I have a class which should be immutable in this class i have only get indexer a private set property so why this is not immutable and i can set some field in array as you could see in main class...
class ImmutableMatice
{
public decimal[,] Array { get; private set; } // immutable Property
public ImmutableMatice(decimal[,] array)
{
Array = array;
}
public decimal this[int index1, int index2]
{
get { return Array[index1, index2]; }
}
........
and in main method if i fill this class with data and change the data
static void Main(string[] args)
{
decimal[,] testData = new[,] {{1m, 2m}, {3m, 4m}};
ImmutableMatice matrix = new ImmutableMatice(testData);
Console.WriteLine(matrix[0,0]); // writes 1
testData[0, 0] = 999;
Console.WriteLine(matrix[0,0]); // writes 999 but i thought it should
// write 1 because class should be immutable?
}
}
Is there any way how to make this class immutable?
Ah yes the solution was copy array to new array in constructor like this:
public ImmutableMatice(decimal[,] array)
{
decimal[,] _array = new decimal[array.GetLength(0),array.GetLength(1)];
//var _array = new decimal[,] { };
for (int i = 0; i < array.GetLength(0); i++)
{
for (int j = 0; j < array.GetLength(1); j++)
{
_array[i, j] = array[i, j];
}
}
Array = _array;
}
That is because you are actually changing the data in the ARRAY, rather than the indexer.
static void Main(string[] args)
{
decimal[,] testData = new[,] {{1m, 2m}, {3m, 4m}};
ImmutableMatice matrix = new ImmutableMatice(testData);
Console.WriteLine(matrix[0,0]); // writes 1
testData[0, 0] = 999; // <--- THATS YOUR PROBLEM
Console.WriteLine(matrix[0,0]); // writes 999 but i thought it should
// write 1 because class should be immutable?
}
You can copy the array into your private property in the constructor to avoid this situation.
Note that you indeed cannot write matrix[0,0] = 999; because the indexer has no setter.
Edit
As Chris pointed out (how could I have missed it myself?) - you shouldn't expose the array as a property at all (which means in most cases it doesn't even have to be a property).
Consider the following code instead:
private decimal[,] _myArray; // That's private stuff - can't go wrong there.
public decimal this[int index1, int index2]
{
// If you only want to allow get data from the array, thats all you ever need
get { return Array[index1, index2]; }
}
Your class is immutable, but the objects inside it aren't.
Having public decimal[,] Array { get; private set; } will only guarantee that you cannot set the property Array to a new instance of Array, but it does not prevent you from accessing the existing object and changing its values (which aren't immutable).
You might want to look into the appropriately named ReadOnlyCollection<T> class.
As #Mike pointed out and I looked past the first time: there's a twist to this because you are accessing the value through the testData object and not through matrix. While the original point still stands, it is more exact to say that the problem you have is that you are changing values in the underlying object which has its reference passed around. You're bypassing the ImmutableMatice object alltogether.
The beforementioned solution of using a ReadOnlyCollection<T> still stands: by creating this read-only wrapper around it, you won't be able to change it anymore afterwards. Howver this is only the case when you actually use it the way its intended: through ImmutableMatice and not through the underlying collection which you still have a reference to.
Another solution that solves this problem is to copy the contents of the original array to another one to "disconnect" it from the array your still have a reference to.
In order to illustrate this, consider the following samples. The first one demonstrates how the underlying reference can still be influenced while the second one shows how it can be solved by copying your values to a new array.
void Main()
{
var arr = new[] { 5 };
var coll = new ReadOnlyCollection<int>(arr);
Console.WriteLine (coll[0]); // 5
arr[0] = 1;
Console.WriteLine (coll[0]); // 1
}
void Main()
{
var arr = new[] { 5 };
var arr2 = new int[] { 0 };
Array.Copy(arr, arr2, arr.Length);
var coll = new ReadOnlyCollection<int>(arr2);
Console.WriteLine (coll[0]); // 5
arr[0] = 1;
Console.WriteLine (coll[0]); // 5
}
Related
I am trying to add a few different members to a list, but when the list is added to it contains copies of only the last member added:
private PotentialSolution tryFirstTrack(PotentialSolution ps, List<PotentialSolution> possibleTracks)
{
for (Track trytrack = Track.Empty + 1; trytrack < Track.MaxVal; trytrack++)
{
if (validMove(ps.nextSide, trytrack))
{
ps.SetCell(trytrack);
possibleTracks.Add(ps);
}
}
return tryNextTrack(ps, possibleTracks);
}
The PotentialSolution class looks like this:
public class PotentialSolution
{
public Track[,] board;
public Side nextSide;
public int h;
public int w;
static int cellsPerSide;
static bool testing;
static int minTracks;
.....
public void SetCell(Track t)
{
board[h, w] = t;
}
}
So we are trying to make several copies of the board which only differ by which 'track' is placed in the current cell.
