if that's the case, are you dealing with non-float vectors?
In that case you need something like:
var genericArray = new T[3]{ X, Y, Z };

You cannot imply the type of a generic parameter from within a method.
And, as I stated before, your posted code does not represent a valid usage of Generic type parameters.
The generic parameter is to be defined in the class or method signature.
public class Class1<T>
{
public T[] Method(params T[] args)
{
return args;
}
}
public class Demo
{
public Demo()
{
var c1 = new Class1<float>();
float[] result = c1.Method(1.1f, 2.2f);
}
}

You said:
// This doesn't work
this._axes = new T[2] { vector2.x, vector2.y };
The following works (since everything can be converted to object, and the subsequent conversion from object to T is permitted but may fail at runtime if the types aren’t compatible, or in this case if unboxing cannot be performed):
this._axes = new T[2] { (T)(object)vector2.x, (T)(object)vector2.y };
That said, it makes absolutely no sense to make the class generic.