First of all, please excuse any typo, English is not my native language.
Here's my question. I'm creating a class that represents approximate values as such:
public sealed class ApproximateValue
{
public double MaxValue { get; private set; }
public double MinValue { get; private set; }
public double Uncertainty { get; private set; }
public double Value { get; private set; }
public ApproximateValue(double value, double uncertainty)
{
if (uncertainty < 0) { throw new ArgumentOutOfRangeException("uncertainty", "Value must be postivie or equal to 0."); }
this.Value = value;
this.Uncertainty = uncertainty;
this.MaxValue = this.Value + this.Uncertainty;
this.MinValue = this.Value - this.Uncertainty;
}
}
I want to use this class for uncertain measurments, like x = 8.31246 +/-0.0045 for example and perform calculations on these values.
I want to overload operators in this class. I don't know how to implement the >, >=, <= and < operators... The first thing I thought of is something like this:
public static bool? operator >(ApproximateValue a, ApproximateValue b)
{
if (a == null || b == null) { return null; }
if (a.MinValue > b.MaxValue) { return true; }
else if (a.MaxValue < b.MinValue) { return false; }
else { return null; }
}
However, in the last case, I'm not satisfied with this 'null' as the accurate result is not 'null'. It may be 'true' or it may be 'false'.
Is there any object in .Net 4 that would help implementing this feature I am not aware of, or am I doing the correct way? I was also thinking about using an object instead of a boolean that would define in what circumstances the value is superior or not to another one rather than implementing comparison operators but I feel it's a bit too complex for what I'm trying to achieve...
I'd probably do something like this. I'd implement IComparable<ApproximateValue> and then define <, >, <=, and >= according to the result of CompareTo():
public int CompareTo(ApproximateValue other)
{
// if other is null, we are greater by default in .NET, so return 1.
if (other == null)
{
return 1;
}
// this is > other
if (MinValue > other.MaxValue)
{
return 1;
}
// this is < other
if (MaxValue < other.MinValue)
{
return -1;
}
// "same"-ish
return 0;
}
public static bool operator <(ApproximateValue left, ApproximateValue right)
{
return (left == null) ? (right != null) : left.CompareTo(right) < 0;
}
public static bool operator >(ApproximateValue left, ApproximateValue right)
{
return (right == null) ? (left != null) : right.CompareTo(left) < 0;
}
public static bool operator <=(ApproximateValue left, ApproximateValue right)
{
return (left == null) || left.CompareTo(right) <= 0;
}
public static bool operator >=(ApproximateValue left, ApproximateValue right)
{
return (right == null) || right.CompareTo(left) <= 0;
}
public static bool operator ==(ApproximateValue left, ApproximateValue right)
{
return (left == null) ? (right == null) : left.CompareTo(right) == 0;
}
public static bool operator !=(ApproximateValue left, ApproximateValue right)
{
return (left == null) ? (right != null) : left.CompareTo(left) != 0;
}
This is one of the rare cases where it may make more sense to define a value type (struct), which then eliminates the null case concern. You can also modify MinValue and MaxValue to be computed properties (just implement a get method that computes the result) rather than storing them upon construction.
On a side note, comparison of approximate values is itself an approximate operation, so you need to consider the use cases for your data type; are you only intending to use comparison to determine when the ranges are non-overlapping? It really depends on the meaning of your type. Is this intended to represent a data point from a normally distributed data set, where the uncertainty is some number of standard deviations for the sampling? If so, it might make more sense for a comparison operation to return a numeric probability (which couldn't be called through the comparison operator, of course.)
It looks to me like you need to check if a.MaxValue == b.MinValue also, in your current implementation that would return null, which seems incorrect, it should either return true or false based on how you want the spec to actually work. I'm not sure of any built in .net functionality for this, so I believe you are going about it the correct way.
return a.Value - a.Uncertainty > b.Value + b.Uncertainty
I wouldn't really mess with the semantics of >: I think bool? is a dangerous return type here. That said, given the uncertainty, you could return true, if a is more likely to be > b.
It seems to me that you're trying to implement some form of Ternary Logic because you want the result of applying the operators to be either True, False or Indeterminate. The problem with doing that is that you really cannot combine the built-in boolean values with your indeterminate value. So whilst you could do some limited form of comparison of two ApproximateValues I think that it's inappropriate to use bool as the result of these comparisons because that implies that the result of the comparisons can be freely combined with other expressions that result in bool values, but the possibility of an indeterminate value undermines that. For example, it makes no sense to do the following when the result of operation on the left of the OR is indeterminate.