If I have a breakpoint at possibleTracks.Add(ps) then I can see by inspecting ps that the required cell contents is changing each time, as required.
But when the code reaches the next line (or the return statement), the cell content is the same in each member of the list (it's the last one that was added).
What I am doing wrong here? I have tried using an ArrayList and also a basic array instead, but get the same result. It's acting as though the board member is decared as static, but it's not.
[edit]
In response to those who suggested making copies of ps, you are correct and I had tried this before - but only tried single-stepping after the change and didn't run the full program (this method is used hundreds of times). When running the full program, making copies of ps certainly makes a difference to the result (although it's still not correct). The problem now, and why I didn't stick with using the copies, is that an added test still shows the list to contain the same versions of ps, even though the debugger has shown 2 or 3 different tracks being deployed:
private PotentialSolution tryFirstTrack(PotentialSolution ps, List<PotentialSolution> possibleTracks)
{
for (Track trytrack = Track.Empty + 1; trytrack < Track.MaxVal; trytrack++)
{
if (validMove(ps.nextSide, trytrack))
{
PotentialSolution newps = new PotentialSolution(ps);
newps.SetCell(trytrack);
possibleTracks.Add(newps);
}
}
// temporary test, can be removed
if (possibleTracks.Count >= 2)
{
PotentialSolution ps1 = new PotentialSolution(possibleTracks.First());
PotentialSolution ps2 = new PotentialSolution(possibleTracks.Last());
if (ps1.GetCell() != ps2.GetCell())
{
// should always get here but never does
int foo = 1;
}
}
return tryNextTrack(ps, possibleTracks);
}
By the way, Track and nextSide are just enum integers, they will be 0-6, and the list will contain 0,1,2,or 3 members, never more.
You are adding references to the same object: ps in possibleTracks.Add(ps)
You could add a constructor to PotentialSolution duplicating the class:
public class PotentialSolution
{
public Track[,] board;
public Side nextSide;
public int h;
public int w;
static int cellsPerSide;
static bool testing;
static int minTracks;
//.....
public PotentialSolution()
{
}
public PotentialSolution(PotentialSolution ps)
{
board = ps.board;
nextSide = ps.nextSide;
h = ps.h;
w = ps.w;
}
//.....
Then use:
private PotentialSolution tryFirstTrack(PotentialSolution ps, List<PotentialSolution> possibleTracks)
{
for (Track trytrack = Track.Empty + 1; trytrack < Track.MaxVal; trytrack++)
{
if (validMove(ps.nextSide, trytrack))
{
ps.SetCell(trytrack);
possibleTracks.Add(new PotentialSolution(ps)); // duplicate object
}
}
return tryNextTrack(ps, possibleTracks);
}
This creates a new instance of the class each time it is added to the list.
Consider giving the PotentialSolution type value semantics by making it a struct and implementing a Clone method, or a constructor that takes another PotentialSolution as an argument. Also, to clone a 2D array of value types, call Object.Clone() and cast the result to T[,].
When making a copy of your PotentialSolution, you'll need to make sure your clone your board array, because, in your case, each PotentialSolution keeps its own representation of the state of the board.
I feel like the critical part you're missing is how to shallow clone a 2D array, which in general, is:
T[,] copy = (T[,])original.Clone();
WARNING: Clone creates a shallow copy of the array. For value-types this copies the values of each element, so for your int-like "Track" type it does what you want, but for other readers who may be using reference-types (like classes) it does not clone each object referred to by each element of the array. The elements of the new array are just object references, and will still refer to the same objects referred to by the elements of the original array. See the documentation.
Full example below that changes the middle cell of a 3x3 board from A to B.
