从列表A中删除元素集并使用Linq添加到列表B中

Question

I am attempting to lean linq by replacing existing code in a project with linq calls. In this method I check for a condition in my list of lines and if the condition is true, move that element from lines to processedLines.

The data structures are just lists:

List<LineSegment2> lines;
List<LineSegment2> processedLines;

The original code was:

for (int i = lines.Count - 1; i >= 0; i--)
{           
   if (lines[i].P2.x < sweepPosition)
   {
      processedLines.Add(lines[i]);
      lines.RemoveAt(i);
   }
}

and my linq code is:

var toMove = lines.FindAll(x => x.P2.x < sweepPosition);
toMove.ForEach(x =>
{
   processedLines.Add(x);
   lines.Remove(x);
});

My question is: Is this linq code less efficient because it is using more memory creating the temporary list 'toMove'. Is there a way to create the linq query without requiring the temporary list or is the original code always more efficient?

Answer 1

A more LINQy solution would be to add all the processed lines at once, then get the remaining lines:

processedLines.AddRange(lines.Where(x => x.P2.x < sweepPosition));
lines = lines.Where(x => x.P2.x >= sweepPosition).ToList();

As for efficiency, it won't be quite as fast as your original code. That's not why you use LINQ.

There is one potential advantage, though. It will make a new list of lines, so if you move a lot of lines to the processed list it will get rid of the unused items in the list.

Answer 2

The "linq" code is less efficient and (more importantly) not necessarily much easier to maintain. Stick with your original code if you must choose between these two alternatives. I'd just recommend you run the for loop forward -- no reason you should run it backwards like you're doing.

As a side note, I wonder if it would be appropriate for your use case to just maintain a single list and add an IsProcessed property to the LineSegment2 class. You might consider that.

Answer 3

I'm not really sure about the efficiency...but in Linq I'll do it like this

processedLines = processedLines.Concat(lines.Where(x => x < sweepPosition)).ToList();
lines.RemoveAll(x => x < sweepPosition);