ApproximateValue approx1 = ...;
ApproximateValue approx2 = ...;
bool result = ...;
bool result = approx1 > approx2 || someBool;
So, in my opinion, I don't think that it's a good idea to implement the comparisons as operators at all if you want to retain the indeterminacy. The solutions offered here eliminate the indeterminacy, which is fine, but not what was originally specified.
Related
I have List<Moves> listOfMoves
ListOfMoves.Add(new Moves()
{
int position1= number1,
int position2= number2,
});
Now I want to check if ListOfMoves contains for example Move(2,3), but also to check if it contains Move(3,2).
I tried if(ListOfMoves.Contains(new Move(2,3))) but this does not work properly.
Method List<T>.Contains(T item) internally uses method Object.Equals to check if objects are equal. Therefore if you want to use method List<T>.Contains(T item) with your type T to check if the specified item is contained in the List<T> then you need to override method Object.Equals in your type T.
When you override Object.Equals you should also override Object.GetHashCode. Here is a good explanation "Why is it important to override GetHashCode when Equals method is overridden?".
Here is how you should override Object.Equals in the Move class to fit your requirement:
class Move
{
public Move(int p1, int p2)
{
position1 = p1;
position2 = p2;
}
public int position1 { get; }
public int position2 { get; }
public override bool Equals(object obj)
{
if (obj == null)
return false;
if (ReferenceEquals(this, obj))
return true;
Move other = obj as Move;
if (other == null)
return false;
// Here we specify how to compare two Moves. Here we implement your
// requirement that two moves are considered equal regardless of the
// order of the properties.
return (position1 == other.position1 && position2 == other.position2) ||
(position1 == other.position2 && position2 == other.position1);
}
public override int GetHashCode()
{
// When implementing GetHashCode we have to follow the next rules:
// 1. If two objects are equal then their hash codes must be equal too.
// 2. Hash code must not change during the lifetime of the object.
// Therefore Move must be immutable. (Thanks to Enigmativity's usefull tip).
return position1 + position2;
}
}
When you override Object.Equals you will be able to use condition ListOfMoves.Contains(new Move(2, 3)) to check if moves Move(2, 3) or Move(3, 2) are contained in the ListOfMoves.
Here is complete sample that demostrates overriding of Object.Equals.
For this you can use LINQ's Any function. If you want both combinations for the positions [ (2,3) or (3,2) ] you'll need two pass in two checks
ListOfMoves.Any(x =>
(x.position1 == 2 && x.position2 == 3)
|| (x.position1 == 3 && x.position2 == 2) )
Any returns a bool so you can wrap this line of code in an if statement or store the result for multiple uses
Potential improvement
If you're going to be doing a lot of these checks (and you're using at least c# version 7) you could consider some minor refactoring and use the built in tuples type: https://learn.microsoft.com/en-us/dotnet/csharp/tuples
Moves would become
public class Moves
{
public (int position1, int position2) positions { get; set; }
}
And the Any call would become
ListOfMoves.Any(x => x.positions == (2,3) || x.positions == (3,2))
Else where in the code you can still access the underlying value of each position as so:
ListOfMoves[0].positions.position1
Obviously depends on what else is going on in your code so totally up to you!
Obviously it won't work cause you can't compare the entity itself rather you will have to compare with property values like below using System.Linq
ListOfMoves.Where(x => x.position1 == 2 && x.position1 == 3)
Note: Your posted code shouldn't compile at all in first place
You said .. I need to get true if either Move(3,2) or (2,3) is in List
Then use Any() using the same predicate like
if(ListOfMoves.Any(x => x.position1 == 2 && x.position1 == 3))
{
// done something here
}
I'm working on an implementation of the A-star algorithm in C# in Unity.
I need to evaluate a collection of Node :
class Node
{
public Cell cell;
public Node previous;
public int f;
public int h;
public Node(Cell cell, Node previous = null, int f = 0, int h = 0)
{
this.cell = cell;
this.previous = previous;
this.f = f;
this.h = h;
}
}
I have a SortedSet which allows me to store several Node, sorted by h property. Though, I need to be able to store two nodes with the same h property. So I've implemented a specific IComparer, in a way that allow me sorting by h property, and triggerring equality only when two nodes are representing the exact same cell.
