How do I sort an array but maintain the original index?

Question

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Hi all,

I've landed in a situation where i need the top N number of items in an array, work on those items and reinsert (or update) them back at the exact same indices. I realized that the easiest way would be to sort an array 1st then pick N number of the highest elements, but I find my logic below a little difficult to implement which is where my dilemma is.

Question:

if the original collection/array P were ...

P()
Index ----> 0 1 2 3 4
Element----> 5 6 16 14 2

and N = 2 (i.e I want the top 2 highest elements)

I could sort P() once and get Q1 but I want to maintain the index as below. *this is where my problem is
(sorted by elements and index maintained)
Index ----> 4 0 1 3 2
Element----> 2 5 6 14 16

then...
pick the top 2 (since N = 2) to create Q2.

Index ----> 3 2
Element---->14 16

then to ensure that index is always sorted ascending I
Sort by index to have

Index ----> 2 3
Element----> 16 14

work on the elements, for instance

change 16 to 17 so that Q2(2) = 17 ( * - new data), change 14 to 19 so that Q2(3) = 19

then after that set the values of Q1 at the index to Q2 and sort i.e.
Q1(2) = Q2(2) and Q1(3) = Q2(3) such that
Q1() is
Index ----> 0 1 2 3 4
Element----> 5 6 17* 19* 2

I don't know whether using keys would help or even how to implement a collection type that uses keys and I am trying to avoid using a parallel array if i can. So please if you have any candid advice, opinions, information or pointers in the right direction kindly share; it would be very much appreciated.

Many thanks for your help and your time

P.S am Using .NET in case there is a collection type better suited to this.

Posted 22-Apr-10 6:34am

CyberSamuraiii

Updated 23-Apr-10 0:50am

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#realJSOP · Accepted Answer · 2010-04-22T07:26:00

Solution 1

With the pitiful amount of info you've provided, the best I can suggest is that you look into using a Dictionary object instead of an Array.

Posted 22-Apr-10 7:26am

#realJSOP

William Winner · Accepted Answer · 2010-04-22T07:50:00

There are a lot of ways that you could approach this.

The simplest is to just have a single array with the numbers. Then, keep an array representing query that you store the index of the max numbers in. You would just have to write a simple for statement to search through and find the index of the two largest numbers..

Something like this:

VB

Dim newArray(4) As Integer
newArray(0) = 5
newArray(1) = 6
newArray(2) = 16
newArray(3) = 14
newArray(4) = 2

Dim q2(1) As Integer
q2(0) = -1 'Will store the index of the largest number
q2(1) = -1 'Will store the index of the second largest number

For i = 0 To newArray.Length - 1
    If q2(0) = -1 Then
         q2(0) = i
     ElseIf newArray(i) >= newArray(q2(0)) Then
         q2(1) = q2(0)
         q2(0) = i
     ElseIf newArray(i) >= newArray(q2(1)) Then
         q2(1) = i
     End If
Next

At this point, with the values you gave, q2(0) = 2 and q2(1) = 3 or the indexes of the two highest numbers in the array. You can always choose to sort q2 too, if for instance newArray(2) = 14 and newArray(3) = 16 which would give q2(0) = 3 and q2(1) = 2 if you wanted to make sure that you were modifying the numbers based on their index order.

Then, to modify the numbers, you just modify newArray(q2(0)).

If you really wanted to do sorting and want to keep track of the original index, you will most likely have to use a multi-dimensional array.

Henry Minute · Accepted Answer · 2010-04-23T02:58:00

Solution 3

To me this looks like a prime candidate for a little bit of LINQ.

If you are not familiar with LINQ, take a look at this[^] article which has some examples of using LINQ with Arrays.

Good luck. :)

Posted 23-Apr-10 2:58am

Henry Minute