c# - index of array entries inside another array using LINQ

Question

I have an array of strings:

string[] stringArray = {"aaa", "bbb", "ccc", "aaa", "ccc", "ddd"};

I would like to get all indexes of this array where a substring of these strings are inside another array:

string[] searchArray = {"a","b"};

The answer I would like to get is then:

index = {0,1,3};

A soultion for just one entry of the searchArray would be:

List<int> index = stringArray.Select((s, i) => new { i, s })
            .Where(t => t.s.Contains(searchArray[1]))
            .Select(t => t.i)
            .ToList();

A solution for all entries would be:

List<int> index = new List<int>();
foreach (string str in searchArray)
            index.AddRange(stringArray.Select((s, i) => new { i, s })
            .Where(t => t.s.Contains(str))
            .Select(t => t.i)
            .ToList());
index.Sort();

But out of curiosity, are there any solutions by just using one command in LINQ?

Jon Skeet · Accepted Answer · 2016-04-05 21:17:04Z

6

Yup, you just need Any to see if "any" of the target strings are contained in the array element:

List<int> index = stringArray
    .Select((Value, Index) => new { Value, Index })
    .Where(pair => searchArray.Any(target => pair.Value.Contains(target)))
    .Select(pair => pair.Index)
    .ToList();

edited Apr 5, 2016 at 21:17

answered Apr 5, 2016 at 21:11

Jon Skeet

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7 Comments

Eser Over a year ago

Whould this code work? Yes, Would I do it this way? No

Jon Skeet Over a year ago

@Eser: Without saying what you don't like about it, that isn't terribly useful. What's the problem? What's your better solution?

Eser Over a year ago

It is an O(n*n) algorithm. ( If we include value.Contains(target) O(n**3)). Here is too late for now, but i'll think of one...

Jon Skeet Over a year ago

@Eser: Given that each element of one array needs to be checked against each element of another, and you can't use hashing to make it O(n+m) because it's a contains condition, not equality, I don't see how you can avoid it being O(n * m) checks. I wouldn't claim that it's not how you'd do it without having at least some reasonably clear idea how you would do it. I'd also point out that there's no indication that the OP will actually have large amounts of data. Even if there were a more efficient approach, it would almost certainly be more complex. Premature optimization and all that...

Eser Over a year ago

I am only also not sure about it but I would try to use some like lucene.apache.org/core/4_1_0/analyzers-stempel/org/egothor/… by preprosessing in a linear time. ( BTW: You updated your comment, and invalidated this :( )

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