How to select array index after Where clause using Linq?

Question

Suppose I have the array string[] weekDays = { "Monday", "Tuesday", "Wednesday", "Thursday", "Friday" }; , and I want to find out the index of array elements containing 's'. How can I do this using Linq ?

I've tried int[] indexOfDaysContainingS = weekDays.Where(day => day.Contains("s")).Select((day, index) => index).ToArray();, but this returns 0,1,2 as presumably it's getting the index of the filtered IEnumberable<string> after the Where() clause instead. If I put the Select() first, then all I have is the index and can't filter by the days.

What do I need to change to make it work and return 1,2,3 instead ?

I know that's not part of the question, but why don't use do this with a for loop instead of Linq. You would have saved many minutes of development. — Aurelien Ribon
– Aurelien Ribon, Commented Nov 20, 2010 at 11:17

Ani · Accepted Answer · 2010-11-20 10:47:50Z

44

You could do it this way:

weekDays.Select((day, index) => new { Day = day, Index = index })
        .Where(x => x.Day.Contains("s"))
        .Select(x => x.Index)
        .ToArray();

Not sure if this is optimal..

edited Nov 20, 2010 at 10:47

Ani

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answered Nov 20, 2010 at 10:42

Patko

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2 Comments

CodeMonkey Over a year ago

@Patko but how did it know to take the index from the array and associate it with the "index" which you put in the Index property?

Patko Over a year ago

@YonatanNir, Select has two overloads, one also has an index of the element as a parameter, see msdn.microsoft.com/en-us/library/bb534869.aspx. That index parameter is then saved to an anonymous object so that it can be again used in the second Select. I hope this answers your question?

Ani · Accepted Answer · 2010-11-20 10:57:31Z

15

Patko's answer is the way to go in the general case.

Here are 2 more options:

// Idea only works with collections that can be accessed quickly by index.
int[] indices = Enumerable.Range(0, weekDays.Length)
                          .Where(index => weekDays[index].Contains("s"))
                          .ToArray();

With MoreLinq:

// Similar to Patko's idea, except using a 'named' type.
int[] indices = weekDays.AsSmartEnumerable()
                        .Where(item => item.Value.Contains("s"))
                        .Select(item => item.Index)
                        .ToArray();

answered Nov 20, 2010 at 10:57

Ani

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2 Comments

Michael Low Over a year ago

Thanks for the alternatives. Is there any reason why performance would be different using your first alternative vs Patko's answer for a large array ? I'm not sure which is the optimal choice.

Ani Over a year ago

@miket2e: Patko's answer is the general one - the Range approach doesn't work well with non-list sequences; so it's hard to make a "fair" comparison. For arrays, I would think the Range approach would be faster - there's probably less heap allocation, it's possibly more cache-friendly etc. (notice the weasel words). Don't believe me though, measure! Maybe you shouldn't even be using LINQ here.

Markus · Accepted Answer · 2016-02-23 08:23:03Z

-3

This should work:

weekDays.Where(a => a.Contains("s")).Select((a, i) => i).ToArray();

edited Feb 23, 2016 at 8:23

Markus

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answered Feb 23, 2016 at 7:16

Ashraf

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1 Comment

Craig Nicholson Over a year ago

I don't believe the logic is correct in this example as the index value returned shall be after the collection has already been filtered rather than the index relative to the original collection.

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