Nested foreach to change list items

Question

I need to change items in a list using nested for/foreach loops. Problem is that I could not get it working using LINQ with or without dot notation. The traditional way worked and goes like this:

foreach (MapObjectLayer mapObjectLayer in map.Objects)
{
    foreach (MapObject mapObject in mapObjectLayer.MapObjects)
    {
        for (int i = 0; i < mapObject.Points.Count; i++)
        {
            mapObject.Points[i] = new Vector2(
                mapObject.Points[i].X * map.Scale,
                mapObject.Points[i].Y * map.Scale);
        }
    }
}

using LINQ, this failed:

var test = (from mol in map.Objects
           from mo in mol.MapObjects
           from p in mo.Points
           select p).ToList();

for (int i = 0; i < test.Count(); i++)
{
    test[i] = new Vector2(
        test[i].X * map.Scale,
        test[i].Y * map.Scale);
}

and this failed:

map.Objects.ForEach(l => l.MapObjects.ForEach(t => t.Points.ForEach(p => p = p * map.Scale)));

If I could get the dot notation variant working I would be very happy, but I do not have a clue on why it fails. Using the debugger it is obvious by examining the Points list that the vectors did not get multiplied using the two LINQ variants.

Update: Vector2 is a struct

Update: Here is two more one-liners that I found (working ones):

map.Objects.SelectMany(m => m.MapObjects).ToList().ForEach(o => o.Points = o.Points.Select(p => p * 2).ToList());
map.Objects.ForEach(l => l.MapObjects.ForEach(t => t.Points = t.Points.Select(p => p * 2).ToList()));

Why do you want to use LINQ? LINQ is for *querying*, not *updating*. — Kendall Frey, May 19 '12 at 16:52
I think LINQ might actually be slower, are you looking for a "cool one-liner" ? — Chris Gessler, May 19 '12 at 16:59
Well, I was just wondering if I could make the code more elegant by using LINQ to get to the actual list, and then do a foreach over it. But do not take this question too serious, its not a must to use LINQ... I am only curious if I could make it work. Yes, a cool one-liner would be great :) — Raymond Holmboe, May 19 '12 at 17:04
Do you need to modify the elements in the original list, or can you just return a new list with new elements? — Roman, May 19 '12 at 17:23

Kendall Frey · Accepted Answer · 2012-05-19T19:48:40.630

2

The regular foreach is the best way. LINQ is designed for querying. You can do it in one line, but it won't be elegant or readable. Here is how:

map.Objects.ForEach(l => l.MapObjects.ForEach(t => Enumerable.Range(0, t.Points.Count).ToList().ForEach(i => t.Points[i] *= map.Scale)));

The reason that your version didn't work is because Vector2 is a value type. In the query, it's value is copied, so when you do p => p = ... you are assigning to a copy of the variable.

Use the original code. LINQ is not a replacement for loops.

edited May 19 '12 at 19:48

answered May 19 '12 at 17:19

Kendall Frey

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This answers my question very well and thanks for the one-liner :) What a mess I got myself into, I will probably use for/foreach loops for this one, thanks. – Raymond Holmboe May 19 '12 at 17:55

Curiosa Globunznik · Answer 2 · 2012-06-01T06:48:49.663

You can use Linq as well to collect items (in fact Resharper will offer a refactoring from nested foreach to linq), but you have to take care what you collect and what you update.

The initial linq query syntax example collects copies of Points into a new List and then replaces each element of this new list with a new instance of Vector2. Even if Vector2 would have been a referency type only the new list would change rather than the original map.Objects-substructure.

It would work the way you want it if

you work with reference rather than value types and
you'd assign the properties of the instead-of-Vector2 items:

test[i].X *= map.Scale

Nested foreach to change list items

2 Answers2