LINQ search though a list of string arrays for a particular string

Question

I have a list of string arrays:

List<String[]> listOfStringArrays = something;

I need to select all objects from a collection that have a value which is equal to the string at the 0th index of any string array in the list.

For example, if I just had a simple list of strings, declared as:

List<String> listOfStrings = something;

I would just do:

var query = someCollection.Where(x => listOfStrings.Contains(x.id_num))

But obviously it's not as simple with a list of string arrays.

I know that I can easily just iterate through the list of string arrays and create a simple list of strings with the 0th value, like this:

List<String[]> listOfStringArrays = something;
List<String> listOfValues = new List<String>();

foreach (string[] s in listOfStringArrays)
    listOfValues.Add(s[0]);

var query = someCollection.Where(x => listOfValues.Contains(x => x.id_num);

But would really like to avoid this and am trying to write it as a one liner without introducing extra lists and loops.

Solution 1

You can put it all into one query:

someCollection.Where(x => listOfValues.Select(y => y[0]).Contains(x => x.id_num);

But it will iterate over listOfValues over and over again.

I would rather go with HashSet<string> to make it faster:

var set = new HashSet<string>(listOfValues.Select(y => y[0]));
someCollection.Where(x => set.Contains(x));

Solution 2

Try the following

var query = someCollection.Where(s => listOfStringArrays.Any(a => a[0] == s));

Solution 3

var firsts = listOfString.Select(x => x[0]);
var query = someCollection.Where(x => firsts.Contains(x));

This will project each array to it's first element, and then match from there

As a one liner:

var query = someCollection.Where(x => listOfString.Select(y => y[0]).Contains(x));

Solution 4

It should be simply:

List<String[]> newListOfStrings = listOfStrings.where(x => x[0].Contains(identifer)).ToList()

The final ToList is needed in this case because I have not used var.

View more solutions

Comments

[dursk]

Ok so I left out some details which are getting in the way while using HashSet. The values in the collection I am comparing them to are integers, so I have to do a cast one way or the other.

[dursk]

Nevermind, just have to call ToString()...I over analyzed and confused myself because I never used HashSet before.

[MarcinJuraszek]

If your values are ints use HashSet<int>.

[dursk]

Well the values from the collection are int, and the List is of course strings. What I did was: someCollection.Where(x => set.Contains(x.ToString()));

[MarcinJuraszek]

So the list contains string representation of ints? That's not good.

[dursk]

It's populated from importing a CSV. It's given to me in this from from another area of the program. After I run this query I never touch it again though, and do all my work on the collection which represents the data correctly, so it's not that bad. I do know what your saying though, but currently don't have the option of going deeper and changing things.