I have class with a List<int> member. If I want to clone an instance of this class, do I need a deep copy or the MemberwiseClone() shallow copy is enough?
We need a deep copy if at least one member is a reference to an object, right? Does that mean having List, DateTime, String, MyClass, ..., will need a deep copy?
This entirely depends on how you plan to use the copy.
If you do a shallow copy like
List<int> x = new List<int>() { 1, 2, 3, 4, 5 };
List<int> y = x;
y[2] = 4;
Then x will contain {1, 2, 4, 4, 5 }
If you do a deep copy of the list:
List<int> x = new List<int> { 1, 2, 3, 4, 5 };
List<int> y = new List<int>(x);
y[2] = 4;
Then x will contain { 1, 2, 3, 4, 5 } and y will contain { 1, 2, 4, 4, 5 }
How you plan on using the copy really determines whether you use shallow or deep copies.
If you have List<int> originalList = new List{1, 2}
then doing this:
List<int> newList = new List<int>();
foreach(int i in originalList)
newList.Add(i);
will get you a cloned list.
However, if you tried what I did above with a List generic on some reference type, then you would not succeed. After altering one of the objects in your list, you would see the altered version whether referencing it from the original list or the new list.
Short answer, yes.
Long answer, you need to determine what the copy in your case actually means with respect to the list. Do you need a copy of the contents of the list as well? For value types like ints, its pretty straight forward, if it were reference types though....yo have some questions to ask regarding what you want your list to contain.
Related
I've been trying to write a program, in which I want to use the intersection of two HashSets. Therefore I wrote the following code (for test purposes):
HashSet<int> test1 = new HashSet<int>() { 1, 3, 5, 7, 9 };
HashSet<int> test2 = new HashSet<int>() { 1, 2, 3, 4, 5, 6};
HashSet<int> intersect = new HashSet<int>();
intersect = test1.Intersect(test2);
Line 5 shows an error (code CS0266) which - that's C#'s suggestion - can be corrected by change the line to:
intersect = (HashSet<int>)test1.Intersect(test2);
But when I run the program, the error appears again. I literally have no clue why, even after searching for an answer.
I want to achieve a intersection in the mathematical sense, so that the result for the variable intersect should be { 1, 3, 5}.
And what I found out - but couldn't test - is, that after using the intersect-method on test1, it changes the list in test1 to the intersection, is that true? If yes, is there any chance to avoid this? In my real program I don't want the variable to change into the intersection.
Should I just make a for-loop with an if-statement, to make my own intersection-method, or does this make the code worse?
As said, I tried to make use of C#'s suggestion, but this doesn't work either.
Because I'm a programming-beginner, I'm not really able to understand the definition of the intersect-method (because of this IEnumerable thing...), so I can't solve the problem using existend methods. And because I think my own method could be very inefficient, I don't to it my own. Furthermore I just want to understand, what's the problem. There are two HashSets, both containing integers, which should be intersected and saved in an extra variable...
Intersect() returns a IEnumerable<T>. You can use IntersectWith(), which modifies the current HashSet<T> object to contain only elements that are present in that object and in the specified collection:
HashSet<int> test1 = new HashSet<int>() { 1, 3, 5, 7, 9 };
HashSet<int> test2 = new HashSet<int>() { 1, 2, 3, 4, 5, 6};
test1.IntersectWith(test2); // we are altering test1 here
// test1 contains now [1, 3, 5]
or use the side-effect free Linq Intersect() to get an IEnumerable<T> and if you want it to be a new HashSet<T> just use a constructor:
HashSet<int> test1 = new HashSet<int>() { 1, 3, 5, 7, 9 };
HashSet<int> test2 = new HashSet<int>() { 1, 2, 3, 4, 5, 6};
HashSet<int> intersect = new HashSet<int>(test1.Intersect(test2));
// intersect contains now [1, 3, 5]
Remarks (from MSDN)
If the collection represented by the other parameter is a HashSet<T> collection with the same equality comparer as the current HashSet<T> object, this method is an O(n) operation. Otherwise, this method is an O(n + m) operation, where n is Count and m is the number of elements in other.
