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Reverse Concat List

Posted on 2011-02-28
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Last Modified: 2013-12-17
Experts,

I have a list, and I need to reverse it and then concat it to itself. This might be really silly question.

Example : Original List  1 2 3 4
                 Altered List  1 2 3 4 4 3 2 1

Reverse() method returns a Void. I was planning on DList.Concat(DList.Reverse());

Whats the fasted way to achieve this?
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Question by:San24
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9 Comments
 
LVL 8

Expert Comment

by:dericstone
ID: 35000242
Just thinking about it without testing:
List<int> list1 = new List<int>();
List<int> list2 = new List<int>();

// fill list1

list1.CopyTo(list2);
list2.Reverse();

list1.AddRange(list2);

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Author Comment

by:San24
ID: 35000559
Is there any other way to do it?
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LVL 8

Expert Comment

by:dericstone
ID: 35000708
What is your ultimate goal? Are you looking for the fastest way to do this? If so, it would be useful to know what the underlying structure of the list is. Is it an Array, List<>, SortedList<>? What is it? I've never heard of a DList.
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Author Comment

by:San24
ID: 35000754
Eric,

Yes, the fastest way to do it. DList is just a simple double list. List<double>

Lets say, I have a list 0 1 2 3 4 - The altered list will look like 0 1 2 3 4 4 3 2 1. So, I have to create a new list, copy, give it a range and then concat it.
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LVL 8

Accepted Solution

by:
dericstone earned 250 total points
ID: 35000885
You could also do it this way, which might be a little more efficient. But I can't think of anything else with a List<double>.
List<double> mylist;
int len = mylist.Count;
List<double> newlist = List<double>(2 * len);
for (int i = 0, j = 2 * len - 1; i < len; i++, j--)
{
  newlist[i] = newlist[j] = mylist[i];
}

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Expert Comment

by:dericstone
ID: 35000947
Oops, I forgot one detail. The items of the list were not allocated. I've changed it a bit.
List<double> mylist;
int len = mylist.Count;
List<double> newlist = List<double>(2 * len);
newlist.AddRange(mylist);
newlist.AddRange(mylist);
for (int i = 0, j = 2 * len - 1; i < len; i++, j--)
{
  newlist[j] = mylist[i];
}

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Assisted Solution

by:anarki_jimbel
anarki_jimbel earned 250 total points
ID: 35001164


Similar to the first but working one solution :)

Output:

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private void button2_Click(object sender, EventArgs e)
        {
            List<int> lst1 = new List<int>(new int[] { 1, 2, 3, 4 });
            List<int> lst2 = new List<int>(lst1);
            lst2.Reverse();
            lst1.AddRange(lst2);
                        foreach (int i in lst1)
            {
                System.Diagnostics.Debug.WriteLine(i.ToString());
            }

        }

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LVL 29

Expert Comment

by:anarki_jimbel
ID: 35001376
Also, my version with loops, might be more efficient than above my code...
private void button2_Click(object sender, EventArgs e)
        {
            List<int> lst1 = new List<int>(new int[] { 1, 2, 3, 4 });
            List<int> lst2 = new List<int>();// straight + reversed list
            for (int i = 0; i < lst1.Count * 2; i++)
            {
                int indexToAdd;
                if (i < lst1.Count)
                    indexToAdd = i;
                else
                    indexToAdd = lst1.Count*2 - 1 - i;

                lst2.Add(lst1[indexToAdd]);
            }

            // print out
            foreach (int i in lst2)
            {
                System.Diagnostics.Debug.WriteLine(i.ToString());
            }

        }

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Author Comment

by:San24
ID: 35001884
Eric and Jimbel...Thanks a lot. This is what I`m doing


            List<double> TempX = new List<double>();
            TempX.AddRange(X);
            TempX.Reverse();
            TempX.RemoveRange(XCnt - 1, 1);
            X.AddRange(TempX);
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