How do I select every 6th element from a list (using Linq)

Question

I've got a list of 'double' values. I need to select every 6th record. It's a list of coordinates, where I need to get the minimum and maximum value of every 6th value.

List of coordinates (sample): [2.1, 4.3, 1.0, 7.1, 10.6, 39.23, 0.5, ... ] with hundrets of coordinates.

Result should look like: [x_min, y_min, z_min, x_max, y_max, z_max] with exactly 6 coordinates.

Following code works, but it takes to long to iterate over all coordinates. I'd like to use Linq instead (maybe faster?)

for (int i = 0; i < 6; i++)
{
    List<double> coordinateRange = new List<double>();

    for (int j = i; j < allCoordinates.Count(); j = j + 6)
        coordinateRange.Add(allCoordinates[j]);

    if (i < 3) boundingBox.Add(coordinateRange.Min());
    else boundingBox.Add(coordinateRange.Max());
}

Any suggestions? Many thanks! Greets!

Duplicate - http://stackoverflow.com/questions/682615/how-can-i-get-every-nth-item-from-a-listt - there is a discussion about the performance there too. — ChrisF, Mar 16 '10 at 11:11
This doesn't make sense, what distinguishes x, y and z from the rest of the list? Why are you only looking at every 6th item in the list? — cjk, Mar 16 '10 at 11:13
@ck Because the list is already sorted. Every point has 3 coordinates. And each 2 points belong together. Like this (x1,y1,z1)(x2,y2,z2) — iDog, Mar 16 '10 at 11:21

score 22 · Accepted Answer · answered Mar 16 '10 at 11:52

22

coordinateRange.Where( ( coordinate, index ) => (index + 1) % 6 == 0 );

The answer from Webleeuw was posted prior to this one, but IMHO it's clearer to use the index as an argument instead of using the IndexOf method.

answered Mar 16 '10 at 11:52

Martin R-L

4,039
3
28
28

Very interesting! Maybe a community wiki about uncommon overloads in the linq library is in order :-) – Dested Mar 16 '10 at 12:06

Hans Keﬆing · Answer 2 · 2010-03-17T08:22:25.500

7

Something like this could help:

public static IEnumerable<T> Every<T>(this IEnumerable<T> source, int count)
{
    int cnt = 0;
    foreach(T item in source)
    {
        cnt++;
        if (cnt == count)
        {
            cnt = 0;
            yield return item;
        }
    }
}

You can use it like this:

    int[] list = new []{1,2,3,4,5,6,7,8,9,10,11,12,13};
    foreach(int i in list.Every(3))
        { Console.WriteLine(i); }

EDIT:

If you want to skip the first few entries, you can use the Skip() extension method:

foreach (int i in list.Skip(2).Every(6))
{ Console.WriteLine(i); }

edited Mar 17 '10 at 08:22

answered Mar 16 '10 at 11:50

Hans Keﬆing

38,117
9
79
111

+1 for this idea, even when the OP probably looked for LINQ explicitly. – Marcel Mar 16 '10 at 12:29
Very good, but I need to get every 6th element with variable start index. Something like: public static IEnumerable Every(this IEnumerable source, int startIndex, int count) I think I'll manage that myself. Thanks!! – iDog Mar 17 '10 at 00:17
That's easy, using the Skip() method. See my extended answer. – Hans Keﬆing Mar 17 '10 at 08:23

score 6 · Answer 3 · answered Mar 16 '10 at 11:51

6

There is an overload of the Where method with lets you use the index directly:

coordinateRange.Where((c,i) => (i + 1) % 6 == 0);

answered Mar 16 '10 at 11:51

codymanix

28,510
21
92
151

score 4 · Answer 4 · answered Mar 16 '10 at 11:13

4

Any particular reason you want to use LINQ to do this?

Why not write a loop that steps with 6 increments each time and get access the value directly?

answered Mar 16 '10 at 11:13

Philip Fourie

111,587
10
63
83

It's a very huge list, and I hoped that Linq is faster than two 'for' loops. – iDog Mar 16 '10 at 11:15
why would linq necessarily be faster? – Mitch Wheat Mar 16 '10 at 11:23
2

Maybe not necessarily but possibly? – iDog Mar 16 '10 at 11:30
4

it would be a for loop in linq to objects. no performance difference. – Muhammad Hasan Khan Mar 16 '10 at 11:34
Why wouldn't he use LINQ? He can one-line anything he needs. – Nicholas Terry Sep 26 '13 at 00:07

Oliver · Answer 5 · 2010-03-16T12:09:58.807

To find a faster solution start a profile!

Measure how long it takes for every step in your for loop and try to avoid the biggest bottleneck.

After making a second look at your code, it seems, your problem is that you run six times over your big list. So the time needed is always six times of the list size.

Instead you should run once over the whole list and put every item into the correct slot.

