I have a query regarding a concept.
Are dynamic arrays in c# also called list? Or they are totally different?
C# does not have a dynamic array type. However, you can use a List as a dynamic array, given that it supports indexed access:
List<string> list = new List<string>();
list.Add("Hello");
list.Add("Goodbye");
Console.WriteLine(list[1]);
Take a look at Generic Lists.
Since it's the first link I found out when looking for this difference I would like to contribute a little.
The Nitin Bisht response is not entirely wrong, but by most of programming definitions a dynamic array is a type of array that can be set when the program is already running, simply put, it's size is set during run time not in compilation.
In C# this is written as:
string[] myDynamicArray;
myDynamicArray = SomeMethodThatReturnsAnCollection().ToArray();
Other way would be a method which returns an existing array like:
string[] myDynamicArray;
myDynamicArray = SomeMethodThatReturnsAnArray();
Those are Dynamic Arrays, you don't tell your program which size it has but inside the logic of that context this will be decided.
Those Arrays in C# has the Resize method, which enables the programmer to grow the array, which I believe it was the Nitin Bisht and so many others interpretation around that question, but this is a wrong assumption, resizable is different than dynamic.
Instead of using Resize in your Array, you could just use a List type, List in C# implements an Array in it's core, which can be seen here, but it also has the ability to grow in an easier approach than the Array implementation enables us with the Add(T item) List method.
List in C# would be a native Dynamic Resizable Array, some languages call similarly implementations of this structure as Vector, which are not a LinkedList with nodes and pointers for example.
Summarizing the difference in code:
var myList = new List<string> { "initial value" };
myList.Add("New value"); // After add, myList has this new value
var myArray = new string[] { "initial value" };
myArray.Append("New value") // after Append myArray doesnt have a new value
With Append() method what happens is another IEnumerable collection is created with the new value, so if you iterate through both collections you will see that the list prints out the new value but the array doesn't, in that case one way you can fix that is doing this:
string[] myArray;
myArray = myArray.Append("New value").ToArray(); // now it will work
The difference here is that myArray variable is being rewritten with the new array generated by .Append().ToArray() line, in the List example the Add() method will resize and then add the new value.
Important to note that this resize is done in a similar manner than what we did with the Append() "fixed" code, the List creates a new Array with increased size and copies it's original contents to this new Array.
You can use the resources given by the Array class to get to the same result as the Add() method implemented by List, but off course if you need that method you should just use an List instead of writing your own implementation, very rarely you would have to do it and to be honest your implementation would be probably less efficient than the already built ones.
In summary that is the difference between them, as you can see they have similarities but I would still treat them very differently.
Usually you wanna use an array when you do not expect to grow any further, if that is your case then an array is more useful since it's has less implementations and that brings a performance gain which can be good in a lot of scenarios.
If not, just use lists.
Related
I know that for object, we can forEach the collection and update the object as we like but for immutable objects like Strings, how can we update the array with new object without converting it into an array again.
For e.g, I have an array of string. I want to iterate through each string and trim them. I would otherwise have to do something like this:
Arrays.stream(str).map(c -> c.trim()).collect(Collectors.toList())
In the end, I will get a List rather then String[] that I initially gave. Its a whole lot of processing. Is there any way I can do something similar to:
for(int i = 0; i < str.length; i++) {
str[i] = str[i].trim();
}
using java streams?
Streams are not intended for manipulating other data structures, especially not for updating their source. But the Java API consists of more than the Stream API.
As Alexis C. has shown in a comment, you could use Arrays.setAll(arr, i -> arr[i].trim());
There’s even a parallelSetAll that you could use when you have a really large array.
However, it might be easier to use just Arrays.asList(arr).replaceAll(String::trim);.
Keep in mind that the wrapper returned by Arrays.asList allows modifications of the wrapped array through the List interface. Only adding and removing is not supported.
Use toArray :
str = Arrays.stream(str).map(c -> c.trim()).toArray(String[]::new);
The disadvantage here (over your original Java 7 loop) is that a new array is created to store the result.
