What Is the Internal Implementation of Lists

What is the internal implementation of lists?

Lists are essentially just arrays of R objects (SEXP). Resizing causes copies of the whole data and name lookup is linear.

Alternatively, you can use environments, which use hash tables internally.

Internal working of List in c#

List is implemented the same way C++'s vector, meaning the implementation allocate an array of a predefined size, fills that array, and when you want to add an element and the array is full the implementation allocate a new array, bigger, copies all values to the new array, and then adds the new value.
This results with an AVERAGE performance of O(1) when adding, but not always.

What's the implementation of List?

The implementation of List<T> can't be shown from Visual Studio because it doesn't have the source code present. It just shows the outline of the class (that is why Visual Studio puts [metadata] on top of the 'code file' when hitting F12).

The actual source can be found on referencesource.microsoft.com.

If List equals ArrayList or something, why .net will allow two class which equals function but different name? If List doesn't equal any other class in .NET, so why give it such an ambiguous name?

No, they are not the same. ArrayList is a non-generic list implementation, while List<T> is generic, and thus strongly typed.

Regarding the ambiguous name: I think Microsoft is right in their naming of List. ArrayList was a terrible name anyway. It emphasizes the implementation too much. You don't care there is an array behind it: to you it is just a List. Given that the name was available, this was a good option for a name.

What is internal implementation of Iteration in java that allows remove() method to work?

You can know that by simply looking at the source code of ArrayList, which comes with the JDK:

ArrayList has this field (inherited from AbstractList):

protected transient int modCount = 0;

It's incremented each time an operation is done on the list. For example, ArrayList.remove() has the following instruction:

modCount++;

The iterator of the ArrayList has the following code:

int expectedModCount = modCount;

public E next() {
   checkForComodification();
   [...]
}

final void checkForComodification() {
    if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
}

So, as you can see, when it iterates, it checks that the modCount of the list hasn't been modified.

And its remove() method does the following:

ArrayList.this.remove(lastRet);
[...];
expectedModCount = modCount;

So it removes the element from the list, and then updates its expectedModCount, so that the next operation done with the iterator doesn't fail when calling checkForComodification().

The key thing to understand is that the method iterator() returns an object that has access to the internal structure of the list. Not only to be able to iterate over its elements, but also to check if that list has been modified between two operations done using the iterator.

How is Python's List Implemented?

It's a dynamic array. Practical proof: Indexing takes (of course with extremely small differences (0.0013 µsecs!)) the same time regardless of index:

...>python -m timeit --setup="x = [None]*1000" "x[500]"
10000000 loops, best of 3: 0.0579 usec per loop

...>python -m timeit --setup="x = [None]*1000" "x[0]"
10000000 loops, best of 3: 0.0566 usec per loop

I would be astounded if IronPython or Jython used linked lists - they would ruin the performance of many many widely-used libraries built on the assumption that lists are dynamic arrays.

How is ListT internally mapped?

List<> is an implementation of a data structure, which takes care of allocating memory on a on-demand basis; it allows for insertion and deletion at any index etc. Therefore it is much more convenient than a simple array.

Under the hood, the current List<> implementation uses an array for storage, and the overhead when doing array-like operations is minimal. The added convenience is usually worth the little (if at all relevant) performance difference. Adding items is typically faster because the list allocates chunks of memory and doesn't require a new allocation and copy on every add (compared to the pure array, where the Length is always bound to the size in memory).

ArrayList internal implementation

An ArrayList only contain references to the objects, not the objects themselves. All references are the same size, so the problem doesn't exist.

The internal type of the reference is surely object.

For generic arrays of value types, the actual value is stored in the array and the size of the element is used as you describe. If you put a value type in an ArrayList it will be boxed into an object and the reference to that object is stored in the ArrayList.

Scala immutable list internal implementation

Yup, this is performance killer, but this is a cost of having immutable structures (which are amazing, safe and makes programs much less buggy). That's why you should often avoid appending list if you can. There is many tricks that can avoid this issue (try to use accumulators).

For example:

Instead of:

val initialList = List(1,2,3,.....1 million)
val myList = List(1,2,3,...,100)
val output = myList.foldLeft(initialList)(_ :+ _)

You can write:

val initialList = List(1,2,3,.....1 million)
val myList = List(1,2,3,...,100)
val output = List(initialList,myList).flatten

Flatten is implemented to copy first line only once instead of copying it for every single append.

P.S.

At least adding element to the front of list works fast (O(1)), cause sharing of old list is possible. Let's Look at this example:

You can see how memory sharing works for immutable linked lists. Computer only keeps one copy of (b,c,d) end. But if you want to append bar to the end of baz you cannot modify baz, cause you would destroy foo, bar and raz! That's why you have to copy first list.

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