How to iterate a collection between two items?-CodePudding

Consider for example a List of objects:

List<MyClass> myList;

I have a method which passes two references to items within the list. I want to iterate all items within the given ones (note: I don't know which of the two items comes first within the list):

privat void MyFunction(MyClass listItem, MyClass anotherListItem)
{
    foreach(var item in ????)
    {
        // do something 
    }
}

Currently I have solved this usecase as follows:

int listItemIdx = myList.IndexOf(listItem);
int anotherListItemIdx = myList.IndexOf(anotherListItem);

if(listItemIdx < anotherListItemIdx )
{
    for(int i = listItemIdx ; i <= anotherListItemIdx ; i  )
    {
        // do stuff
    }
}
else
{
    for (int i = anotherListItemIdx ; i < listItemIdx ; i  )
    {
        // do stuff
    }
}

I was wondering if there is a more elegant, efficient or built-in solution to this problem?

CodePudding user response：

If you are looking for performance (IndexOf twice can be a bit slow) and generalization (when myList is not necessary List<T> but IEnumerable<T> only) you can put it as

bool proceed = false;
MyClass lastItem = default; 

foreach (var item in myList) {
  if (!proceed) {
    if (proceed = item == listItem) 
      lastItem = anotherListItem; 
    else if (proceed = item == anotherListItem) 
      lastItem = listItem; 
  }

  if (proceed) {
    //TODO: do staff here

    if (item == lastItem)
      break; 
  } 
}

CodePudding user response：

You iterate three times over the list: Two times in IndexOf, and then once again in your loop. You can make your code more efficient with this code, which iterates only once over the list.

privat void MyFunction(MyClass listItem, MyClass anotherListItem)
{
    bool betweenTwoItems = false;
    foreach(var item in myList)
    {
        if(item == listItem || item == anotherListItem)
        {
            betweenTwoItems = !betweenTwoItems;
            if(!betweenTwoItems)
            {
               break;
            }
        }

        if(betweenTwoItems )
        {
            // do stuff
        }
    }
}

We set a bool variable if we are between the two items. In the beginning, it is false. The we iterate over the list and check whether the current item is one of the two method parameters. If this is the case, we invert the value of the bool. If after the inversion of the bool the value is false, we can leave the list. After that, we check whether the bool is true. If so, we can do stuff.

Online demo: https://dotnetfiddle.net/xYcr7V

CodePudding user response：

Is the list sorted? The you can use that fact to realize which item must be first. Nevertheless you can do with one for-loop, if you prefer:

private static void MyFunction(string item1, string item2)
{
    List<string> input = new() {"A", "B", "C", "D", "E"};

    int index1 = input.IndexOf(item1);
    int index2 = input.IndexOf(item2);
    int beginIndex = Math.Min(index1, index2);
    int count = Math.Abs(index1 - index2)   1;

    foreach (string item in input.GetRange(beginIndex, count))
    {
        Console.Write(item);
    }
}