Basically in your case IntersectWith() is going to be more efficient!
Complete demo:
using System;
using System.Linq;
using System.Collections.Generic;
public class Program
{
public static void Main()
{
HashSet<int> test1 = new HashSet<int>() {1, 3, 5, 7, 9};
HashSet<int> test2 = new HashSet<int>() {1, 2, 3, 4, 5, 6};
HashSet<int> intersect = new HashSet<int>(test1.Intersect(test2));
intersect.Dump();
test1.IntersectWith(test2);
test1.Dump();
}
}
Try it Online!
Enumerable.Intersect is a LINQ extension method that works with any kind of IEnumerable<T>. It returns IEnumerable<T>, so not a HashSet<T>. But since you have alrady two sets you want to use HashSet.IntersectWith(more efficient since O(n)) with manipulates the first HashSet<T>:
test1.IntersectWith(test2); // test1 contains now [1, 3, 5]
I can do this with an integer:
int a;
a = 5;
But I can't do this with an integer array:
int[] a;
a = { 1, 2, 3, 4, 5 };
Why not?
To clarify, I am not looking for the correct syntax (I can look that up); I know that this works:
int[] a = { 1, 2, 3, 4, 5 };
Which would be the equivalent of:
int a = 5;
What I am trying to understand is, why does the code fail for arrays? What is the reason behind the code failing to be recognised as valid?
The reason there is a difference is that the folks at Microsoft decided to lighten the syntax when declaring and initializing the array in the same statement, but did not add the required syntax to allow you to assign a new array to it later.
This is why this works:
int[] a = { 1, 2, 3, 4, 5 };
but this does not:
int[] a;
a = { 1, 2, 3, 4, 5 };
Could they have added the syntax to allow this? Sure, but they didn't. Most likely they felt that this use-case is so seldom used that it didn't warrant prioritizing over other features. All new features start with minus 100 points and this probably just didn't rank high enough on the priority list.
Note that { 1, 2, 3, 4, 5 } by itself has no meaning; it can only appear in two places:
As part of an array variable declaration:
int[] a = { 1, 2, 3, 4, 5 };
As part of an array creation expression:
new int[] { 1, 2, 3, 4, 5 }
The number 5, on the other hand, has a meaning everywhere it appears in C#, which is why this works:
int a;
a = 5;
So this is just special syntax the designers of C# decided to support, nothing more.
This syntax is documented in the C# specification, section 12.6 Array Initializers.
The reason your array example doesn't work is because of the difference between value and reference types. An int is a value type. It is a single location in memory whose value can be changed.
Your integer array is a reference type. It is not equivalent to a constant number of bytes in memory. Therefore, it is a pointer to the bytes where that data is stored.
In this first line, you are assigning null to a.
int[] a;
In the next line, if you want to change the value of the array, you need to assign it to a new array.
a = new[] {1, 2, 3, 4, 5};
That is why you need the new[] before the list of values within the array if you strongly type your declaration.
int[] a = {1, 2, 3, 4, 5}; // This will work.
var a = {1, 2, 3, 4, 5}; // This will not.
However, as many of the other answers have said, if you declare it in a single line, then you do not need the new[]. If you separate the declaration and initialization, then you are required to use new[].
{} syntax is available for array initialization, not to be used after declaration.
To initialize an array you should try like this:
int[] a = { 1, 2, 3, 4, 5 };
Other ways to Initializing a Single-dimensional array:
int[] a = new int[] { 1, 2, 3, 4, 5 };
int[] a = new int[5] { 1, 2, 3, 4, 5 };
Have a look at this: different ways to initialize different kinds of arrays
I have got this structure
var values = new Dictionary<int, List<Guid>>();
And I have to say if all dictionary elements has the same set of List<Guid>.
I dont need to know which are exactly are different, just to answer the question.
So it looks like
List A { 1, 2, 3} List B { 1, 2, 3} List C { 1, 2, 3} the same and have no difference.
and
List A { 3, 2, 3} List B { 1, 2, 3} List C { 1, 2, 3} are not the same.
I have no clue where I can start it.