class ByHCost : IComparer<Node>
{
public int Compare(Node n1, Node n2)
{
int result = n1.h.CompareTo(n2.h);
result = (result == 0) ? 1 : result;
result = (n1.cell == n2.cell) ? 0 : result;
return result;
}
}
My problem : I have a hard time to remove things from my SortedSet (I named it openSet).Here is an example:
At some point in the algorithm, I need to remove a node from the list based on some criteria (NB: I use isCell127 variable to focus my debug on an unique cell)
int removedNodesNb = openSet.RemoveWhere((Node n) => {
bool isSame = n.cell == candidateNode.cell;
bool hasWorseCost = n.f > candidateNode.f;
if(isCell127)
{
Debug.Log(isSame && hasWorseCost); // the predicate match exactly one time and debug.log return true
}
return isSame && hasWorseCost;
});
if(isCell127)
{
Debug.Log($"removed {removedNodesNb}"); // 0 nodes where removed
}
Here, the removeWhere method seems to find a match, but doesn't remove the node.
I tried another way :
Node worseNode = openSet.SingleOrDefault(n => {
bool isSame = n.cell == candidateNode.cell;
bool hasWorseCost = n.f > candidateNode.f;
return isSame && hasWorseCost;
});
if(isCell127)
{
Debug.Log($"does worseNode exists ? {worseNode != null}"); // Debug returns true, it does exist.
}
if(worseNode != null)
{
if(isCell127)
{
Debug.Log($"openSet length {openSet.Count}"); // 10
}
openSet.Remove(worseNode);
if(isCell127)
{
Debug.Log($"openSet length {openSet.Count}"); // 10 - It should have been 9.
}
}
I think the problem is related to my pretty unusual IComparer, but I can't figure whats exatcly the problem.
Also, I would like to know if there is a significative performance improvment about using an auto SortedSet instead of a manually sorted List, especially in the A-star algorithm use case.
If i write your test you do:
n1.h < n2.h
n1.cell = n2.cell -> final result = 0
n1.h > n2.h
n1.cell = n2.cell -> final result = 0
n1.h = n2.h
n1.cell != n2.cell -> final result = 1
n1.h < n2.h
n1.cell != n2.cell -> final result = -1
n1.h > n2.h
n1.cell != n2.cell -> final result = 1
when you have equality on h value (test number 3) you choose to have always the same result -> 1. so its no good you have to have another test on cell to clarify the position bacause there is a confusion with other test which gives the same result (test number 5)
So i could test with sample, but i am pretty sure you break the Sort.
So if you clarify the test, i suggest you to use Linq with a list...its best performance.
I'll answer my own topic because I've a pretty complete one.
Comparison
The comparison of the IComparer interface needs to follow some rules. Like #frenchy said, my own comparison was broken. Here are math fundamentals of a comparison I totally forgot (I found them here):
1) A.CompareTo(A) must return zero.
2) If A.CompareTo(B) returns zero, then B.CompareTo(A) must return zero.
3) If A.CompareTo(B) returns zero and B.CompareTo(C) returns zero, then A.CompareTo(C) must return zero.
4) If A.CompareTo(B) returns a value other than zero, then B.CompareTo(A) must return a value of the opposite sign.
5) If A.CompareTo(B) returns a value x not equal to zero, and B.CompareTo(C) returns a value y of the same sign as x, then A.CompareTo(C) must return a value of the same sign as x and y.
6) By definition, any object compares greater than (or follows) null, and two null references compare equal to each other.
In my case, rule 4) - symetry - was broken.
I needed to store multiple node with the same h property, but also to sort by that h property. So, I needed to avoid equality when h property are the same.
What I decided to do, instead of a default value when h comparison lead to 0 (which broke 4th rule), is refine the comparison in a way that never lead to 0 with a unique value foreach node instance. Well, this implementation is probably not the best, maybe there is something better to do for a unique value, but here is what I did.
private class Node
{
private static int globalIncrement = 0;
public Cell cell;
public Node previous;
public int f;
public int h;
public int uid;
public Node(Cell cell, Node previous = null, int f = 0, int h = 0)
{
Node.globalIncrement++;
this.cell = cell;
this.previous = previous;
this.f = f;
this.h = h;
this.uid = Node.globalIncrement;
}
}
private class ByHCost : IComparer<Node>
{
public int Compare(Node n1, Node n2)
{
if(n1.cell == n2.cell)
{
return 0;
}
int result = n1.h.CompareTo(n2.h);
result = (result == 0) ? n1.uid.CompareTo(n2.uid) : result; // Here is the additional comparison which never lead to 0. Depending on use case and number of object, it would be better to use another system of unique values.
return result;
}
}
RemoveWhere method
RemoveWhere use a predicate to look into the collection so I didn't think it cares about comparison. But RemoveWhere use internally Remove method, which do care about the comparison. So, even if the RemoveWhere have found one element, if your comparison is inconstent, it will silently pass its way. That's a pretty weird implementation, no ?
