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Tech Tip: A method to copy elements with the same collection
Published On: April 18, 2019
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4D V17 offers a rich selection of collection methods to add and remove element to and from collections. Developers can also use concat() and reduce() to add/remove multiple elements at one time. However, it does not yet offer a member method to move element(s) inside the same collection.
Below is a method used to copy any number of elements of a collection to a target location within the same collection. The copied elements will replace elements at target locations. The size of collection is not modified.

// Name: Collection_CopyWithIn
// Description: Method will return a collection in which a part of collection is copied
// to another location of the same acollection without changing its size
// Parameters:
// $1 (Collection) - target collection
// $2 (Longint)- Zero based index to copy elements to (included)
// $3 (Longint)- Zero based index to start copying elements from (included)
// $4 (Longint)- Zero based index to end copying elements from (not included)
// Return
// $0(Collection)- a modified collection with copied elements


//If target is bigger than collection length, nothing is copied
If ($2>=$1.length)
End if
//If start is smaller than 0 then add it to length of collection
While ($2<0)
End while
//Get the elements to copy from
//Calculate the start and end to copy to
If($tempCollection.length > ($1.length - $2))
End if
//Remove original elements and copy new elements in

This method is useful to move elements around in a collection. Instead of creating a temporary collection, or split current collection and concat them, this CopyWithIn method offers an easy way to move elements. The copying process is performed at high performance because it does not involve iterating through the collection

Below is an example:

$c:=New collection(1;2;3;4;5)

$c::=Collection_CopyWithIn ($c;0;3;4) //[4,2,3,4,5]

$c::=Collection_CopyWithIn($c;-2; -3; -1) //[1,2,3,3,4]

$c::=Collection_CopyWithIn ($c;-2;0;$c.length)//[1, 2, 3, 1, 2]

$c::=Collection_CopyWithIn ($c;0;2;$c.length)//[3, 4, 5, 4, 5]