To update the original array, you can re-write your loop with Streams, though I'm not sure what's the point :
IntStream.range (0, str.length).forEach (i -> {str[i] = str[i].trim();});
It's not that much processing as you might think, the array has a known size and the spliterator from it will be SIZED, thus the resulting collection size will be known before processing and the space for it can be allocated ahead of time, without having to re-size the collection.
It's also always interesting that in the absence of actual tests we almost always assume that this is slow or memory hungry.
of course if you want an array as the result there is a method for that :
.toArray(String[]::new);
Out of pure interest, why do most programming languages not allow the programmer to reinitialise an array after it's creation.
Example
int apples[4][4]
apples[0][1] = "blue"
apples = apples[8][8] // Reinitialise the array with a new size of 8x8
apples[7][4] = "purple"
Explanation of what I mean
As you can see above, I create an array that is 4x4, then I assign a value, then I reinitialise that same array with a new size of 8x8, then I assign another value. In theory, I'd prefer that it destroy the contents of the old array (so my new 8x8 array doesn't have that value at 0x1).
However, I've searched high and low, yet I've not managed to find anything that explains why programming languages enforce this restriction. In my eyes it seems greatly beneficial to allow this and I can't see any immediate issues. But clearly there is an issue otherwise this would be allowed.
Question
So my question is: What's the reason that programming languages do not allow programmers to reinitialise an array after it's creation?
Initialization can be done until code is not in running state, that's why initialization can take place only one time in a code, for the second time if you are initializing same array, its already out of creation phase and now in running phase.
for better clarity size of array needs to be a constant at compile time because it might be getting used in code at somewhere.
int i=arr.lenght;
if(i<5)
{
//do something}
you can resize a dynamic array by realloc() in c and by using collection properties with arrayList, but its not re initialization.
I was reading an article about array vs list, and the author says that an array is worse than a list, because (among other things) an array is a list of variables, but to me a list is also a list of variables. I mean, I can still do list[3] = new Item().
Actually, I have always somehow saw a List<T> like a wrapper for an array that allows me to use it easily without caring about handling its structure.
What are the internal differences between a T[] and a List<T> in terms of heap/stack memory usage?
Since an array is a static structure, after the initialization, it allocates the memory that you've demanded.
int arr[5];
For example here there are 5 int objects created in memory. But when you use lists, according to its implementation, it gives you first an array with predefined capacity. And while you are adding your elements, if you exceed the capacity then it scales up. In some implementations it just doubles its size, or in some implementations it enlarges itself when the granted capacity is half full.
The author's point about a "list of variables" wasn't about memory. It's that an array contains your internal variables, and returning it allows them to be reassigned by the caller. It comes down to this:
Only pass out an array if it is wrapped up by a read-only object.
If you pass out an internal List<T>, you have the same problem, but here's the key:
We have an extensibility model for lists because lists are classes. We
have no ability to make an “immutable array”. Arrays are what they are
and they’re never going to change.
And, at the time the article was written, the IReadOnlyList interface didn't exist yet (.NET 4.5), though he probably would have mentioned it if it had. I believe he was advocating implementing an IList<T> that would simply throw an exception if you tried to use the setter. Of course, if the user doesn't need the ability to access elements by index, you don't need a list interface at all -- you can just wrap it in a ReadOnlyCollection<T> and return it as an IEnumerable<T>.
I am playing around with OOP in MATLAB, and I have the following constructor:
function obj = Squadron(num_fighters, num_targets, time_steps)
if nargin == 0
num_targets = 100;
time_steps = 100;
num_fighters = 10;
end
obj.num_shooters = num_fighters;
for iShooter = 1:obj.num_shooters
a(iShooter) = Shooter(num_targets, time_steps);
end
obj.ShooterArray = a;
obj.current_detections = zeros(num_fighters, num_targets);
end
That temporary variable 'a' smells terrible. Is there a better way to initialize an array of objects, I wish there was a push/pop method. I am sure there is a better way to do this.
Looks like you are trying to create an array of handle objects (Shooters) and store it inside the property of another handle object (a Squardron). I have had a very similar problem discussion that might help you.
In short: What you are doing might not be pretty - but might be pretty good already.
When creating an array in Matlab it is usually a good Idea to do some pre-allocation to reserve memory which speeds up performance significantly.