I'm having a little difficulty with the array.sort. I have a class and this class has two fields, one is a random string the other one is a random number. If i want to sort it with one parameter it just works fine. But i would like to sort it with two parameters. The first one is the SUM of the numbers(from low to high), and THEN if these numbers are equal by the random string that is give to them(from low to high).
Can you give some hint and tips how may i can "merge" these two kinds of sort?
Array.Sort(Phonebook, delegate(PBook user1, PBook user2)
{ return user1.Sum().CompareTo(user2.Sum()); });
Console.WriteLine("ORDER");
foreach (PBook user in Phonebook)
{
Console.WriteLine(user.name);
}
That's how i order it with one parameter.
i think this is what you are after:
sourcearray.OrderBy(a=> a.sum).ThenBy(a => a.random)
Here is the general algorithm that you'll use for comparing multiple fields in a CompareTo method:
public int compare(MyClass first, MyClass second)
{
int firstComparison = first.FirstValue.CompareTo(second.SecondValue);
if (firstComparison != 0)
{
return firstComparison;
}
else
{
return first.SecondValue.CompareTo(second.SecondValue);
}
}
However, LINQ does make the syntax for doing this much easier, allowing you to only write:
Phonebook = Phonebook.OrderBy(book=> book.Sum())
.ThenBy(book => book.OtherProperty)
.ToArray();
You can do this in-place by using a custom IComparer<PBook>. The following should order your array as per your original code, but if two sums are equal it should fall back on the random string (which I've called RandomString):
public class PBookComparer : IComparer<PBook>
{
public int Compare(PBook x, PBook y)
{
// Sort null items to the top; you can drop this
// if you don't care about null items.
if (x == null)
return y == null ? 0 : -1;
else if (y == null)
return 1;
// Comparison of sums.
var sumCompare = x.Sum().CompareTo(y.Sum());
if (sumCompare != 0)
return sumCompare;
// Sums are the same; return comparison of strings
return String.Compare(x.RandomString, y.RandomString);
}
}
You call this as
Array.Sort(Phonebook, new PBookComparer());
You could just do this inline but it gets a bit hard to follow:
Array.Sort(Phonebook, (x, y) => {
int sc = x.Sum().CompareTo(y.Sum());
return sc != 0 ? sc : string.Compare(x.RandomString, y.RandomString); });
... Actually, that isn't too bad, although I have dropped the null checks.
In our application we work with DataTables a lot. This is dictated by the interface to a another system. Often a column in one of these DataTable's is in fact an enumeration, which is then of a Int16 datatype. Currently we use magic constants all over the place, but that isn't pretty. A real enum would be much better, but how can you write an elegant comparison? Especially considering that a DBNull is also sometimes a valid value.
Ideally we would write this:
if ( tbl.Rows[0]["EnumColumn"] == MyEnum.SomeValue )
// Do stuff
But, naturally, that will not work. The closest to what I can come is:
if ( tbl.Rows[0]["EnumColumn"] != DBNull.Value && Convert.ToInt32(tbl.Rows[0]["EnumColumn") == (int)MyEnum.SomeValue )
// DO stuff
Which looks plain ugly. Any ideas on how to make this prettier and easier to write?