using System;
using System.Linq;
public enum Track { A, B, C }
public enum Side { X, Y, Z }
public struct PotentialSolution
{
public Track[,] board;
public Side nextSide;
public int h;
public int w;
public void SetCell(Track t)
{
board[h, w] = t;
}
public PotentialSolution(Track[,] board, Side nextSide, int h, int w)
{
this.board = (Track[,])board.Clone();
this.nextSide = nextSide;
this.h = h;
this.w = w;
}
public PotentialSolution Clone()
{
return new PotentialSolution(board, nextSide, h, w);
}
// This `ToString` is provided for illustration only
public override string ToString()
{
var range0 = board.GetLength(0);
var range1 = board.GetLength(1);
var b = board;
return string.Join(",",
Enumerable.Range(0, range0)
.Select(x => Enumerable.Range(0, range1)
.Select(y => b[x, y]))
.Select(z => "[" + string.Join(",", z) + "]"));
}
}
class Program
{
static void Main(string[] args)
{
Track[,] someBoard = new Track[3, 3];
PotentialSolution ps1 = new PotentialSolution(someBoard, Side.X, 1, 1);
ps1.SetCell(Track.A);
PotentialSolution ps2 = ps1.Clone();
ps2.SetCell(Track.B);
Console.WriteLine(ps1);
Console.WriteLine(ps2);
}
}
I'm filling in the blanks liberally, so please excuse any assumptions I have made that differ from your actual situation, because I have done so only to make this example self-contained. My ToString implementation and its usage of System.Linq is not necessary; it's purely for the purposes of displaying the 2D array in my example.
You always call SetCell on the same ps object you received as a parameter then add the same instance to the possibleTracks list. The result is: possibleTrack contains ps n times and because it is the same instance you used in each cycle it will have the last change you applied via SetCell call.
Not sure what you wanted to achieve but it looks you need a modified copy of ps in each cycle for adding to possibleTrack list. Making PotentialSolution a struct instead of class could be enough? Structs are copied in such a way but may hit your performance if PotentialSolution is big.
The board member will still generate the same problem, because despite ps will be copied but the board inside it will contain same Track references. The trick can be applied to Track too, but the performance issues may raise more.
Just implement a Clone on PotentialSolution to have fully detached instances of it, then call ````SetCell``` on cloned instance and add that instance to the list.
I'm looking for a way to replace some of my List objects with arrays in my project to boost performance. The reasoning is that the List I would replace do not change size often (sometimes only once), so it would make sense to swap them out with arrays. Also, I would like to prevent allocations on the heap as much as possible.
So my idea is to create a struct (ArrayStruct) with two members "Count" and "Array". The struct would have some functions to add/remove/get/set/ect... elements in the array. The count would keep count of the elements that are usable in the array, and the array would only increase in size, never decrease.
Note that this is for a very particular case because I know the array size won't be very big.
The main problem I have is when I pass the reference to an already existing ArrayStruct object (named a in the example) to another (named b in the example) - I can edit the array in it, but once I resize it in a, I get an out of range exception in b. Is this because of the way I'm updating the reference via System.Array.Resize, so that the object b somehow does not attribute this change?
The only thing I could come up with as an idea was trying to override the assignment operator so that it would create a new array on assignment... but obviously that does not work.
My ArrayStruct
public struct ArrayStruct<T>
{
[SerializeField] private int m_Count;
[SerializeField] private T[] m_Array;
public int Count => m_Count;
public void Initialize(int startSize)
{
m_Count = 0;
m_Array = new T[startSize];
}
public T GetAt(int index)
{
return m_Array[index];
}
public void SetAt(int index, T newValue)
{
m_Array[index] = newValue;
}
public void Add(T newElement)
{
if (m_Array == null) {
m_Array = new T[1];
} else if (m_Array.Length == m_Count) {
System.Array.Resize(ref m_Array, m_Count + 1);
}
m_Array[m_Count] = newElement;
m_Count++;
}
public void Remove(T element)
{
for (int i = 0; i < m_Count; ++i) {
if (!element.Equals(m_Array[i])) {
continue;
}
for (int j = index; j < m_Count - 1; ++j) {
m_Array[j] = m_Array[j + 1];
}
m_Count--;
m_Array[m_Count] = default(T);
break;
}
}
//Trying to overload the = operating by creating a auto cast, this gives a compile error
/*public static implicit operator ArrayStruct<T>(ArrayStruct<T> otherArray)
{
var newArray = new ArrayStruct<T>();
newArray.m_Count = otherArray.Count;
newArray.m_Array = new T[otherArray.Length];
otherArray.m_Array.CopyTo(newArray.m_Array);
return newArray;
}*/
An example showcasing the problem
var a = new ArrayStruct<string>()
a.Add("hello");
var b = a;
print (b.GetAt(0)); //"hello"
a.SetAt(0, "new value for a");
print(b.GetAt(0));//"new value for a" , changing a changed b because the array is the same in both, this is bad
a.Add("resizing array");
print (b.GetAt(1)); //"Out of range exception" , because the array was resized for a but not for b therefore the connection broke
So do any of you have an idea of how I could make a new copy the array when I assign the struct to another variable?