In a normal case something like this:
a=zeros(1,1000);
for n=1:1000
a(n)=n;
end
(here a=1:1000; would be even better)
For objects the pre-allocation works by assigning one of the objects to the very last field in the array. Matlab then fills the other fields before that with objects (handles) that it creates by calling the constructor of that object with no arguments (see Matlab help). Hence a pre-allocation for objects could look like this:
a(1,1000)=ObjectConstructor();
for n=1:1000
a(n)=ObjectConstructor();
end
or simply
for n=1000:-1:1
a(n)=ObjectConstructor();
end
Making sure Shooter can be called with no arguments you should be able to do something like:
for iShooter = obj.num_shooters:-1:1
obj.ShooterArray(iShooter) = Shooter(num_targets, time_steps);
end
However, it turns out that for some reason this direct storing of an array of objects in another object's property creates very bad performance. (Probably the array pre-allocation does not work well in this case). Hence using an auxiliary variable and allocating the full array at once to the property is in this case is a good idea to increase performance.
I would try:
for iShooter = obj.num_shooters:-1:1
a(iShooter) = Shooter(num_targets, time_steps);
end
obj.ShooterArray = a;
Again - for more detail see this discussion
There are a couple of ways to handle this situation...
Building object arrays in the constructor:
You could modify your Shooter class such that when you pass arrays of values it creates an array of objects. Then you could initialize ShooterArray like so:
obj.ShooterArray = Shooter(repmat(num_targets,1,num_fighters),...
repmat(time_steps,1,num_fighters));
Replicating instances of a value class:
If Shooter is a value class, and each object is going to be exactly the same (i.e. you don't initialize any of its default properties to random values), then you can create just one object and replicate it using REPMAT:
obj.ShooterArray = repmat(Shooter(num_targets,time_steps),1,num_fighters);
Unfortunately, if Shooter is a subclass of the handle class, you can't just replicate it as you can with a value class. You would actually be replicating references to just one object, when you really need a number of separate objects each with their own unique reference. In such a case, your current code is likely the best solution.
I have an array in Scala with the class ArrayBuffer[Actor], where Actor is a class that implements the Ordered[Actor] trait. How do I sort this array without coding it manually?
I know there is an object called Sorting, but it doesnt seem to work since ArrayBuffer doesn't implement/extend the right classes.
How do I sort ArrayBuffer[A] type arrays?
If you are using Scala 2.8, you could use the sortWith method of the ArrayBuffer[T] class, which is inherited from the SeqLike trait.
The following code snippet sorts an ArrayBuffer[T] object in ascending order:
def ascendingSort[T <% Ordered[T]](xs: ArrayBuffer[T]) = xs.sortWith(_ < _)
Note that this does not mutate the actual ArrayBuffer, but creates a new one with the elements in the right order.
If you are using Scala 2.7, you could use the stableSort method of the Sorting object. This takes the elements of the ArrayBuffer and produces an array of elements sorted in the right order (given by a closure as an argument, ascending as a default).
For example:
val a = new scala.collection.mutable.ArrayBuffer[Int]()
a += 5
a += 2
a += 3
scala.util.Sorting.stableSort(a)
The important question is what do you want to do with the ArrayBuffer. Usually, a Buffer is used internally in different algorithms in order to increase the performance of intermediate results. If you are using it for that, have a look at the ways of sorting the collection you want to return at the end of your algorithm. The Sorting object already provides a way of transforming an ArrayBuffer into a sorted Array.
From the scaladoc of the Buffer class:
Buffers are used to create sequences of elements incrementally
As you are using it with Actors, it might be used for some kind of actor queue - in which case, you might want to have a look at the Queue collection.
Hope it helps,
-- Flaviu Cipcigan
Btw, the Actor class here is my own class used for "Actors" in a world created using my new game engine for scala ("Awesome Game Engine for Scala ~ AGES"), so it has nothing to do with the concurrency actor class. Also, implementations of lists in scala are a jungle, everything is either deprecated or implemented in a lot of different ways...ArrayBuffer works for my need (my need being a variable size array for containing actors).
Hope this clarifies :)