How do I expand an array in C# without using the new keyword

Question

I have a char array in C#.

var arr = new char[3] { 'a','b','c' };

How do I add spaces to the end of it without creating a new array?

result: arr = { 'a', 'b', 'c', ' ', ' ', ' ' };

This might sound similar to VB.NET's ReDim. But I'm not sure that is what I want either. I want to preserve the elements inside of it and not instantiate a new array behind the scenes.

Is this only possible with Generic Collections and ArrayList?

Thanks

"I know its possible in Java to do it in place." I'd be really interested in knowing how, I'm fairly sure it's not. — millimoose
– millimoose, Commented Aug 15, 2012 at 20:39
Are the arrays we're talking about large enough or so large in number that ones created in the background really do matter for as long as it'll take gc to grab the old ones? — jwrush
– jwrush, Commented Aug 15, 2012 at 20:41
@millimoose You know what, you're right. It's not possible in Java. +1 — The Internet
– The Internet, Commented Aug 15, 2012 at 20:42
@jwrush Yes, reading/setting are both O(1). That's not what you said. You said adding/removing, both of which are O(n). — Servy
– Servy, Commented Aug 15, 2012 at 21:05

Daniel · Accepted Answer · 2012-08-15 20:50:25Z

8

No, this is not possible using an array, generic or otherwise.. AFAIK, there is no way to dynamically resize an array. Use a List instead.

As Martin pointed out in the comments, even the List class uses an array in its internal implementation. If you want to truly be able to dynamically resize a data structure without reinitializing it, you must implement your own version of a linked list.

System.Collections.Generic contains a class called LinkedList that represents a doubly-linked list (meaning that each node has a reference to both the next and the previous node), but I'm not sure if its internal implementation uses an array..

edited Aug 15, 2012 at 20:50

answered Aug 15, 2012 at 20:43

Daniel

2,9743 gold badges25 silver badges40 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Martin Liversage Over a year ago

List uses an array as the backing store so that doesn't help. There is no way to dynamically resize a block of memory allocated on the managed heap.

Daniel Over a year ago

Well, honestly, when I think List I think linked list, which is dynamically resizeable (right?).. If its internal implementation uses an array, however, then you're correct. I'll edit my post to reflect this.

Jonathan Prates Over a year ago

Agreed. When an array is created, the property IsFixedSize (IList) is True. According [msdn.microsoft.com/en-us/library/… "An array with a fixed size does not allow the addition or removal of elements after the array is created, but it allows the modification of existing elements." So use other Collection.

Servy Over a year ago

@MartinLiversage While List technically violates the OPs requirement of resizing an array, it probably is what he actually wants to use in practice, because odds are he doesn't really need to dynamically resize an array. Suggesting it, as done in this answer, is correct.

Andre Calil · Accepted Answer · 2012-08-16 11:20:43Z

8

Unfortunately, arrays are pre-fixed by design. This is important because it will reserve the necessary amout of memory at the heap.

So, to answer your requirement about not creating a new one: it won't be possible.

There is, however, a work-around. Look the following method:

Array.Resize(ref myArr, myArr.Length + 5);

It works as described at the source:

This method allocates a new array with the specified size, copies elements from the old array to the new one, and then replaces the old array with the new one.

If array is null, this method creates a new array with the specified size.

If newSize is greater than the Length of the old array, a new array is allocated and all the elements are copied from the old array to the new one. If newSize is less than the Length of the old array, a new array is allocated and elements are copied from the old array to the new one until the new one is filled; the rest of the elements in the old array are ignored. If newSize is equal to the Length of the old array, this method does nothing.

This method is an O(n) operation, where n is newSize.

This means that myArr will be updated to reference the new array. However, if there is another reference to the original array, this won't be updated (it will keep referencing the older version).

Source: MSDN

edited Aug 16, 2012 at 11:20

answered Aug 15, 2012 at 20:41

Andre Calil

7,69236 silver badges41 bronze badges

2 Comments

Daniel Mann Over a year ago

The original question states "I want to preserve the elements inside of it and not instantiate a new array behind the scenes". If you read the MSDN page, it explicitly says that it creates a new array and copies the elements into it.

Andre Calil Over a year ago

@DanielMann I didn't read the question carefuly, will update the answer

Collectives™ on Stack Overflow

How do I expand an array in C# without using the new keyword

2 Answers 2

4 Comments

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related