Of course, I know I could use a function to do something like so
b = new ArrayStruct(a);
But I want it to be implicit. Or if there was a way to prevent assignments that would also work for me.
var a = new ArrayStruct();//let this work
var b = a; //Prevent this from happening
If you have any other solutions for replacing Lists with arrays conveniently please let me know
So do any of you have an idea of how I could make a new copy the array when I assign the struct to another variable? Of course, I know I could use a function to do something like so b = new ArrayStruct(a); But I want it to be implicit.
You can't do that. If a is a struct and you execute var b = a, then it will always just set b to a shallow copy of a; there's no way to change that behavior.
Doing something like b = new ArrayStruct(a) is the best you can do.
You're not understanding what the "=" operator is doing there. Objects are defined by reference, meaning when you write var b = a; you are actually asking for what you wish to avoid to happen, that is - object "b", pointing to the same object, which is "a".
You're passing the object "b" to point to the reference of object "a", which object holds the address to the object in question you defined, using the new keyword var a = new ArrayStruct<string>()
Your operator overloading does not work because you can't use it for conversions of the same type as the class you're writing the operator in. You'd either have to make a separate object (class), which would defeat the point you're trying to fix, or pass the m_array instead: public static implicit operator ArrayStruct<T>(T[] array)
You can also do something like this:
public static ArrayStruct<T> CopyFrom (ArrayStruct<T> array)
{
var newArray = new ArrayStruct<T>();
newArray.m_Array = new T[array.Count + 1];
newArray.Count = array.Count;
Array.Copy(array.m_Array, 0, newArray.m_Array, 0, array.m_Array.Length);
return newArray;
}
Now it should work exactly as you wanted.
var a = new ArrayStruct<string>();
a.Add("hello");
var b = ArrayStruct<string>.CopyFrom(a);
a.SetAt(0, "new value for a");
a.Add("resizing array");
Console.WriteLine(b.Count); // b returns 1
Console.WriteLine(a.Count); // a returns 2
As to why your index was going out of bounds, i.e it did not preserve the reference found in "a", this is all in correlation with references, yet again.
In the following code System.Array.Resize(ref m_Array, Count + 1); you're changing the reference found for object m_array in "a", when you're adding the new element with the line a.Add("resizing array");. That essentially means that your "a" object's m_array (which is another object of type array[]) is now pointing to a new object (of type array[]), which is of a new size.
Okay, but object "a" is now using the new reference, whilst object "b" is still using the old one. m_array in object "b" only has a single element, while the new m_array object in "a" has the newly added element "resizing array", along with any previously added ones.
I believe that's how System.Array.Resize works? Edit me if I'm wrong.
Below is a resource explaining how to do a deep cloning of an object, if that's what you truly wish to do (i.e create a completely new copy of an object, as well as any other objects inside)
Deep cloning of objects
I have a 'Movie' class in my C# code that has an int[] ratings = new int[10]; as a field. I would like to place numbers in this empty array from my main program.
For that, I would need a method, that could point to the actual free index of the array to put the integer there, but the other integer that would point to the free index would be reset to 0 everytime the method is called. Thus, my question is, that how can I place an integer in my method that is increased everytime the method was called.
This is the method in the class:
public void Rate(int rating)
{
int x = 0;
ratings[x] = rating;
}
This is how I call it in the main program
Movie asd = new Movie(blabla...);
Rate.asd(1);
Rate.asd(1);
Rate.asd(1);
So I called it 3 times, and I would want the 'x' integer in the class's method to increase.
Thanks in advance.
First of all, you have an error in the code you have posted.
As I suppose rather than:
Movie asd = new Movie(blabla...);
Rate.asd(1);
Rate.asd(1);
Rate.asd(1);
you want to paste here:
Movie asd = new Movie(blabla...);
asd.Rate(1);
asd.Rate(1);
asd.Rate(1);
As C# does not allow to use static method variables (like i.e. C++ does) you have two options:
first, make x value (from Rate method) a Movie's class variable, so Rate method will "remember" the next index,
second (and better) rather than intiger array - if possible use any kind of list or queue (which can manage indexing for you).
The problem is that local variables are discarded when exiting a method.
class SomeClass
{
private int x = 42;
public void DoSometing(int y)
{
int a = y + 5;
x += a * a;
// a stops to exist here
}
}
Solution is to store the variable in the containing class as well
class SomeOtherClass
{
private int x = 42;
private int a = 0;
public void DoSomething(int y)
{
a = y + 5;
x += a * a;
}
}
Now SomeOtherClass remembers the value of a. That's basically the point of member variables a.k.a. fields - to store the state of the object.