Initially i guessed to convert List<Guid> to string and just do distinct operation over it.
But is this a good approach?
Thank you!
I'd create a HashSet<Guid> from one of the values (any) and then check that all of the others are equal to it:
// TODO: Handle the dictionary being empty
var firstSet = new HashSet<Guid>(values.First().Value);
var allEqual = values.All(pair => firstSet.SetEquals(pair.Value));
This assumes that:
The order within each list is unimportant
The number of times each GUID appears in the list is unimportant
(i.e. you really are thinking of them as sets, not lists, at least for this part of the code)
In other words, if you have guids A and B, the code above assumes that { A, B, B } is equivalent to { B, A }.
SequenceEquals() might be what you're looking for. Combine it with IEnumerable.All() and you can get a boolean answer whether all elements of your dictionary contain the same Lists. For instance:
values.All(list => values.All(list2 => list2.SequenceEquals(list));
i'm new to c# with python background.
In python, i can do this way:
a = [1, 5, 10] #initialize
a = [1, 3, 5] #change the value
How can i perform a = {1, 3, 5} on c#? Currently i found this way.
int[] a = {1, 5, 10}; //initialize
int[] b = {1 ,3, 5}; //create new reference type
a = (int[])b.Clone(); //change the value
Is there any better way to to change entire array a from {1, 5, 10} to {1 ,3, 5}?
i.e. a = [1, 3, 5] on python to C#?
Maybe I am missing something, but surely just...
a = new int[]{1, 3, 5};
a is a variable like in python.
When you assign to the variable a = {1, 5, 10} it assigns the value of the reference to the array in the variable. This is the shorthand syntax for assigning arrays.
To reassign the variable you simply assign to it again using the new operator a = new[] {1, 5, 5}.
Sample:
int[] a = {1, 5, 10}; // Shorthand syntax for creating array
a = new[] {1, 5, 5}; // Reassign using new operator
See Array initializers
[EDIT: It seems that the OP didn't want to make a copy of an existing array, but just to create an entirely new one. But I'll leave this here in case.]
That's the normal way to create a copy of an array that isn't just a reference to the original array.
There is a faster way of doing it, which is to use Array.Copy() directly. You can wrap it in an extension method to make it easier to use:
/// <summary>Shallow clones an array.</summary>
/// <typeparam name="T">The type of array elements.</typeparam>
/// <param name="array">An array to clone.</param>
/// <returns>The cloned array.</returns>
/// <remarks>This is much faster than the built-in <see cref="Array.Clone"/> method.</remarks>
public static T[] ShallowClone<T>(this T[] array)
{
if (array == null)
{
throw new ArgumentNullException("array");
}
T[] result = new T[array.Length];
Array.Copy(array, result, array.Length);
return result;
}
Normally you wouldn't worry about performance, but this does happen to be faster than using Array.Copy().
You can use it like so:
int[] a = {1, 2, 3};
int[] b = a.ShallowClone();
This way, it's both faster and you don't need to cast (although you could of course also write an extension method that used Array.Clone().)
Yes, you can use indexer (which is zero based)
a[2] = 5;
If you want to change whole array, then assign a new reference for it, like
int[] a = new[] {1, 2, 10};
a = new[] {1,2,5};
Your original array will lose all the references to it and become subject to GC. The second line assigns a a reference to the new array.
This should be a pretty basic question, but I've been having a little trouble finding a definite answer.
When you have an array of values and you use the .ToArray() method does it create a deep or shallow copy of the array?
No.
You can easily verify this by writing a small program to test.
Not strictly speaking, ICollection.ToArray() creates a new T[] and assigns each element in the original collection to the new array using Array.CopyTo().
Note:
If T is a value type, values are assigned and not references. This will behave as one would expect of a "Deep" copy.
int[] foo = { 1, 2, 3, 4 };
int[] bar = foo.ToArray();
for (int i = 0; i < foo.Length; i++) foo[i] += 10;
Console.WriteLine(string.Join(',', foo)); // 11, 12, 13, 14
Console.WriteLine(string.Join(',', bar)); // 1, 2, 3, 4