It should be something like this:
tbl.Rows[0]["EnumColumn"] != DbNull.Value && Convert.ToInt32(tbl.Rows[0]["EnumColumn"]) == MyEnum.SomeValue
I would make a static method for it:
public enum TestEnum
{
A = 1,
B = 2
}
public static bool EqualsTestEnum(object value, TestEnum enumValue)
{
if (value == null || value == DBNull.Value)
{
return false;
}
int i;
if (int.TryParse(value.ToString(), out i))
{
return i == (int) enumValue;
}
return false;
}
Take a look at this class:
public class MemorialPoint:IMemorialPoint,IEqualityComparer<MemorialPoint>
{
private string _PointName;
private IPoint _PointLocation;
private MemorialPointType _PointType;
private DateTime _PointStartTime;
private DateTime _PointFinishTime;
private string _NeighborName;
private double _Rms;
private double _PointPdop;
private double _PointHdop;
private double _PointVdop;
// getters and setters omitted
public bool Equals(MemorialPoint x, MemorialPoint y)
{
if (x.PointName == y.PointName)
return true;
else if (x.PointName == y.PointName && x.PointLocation.X == y.PointLocation.X && x.PointLocation.Y == y.PointLocation.Y)
return true;
else
return false;
}
public int GetHashCode(MemorialPoint obj)
{
return (obj.PointLocation.X.ToString() + obj.PointLocation.Y.ToString() + obj.PointName).GetHashCode();
}
}
I also have a Vector class, which is merely two points and some other atributes. I don't want to have equal points in my Vector, so I came up with this method:
public void RecalculateVector(IMemorialPoint fromPoint, IMemorialPoint toPoint, int partIndex)
{
if (fromPoint.Equals(toPoint))
throw new ArgumentException(Messages.VectorWithEqualPoints);
this.FromPoint = FromPoint;
this.ToPoint = ToPoint;
this.PartIndex = partIndex;
// the constructDifference method has a weird way of working:
// difference of Point1 and Point 2, so point2 > point1 is the direction
IVector3D vector = new Vector3DClass();
vector.ConstructDifference(toPoint.PointLocation, fromPoint.PointLocation);
this.Azimuth = MathUtilities.RadiansToDegrees(vector.Azimuth);
IPointCollection pointCollection = new PolylineClass();
pointCollection.AddPoint(fromPoint.PointLocation, ref _missing, ref _missing);
pointCollection.AddPoint(toPoint.PointLocation, ref _missing, ref _missing);
this._ResultingPolyline = pointCollection as IPolyline;
}
And this unit test, which should give me an exception:
[TestMethod]
[ExpectedException(typeof(ArgumentException), Messages.VectorWithEqualPoints)]
public void TestMemoriaVector_EqualPoints()
{
IPoint p1 = PointPolygonBuilder.BuildPoint(0, 0);
IPoint p2 = PointPolygonBuilder.BuildPoint(0, 0);
IMemorialPoint mPoint1 = new MemorialPoint("teste1", p1);
IMemorialPoint mPoint2 = new MemorialPoint("teste1", p2);
Console.WriteLine(mPoint1.GetHashCode().ToString());
Console.WriteLine(mPoint2.GetHashCode().ToString());
vector = new MemorialVector(mPoint1, mPoint1, 0);
}
When i use the same point, that is, mPoint1, as in the code the exception is thrown. When I use mPoint2, even their name and coordinates being the same, the exception is not thrown. I checked their hash codes, and they are in fact different. Based on the code I created in GetHashCode, I tought these two point would have the same hashcode.
Can someone explain to me why this is not working as I tought it would? I'm not sure I explained this well, but.. I appreciate the help :D
George
You're implementing IEqualityComparer<T> within the type it's trying to compare - which is very odd. You should almost certainly just be implementing IEquatable<T> and overriding Equals(object) instead. That would definitely make your unit test work.
The difference between IEquatable<T> and IEqualityComparer<T> is that the former is implemented by a class to say, "I can compare myself with another instance of the same type." (It doesn't have to be the same type, but it usually is.) This is appropriate if there's a natural comparison - for example, the comparison chosen by string is ordinal equality - it's got to be exactly the same sequence of char values.
Now IEqualityComparer<T> is different - it can compare any two instances of a type. There can be multiple different implementations of this for a given type, so it doesn't matter whether or not a particular comparison is "the natural one" - it's just got to be the right one for your job. So for example, you could have a Shape class, and different equality comparers to compare shapes by colour, area or something like that.
You need to override Object.Equals as well.
Add this to your implementation:
// In MemorialPoint:
public override bool Equals(object obj)
{
if (obj == null || GetType() != obj.GetType())
return false;
MemorialPoint y = obj as MemorialPoint;
if (this.PointName == y.PointName)
return true;
else if (this.PointName == y.PointName && this.PointLocation.X == y.PointLocation.X && this.PointLocation.Y == y.PointLocation.Y)
return true;
else
return false;
}
I'd then rework your other implementation to use the first, plus add the appropriate null checks.
public bool Equals(MemorialPoint x, MemorialPoint y)
{
if (x == null)
return (y == null);
return x.Equals(y);
}
You also need to rethink your concept of "equality", since it's not currently meeting .NET framework requirements.
If at all possible, I recommend a re-design with a Repository of memorial point objects (possibly keyed by name), so that simple reference equality can be used.
You've put an arcobjects tag on this, so I just thought I'd mention IRelationalOperator.Equals. I've never tested to see if this method honors the cluster tolerance of the geometries' spatial references. This can be adjusted using ISpatialReferenceTolerance.XYTolerance.