More appropriate for your problem:
class ClassWithAnArrayAndCount
{
private int[] values = new int[10];
private int taken = 0;
public void Add(int value)
{
if (taken == 10)
throw new ArgumentOutOfRangeException(); // sorry, no more space
values[taken++] = value;
}
public int Taken { get { return taken; } }
}
Here's a piece of code coming from a C# project using a 2d array. For a reason I don't understand my program compiles perfectly but during run-time it crashes.
public class Tile_Info
{
public int id;
public Tile_Info(int _id)
{
id = _id;
}
}
class Program
{
public static void Main(string[] args)
{
int width = 20;
int height = 30;
Tile_Info[,] my_tile;
my_tile = new Tile_Info[width, height];
for(int y = 0; y < height; y++)
{
for(int x = 0; x < width; x++)
{
my_tile[x, y].id = 0;
}
}
}
}
According to the debugger it's because "Object reference not set to an instance of an object", but I'm pretty sure it's what I'm doing here: my_tile = new Tile_Info[width, height];.
Anyone can tell what's wrong? Thank you for your support!
The creation of the array does not create the objects themselves, just like a creation of a parking lot does not create the cars that park there.
You still need to create the objects yourself. Change
my_tile[x, y].id = 0;
to
my_tile[x, y] = new Tile_Info(0);
This only happens when reference types (class) are used because the thing that is stored in the array is a reference to an instance, instead of the instance itself. On a lower level this (more or less) means that the memory for the instance is not yet allocated, just the memory for its reference, so you must new up an instance to initialize it. On the other hand if Tile_Info is a value type (struct) then the array will contain the actual instance, and the new Tile_Info[width, height] would have initialized the allocated memory to valid start state (all zeroes), which is exactly what the default parameterless constructor of a value type does.
So, if you had defined Tile_Info like this:
public struct Tile_Info
{
public int id; // this should be a property, public fields are baaad
public Tile_Info(int _id){ id = _id;}
}
both my_tile[x, y].id = 0 and my_tile[x, y] = new Tile_Info(0) would have been legal.
You have created a new 2D Tile_Info array by the first new keyword.
You have only created an empty array that can hold items of type Tile_Info. Its your job to create individual items and put them in the array.
typedef struct {
int e1;
int e2;
int e3;
int e4;
int e5;
} abc;
void Hello(abc * a, int index)
{
int * post = (&(a->e1) + index);
int i;
for(i = 0; i<5; i++)
{
*(post + i) = i;
}
}
The problem I face here is how they able to access the next element in the struct by
*(post + i)
I'm not sure how all these would be done in C# and moreover, I don't want to use unsafe pointers in C#, but something alternate to it.
Thanks!
You should replace the struct with an array of 5 elements.
If you want to, you can wrap the array in a class with five properties.
edit...
When you say 'Wrap,' it generally means to write properties in a class that set or get the value of either a single variable, an array element, or a member of another class whose instance lives inside your class (the usual usage here = 'wrap an object'). Very useful for separating concerns and joining functionality of multiple objects. Technically, all simple properties just 'wrap' their private member variables.
Sample per comment:
class test
{
int[] e = new int[5];
public void Hello(int index)
{
for (int i = 0; i <= 4; i++) {
// will always happen if index != 0
if (i + index > 4) {
MsgBox("Original code would have overwritten memory. .Net will now blow up.");
}
e[i + index] = i;
}
}
public int e1 {
get { return e[0]; }
set { e[0] = value; }
}
public int e2 {
get { return e[1]; }
set { e[1] = value; }
}
//' ETC etc etc with e3-e5 ...
}
The problem with the C code is that if index is greater than 0 it runs off the end of the abc struct, thus overwriting random memory. This is exactly why C#, a safer language, does not allow these sorts of things. The way I'd implement your code in C# would be:
struct abc
{
public int[] e;
}
void Hello(ref abc a, int index)
{
a.e = new int[5];
for (int i = 0; i < 5; ++i)
a.e[index + i] = i;
}
Note that if index > 0, you'll get an out of bounds exception instead of possibly silent memory overwriting as you would in the C snippet.
The thinking behind the C codes is an ill fit for C#. The C code is based on the assumption that the fields of the struct will be placed sequentially in memory in the order defined the fields are defined in.
The above looks like either homework or a contrived example. Without knowing the real intent it's hard to give a concrete example in C#.
other examples here suggest changing the data structure but if you can't/don't want to do that, you can use reflection combined with an array of objects of the struct type to accomplish the same result as above.
void Hello(abc currentObj){
var fields = typeof(abc).GetFields();
for(var i = 0;i<fields.Length;i++){
fields[i].SetValue(currentObj,i);
}
}