Just to make a performance test for yourself you should test these two approaches:

Sample class to hold data

public class Coordinates
{
    public double x1 { get; set; }
    public double x2 { get; set; }

    public double y1 { get; set; }
    public double y2 { get; set; }

    public double z1 { get; set; }
    public double z2 { get; set; }
}

Initializing of the value holder

Coordinates minCoordinates = new Coordinates();

//Cause we want to hold the minimum value, it will be initialized with
//value that is definitely greater or equal than the greatest in the list
minCoordinates.x1 = Double.MaxValue;
minCoordinates.x2 = Double.MaxValue;
minCoordinates.y1 = Double.MaxValue;
minCoordinates.y2 = Double.MaxValue;
minCoordinates.z1 = Double.MaxValue;
minCoordinates.z2 = Double.MaxValue;

Using a for loop if the index operator is O(1)

for (int i = 0; i < allCoordinates.Count; i++)
{
    switch (i % 6)
    {
        case 0:
            minCoordinates.x1 = Math.Min(minCoordinates.x1, allCoordinates[i]);
            break;
        case 1:
            minCoordinates.x2 = Math.Min(minCoordinates.x2, allCoordinates[i]);
            break;
        case 2:
            minCoordinates.y1 = Math.Min(minCoordinates.y1, allCoordinates[i]);
            break;
        case 3:
            minCoordinates.y2 = Math.Min(minCoordinates.y2, allCoordinates[i]);
            break;
        case 4:
            minCoordinates.z1 = Math.Min(minCoordinates.z1, allCoordinates[i]);
            break;
        case 5:
            minCoordinates.z2 = Math.Min(minCoordinates.z2, allCoordinates[i]);
            break;
    }
}

Using foreach if IEnumerator is O(1)

int count = 0;
foreach (var item in allCoordinates)
{
    switch (count % 6)
    {
        case 0:
            minCoordinates.x1 = Math.Min(minCoordinates.x1, item);
            break;
        case 1:
            minCoordinates.x2 = Math.Min(minCoordinates.x2, item);
            break;
        case 2:
            minCoordinates.y1 = Math.Min(minCoordinates.y1, item);
            break;
        case 3:
            minCoordinates.y2 = Math.Min(minCoordinates.y2, item);
            break;
        case 4:
            minCoordinates.z1 = Math.Min(minCoordinates.z1, item);
            break;
        case 5:
            minCoordinates.z2 = Math.Min(minCoordinates.z2, item);
            break;
    }
    count++;
}

I'll think about that. Thank you – iDog Mar 16 '10 at 12:00 — iDog, Mar 16 '10 at 12:00

score 2 · Answer 6 · answered Mar 16 '10 at 12:15

Well its not LINQ, but If you are worrying about performance, this might help.

public static class ListExtensions
{
    public static IEnumerable<T> Every<T>(this IList<T> list, int stepSize, int startIndex)
    {
        if (stepSize <= 0)
            throw new ArgumentException();

        for (int i = startIndex; i < list.Count; i += stepSize)
            yield return list[i];
    }
}

Webleeuw · Answer 7 · 2010-03-16T12:03:39.833

1

Suggestion:

~~coordinateRange.Where(c => (coordinateRange.IndexOf(c) + 1) % 6 == 0);~~

I stand corrected, thanks for the comments. As stated by codymanix, the correct answer is indeed:

coordinateRange.Where((c, i) => (i + 1) % 6 == 0);

edited Mar 16 '10 at 12:03

answered Mar 16 '10 at 11:14

Webleeuw

7,222
33
34

is this faster? i'm curious about the speed difference... i think it should be much slower. – stmax Mar 16 '10 at 11:45
calling IndexOf every iteration is overkill. There is an overload of Where which let you use the index directly. See my answer. – codymanix Mar 16 '10 at 11:52
This is a horrible solution to the problem. It's correct, but slow as hell – Philippe Leybaert Mar 16 '10 at 11:54

David Schmitt · Answer 8 · 2010-03-16T12:09:56.873

The best way to do this would be refactoring the datastructure so that every dimension would be its own array. That way x1_max would be just x1.Max(). If you cannot change the input data structure, the next best way is to iterate over the array once and do all accesses locally. This helps with staying within the L1-cached memory:

var minValues = new double[] { Double.MaxValue, Double.MaxValue, Double.MaxValue };
var maxValues = new double[] { Double.MinValue, Double.MinValue, Double.MinValue };

for (int i = 0; i < allCoordinates.Length; i += 6) 
{
    for (int j = 0; i < 3; i++)
    {
        if (allCoordinates[i+j] < minValues[j])
            minValues[j] = allCoordinates[i+j];
        if (allCoordinates[i+j+3] > maxValues[j])
            maxValues[j] = allCoordinates[i+j+3];
    }
}

score 1 · Answer 9 · answered May 15 '19 at 02:32

1

Use Skip and combintaion with Take.

coordinateRange.Skip(6).Take(1).First();

I recommend you move this to a extension method.

answered May 15 '19 at 02:32

Ewald

698
7
10

How do I select every 6th element from a list (using Linq)

9 Answers9

Sample class to hold data

Initializing of the value holder

Using a for loop if the index operator is O(1)

Using foreach if IEnumerator is O(1)

Linked

Related