How can I copy a string[] from another string[]?
Suppose I have string[] args. How can I copy it to another array string[] args1?
Add a comment
|
3 Answers
- To create a completely new array with the same contents (as a shallow copy): call
Array.Cloneand just cast the result. - To copy a portion of a string array into another string array: call
Array.CopyorArray.CopyTo
For example:
using System;
class Test
{
static void Main(string[] args)
{
// Clone the whole array
string[] args2 = (string[]) args.Clone();
// Copy the five elements with indexes 2-6
// from args into args3, stating from
// index 2 of args3.
string[] args3 = new string[5];
Array.Copy(args, 2, args3, 0, 5);
// Copy whole of args into args4, starting from
// index 2 (of args4)
string[] args4 = new string[args.Length+2];
args.CopyTo(args4, 2);
}
}
Assuming we start off with args = { "a", "b", "c", "d", "e", "f", "g", "h" } the results are:
args2 = { "a", "b", "c", "d", "e", "f", "g", "h" }
args3 = { "c", "d", "e", "f", "g" }
args4 = { null, null, "a", "b", "c", "d", "e", "f", "g", "h" }
5 Comments
dragonfly02
Hi Jon, why is string array clone a 'deep' clone? I mean I know Array.Clone() is a shallow copy; while it does do a shallow copy on other reference types (e.g. CultureInfo) it somehow does a 'deep' copy on string array. For example, if I clone an array containing "Hello", "World"; after the clone, I set the first element to null. Then I expect the 1st element of the cloned array should also be null but it's still Hello. Hope you know what I mean without being able to show the code sample in the comment.
Jon Skeet
@stt106: Why would you expect it to be null? It's creating a shallow copy, i.e. creating a new array of the same size, and copying each value from the original array to the new array. You'd see exactly the same behaviour with
CultureInfo.dragonfly02
But the references in the new Array point to the same objects that the references in the original Array point to; hence if either the original or cloned (new) array is modified then the other array should reflect the modification as well?
Jon Skeet
@stt106: No, because modifying the array isn't the same as modifying the object that a reference refers to. If you write
array[0] = null that's saying "Ignore the old value in the array; the new value is null". Why would that affect the other array? Imagine I have a printed page with a list of URLs on, and I photocopy that and give you the photocopy. If I scribble out one of the URLs, that doesn't affect your copy at all. If on the other hand I modify the page that one of the URLs refers to, you will see the difference.dragonfly02
Ah got you. Thanks for the example in your explanation. I guess string isn't the best reference type example to test array shallow copy due to its immutability... probably that's why MSDN uses an example of CultureInfo rather than string
Allocate space for the target array, that use Array.CopyTo():
targetArray = new string[sourceArray.Length];
sourceArray.CopyTo( targetArray, 0 );
1 Comment
Jon Skeet
Using Clone() makes this particular case a one-liner though.
The above answers show a shallow clone; so I thought I add a deep clone example using serialization; of course a deep clone can also be done by looping through the original array and copy each element into a brand new array.
private static T[] ArrayDeepCopy<T>(T[] source)
{
using (var ms = new MemoryStream())
{
var bf = new BinaryFormatter{Context = new StreamingContext(StreamingContextStates.Clone)};
bf.Serialize(ms, source);
ms.Position = 0;
return (T[]) bf.Deserialize(ms);
}
}
Testing the deep clone:
private static void ArrayDeepCloneTest()
{
//a testing array
CultureInfo[] secTestArray = { new CultureInfo("en-US", false), new CultureInfo("fr-FR") };
//deep clone
var secCloneArray = ArrayDeepCopy(secTestArray);
//print out the cloned array
Array.ForEach(secCloneArray, x => Console.WriteLine(x.DateTimeFormat.DateSeparator));
//modify the original array
secTestArray[0].DateTimeFormat.DateSeparator = "-";
Console.WriteLine();
//show the (deep) cloned array unchanged whereas a shallow clone would reflect the change...
Array.ForEach(secCloneArray, x => Console.WriteLine(x.DateTimeFormat.DateSeparator));
}