C#•16mo ago

❔ Filing empty list with equal items left and right of selected item from another list

Given a list and selected index of an item in that list, I want to fill another list such that items will try to be inserted, in order, to the left and right of that selected item without going past a given max count for the new list. A little confusing, but here's an example: I have a list with the following strings { "red", "blue", "green", "yellow", "black", "brown" }. I am given the index 3 which corresponds to the element yellow in the list. I want to populate a new list that, given a max of 5, looks like the following: { "blue", "green", "yellow", "black", "brown" } So essentially I try to keep the original selected item in the middle of a new list and insert items from the original list to the left and right until I hit the max. If there's not enough elements to reach the max then it's fine if it's not in the middle.
This is what I have so far, and it's fine and works, but I'm not sure if there's a more efficient way or using less for loops and if statements.

int idx = originalList.IndexOf(selectedItem);
int count = 1;
newList.Add(selectedItem); // first add the selecteditem to the new list

// first try to get two elements before the selected item
int lastBeforeIndx = idx;
for (int i = idx - 1; i >= 0; i--)
{
    if (count == 3)
    {
        break;
    }
    newList.Insert(0, originalList[i]);
    lastBeforeIndx = i;
    count++;
}

// then try to get 2 elements to the right of the selected item
for (int i = idx + 1; i <= originalList.Count; i++)
{
    if (count == 5)
    {
        break;
    }
    newList.Add(originalList[i]);
    count++;
}

// all finished. Only want to show five at a time in this example
if (count == 5)
{
    return;
}

// if not at 5, try adding to the left again
for (int i = lastBeforeIndx - 1; i >= 0; i--)
{
    if (count == 5)
    {
        break;
    }
    newList.Insert(0, originalList[i]);
    count++;
}

4 Replies

lycian•16mo ago

you should be able to do it in one loop. Just think about it as distance:

var right = midPoint + 1;
var left = midPoint - 1;
for (; (right < newList.Length) && (left > 0); left--, right++)
{
  newList.Insert(left, originalList[left]);
  newList.Insert(right, originalList[right]);
} 

if (newList.Length % 2 == 0)
{
  // if it's even, you need to insert one extra on either left or right. 
  // it only depends on how to determine "midPoint" for even
}

var right = midPoint + 1;
var left = midPoint - 1;
for (; (right < newList.Length) && (left > 0); left--, right++)
{
  newList.Insert(left, originalList[left]);
  newList.Insert(right, originalList[right]);
} 

if (newList.Length % 2 == 0)
{
  // if it's even, you need to insert one extra on either left or right. 
  // it only depends on how to determine "midPoint" for even
}

This is also assuming that for(;; left--, right++) is valid C#. Which I didn't check 🙂 it also assumes the new list is not larger than the original the condition would change accordingly

Henkypenky•16mo ago

that's valid i really like it btw just a comment xd

Nacho Man Randy Cabbage•16mo ago

ahhhh thank you!

Accord•16mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Filing empty list with equal items left and right of selected item from another list