How to find a specific element in a List

Question

I want to filter my List and if an item already exists, I don't want to add that item to that List. That's why I try to filter by the item property.

if(!(PolyLineList.Contains(PolyLineList.Find(x => x.item == item))))
{
    cPolyline currentPolyLine = new cPolyline(pointlist,item);
    PolyLineList.Add(currentPolyLine);
}

When an item already exists in PolyLineList, that is equal to item, it should skip the if-statement.

So what's wrong with your code? – Gert Arnold Nov 02 '19 at 19:48 — Gert Arnold, Nov 02 '19 at 19:48

score 1 · Answer 1 · answered Oct 30 '19 at 09:55

1

Use Linq's Any method:

if (!PolyLineList.Any(x => x.item == item))
{
    ...
}

answered Oct 30 '19 at 09:55

canton7

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This will slow down greatly as the list grows. https://stackoverflow.com/a/10762995/9945524 – Mark Cilia Vincenti Oct 30 '19 at 13:12
@MarkCiliaVincenti Of course it will, that goes without saying. I did have an edit which suggested keeping a separate HashSet of the `item` property, but it was hard to articulate clearly without knowing much of the context (such as the type of the `item` property), given that the OP is obviously fairly inexperienced. – canton7 Oct 30 '19 at 13:14
that's just what my answer is doing, unless I misunderstood you. – Mark Cilia Vincenti Oct 30 '19 at 13:15
1

@MarkCiliaVincenti It is, yes. Although I think that could be unclear to the OP - you need to make it clear that items are removed from `itemSet` when they're removed from `PolyLineList`, and that `itemSet` and `PolyLineList` need to have the same storage duration, for instance. – canton7 Oct 30 '19 at 13:15

Dmitry Bychenko · Answer 2 · 2019-10-30T10:20:41.607

1

You may want to change List<T> to HashSet<T> and just Add (providing that PolyLine implements Equals and GetHashCode methods for PolyLine instances to be compared):

 //TODO: you, probably, want to rename PolyLineList into PolyLineSet or something 
 HashSet<cPolyline> PolyLineList = new HashSet<cPolyline>();

 ...

 PolyLineList.Add(currentPolyLine);

edited Oct 30 '19 at 10:20

answered Oct 30 '19 at 10:01

Dmitry Bychenko

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Note that 1) this relies on the OP overriding `Equals` and `GetHashCode` in `cPolyline` to do equality based only on the `name` member, and 2) assumes that order in `PolyLineList` is not relevant. – canton7 Oct 30 '19 at 10:04

Mark Cilia Vincenti · Answer 3 · 2019-10-30T13:18:26.610

1

Assuming the type of item is ItemType:

var itemSet = new HashSet<ItemType>();

...

if (itemSet.Add(item))
{ // item is unique
    PolyLineList.Add(new cPolyline(pointlist,item));
}

edited Oct 30 '19 at 13:18

answered Oct 30 '19 at 13:08

Mark Cilia Vincenti

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How to find a specific element in a List

3 Answers3