c# - How to find index of list in the list

Question

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i got a list

C#

List<List<string>> myList

myList contains 3 lists
list1 {a,b,c}
list2 {d,e,f}
list3 {g,h,k}

how to find index of list that contains string "d"

Posted 29-Jan-16 8:47am

Nguyen Khoa Beo

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Comments

Sergey Alexandrovich Kryukov 29-Jan-16 14:53pm

What's wrong with just reading original MSDN documentation on this and other classes?
What have you tried so far?
—SA

3 solutions

Solution 1

Please see:
List<T>.Find Method (Predicate<T>) (System.Collections.Generic)[^],
List<T>.FindAll Method (Predicate<T>) (System.Collections.Generic)[^];
see also other System.Collections.Generic.List<>.Find* methods:
List<T> Class (System.Collections.Generic)[^].

Of course, your can also explicitly traverse all the elements in a foreach or for loop until you find what you need to.

Alternatively, you can use LINQ:
LINQ (Language-Integrated Query)[^],
LINQ to Objects[^].

Note that it gives you slow result, with the time complexity of O(N). For big sets, it can be prohibitively slow. Then you may need to use the collection classes which give you O(1), such as Dictionary, HashSet and so on:
Dictionary(TKey, TValue) Class (System.Collections.Generic)[^],
HashSet(T) Class (System.Collections.Generic)[^],
SortedDictionary(TKey, TValue) Class (System.Collections.Generic)[^],
SortedList(TKey, TValue) Class (System.Collections.Generic)[^].

See also:
Big O notation — Wikipedia, the free encyclopedia[^],
Time complexity — Wikipedia, the free encyclopedia[^].

—SA

Posted 29-Jan-16 8:59am

Sergey Alexandrovich Kryukov

Updated 29-Jan-16 13:48pm

v7

Solution 2

First of all, there is no solution to your problem because your 3 lists are wrong and must be replaced by:

C#

list1 {"a","b","c"}
list2 {"d","e","f"}
list3 {"g","h","k"}

The job of programmer means that sometimes, you have to get your hands dirty and create yourself what you need. This is such a case.

This case is very simple because using brute force is also the best way to go.
You have to scan your list in order to scan each element of current sub-list, when you get a match, simply return your index and you are done.

I let you write the code as it is probably your assignment.

Posted 29-Jan-16 9:30am

Patrice T

Comments

BillWoodruff 29-Jan-16 22:40pm

Since this person is a newcomer here, why not write a comment asking them to correct the format of the lists, rather than a solution which unnecessarily elaborates your view they don't know how to as a question ? I question your statement: "brute force is the best way to go."

Patrice T 1-Feb-16 13:40pm

Thank for your comment.
In this case, "brut force" is simply testing all possibilities.
I see that the OP have made no comment, either everything is obvious for hil or he just wanted the source code of solution.

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BillWoodruff · Accepted Answer · 2016-01-29T17:02:00

You could implement finding a match using a for-loop:

C#

private List<string> LoopFindExactMatch(List<List<string>> listolists, string matchtofind)
{
    foreach(var listostring in listolists)
    {
        if(listostring.Contains(matchtofind))
        {
             return listostring;     
        }
    }

    return null;

Or, you could use Linq to simplify the code:

C#

using System.Linq

private List<string> LinqFindExactMatch(List<List<string>> listolists, string matchtofind)
{
    return listolists.FirstOrDefault(ll => ll.Contains(matchtofind));
}

Test:

C#

private void testFindMatch()
{
    List<string> list1 = new List<string> { "a", "b", "c" };
    List<string> list2 = new List<string> { "d", "e", "f" };
    List<string> list3 = new List<string> { "g", "h", "k" };

    List<List<string>> ListOLists = new List<List<string>> {list1, list2, list3};

    List<string> result1 = LoopFindExactMatch(ListOLists, "d");

    List<string> result2 = LinqFindExactMatch(ListOLists, "d");

    // test with string not in list

    List<string> result3 = LoopFindExactMatch(ListOLists, "x");

    List<string> result4 = LinqFindExactMatch(ListOLists, "y");
}

Note that in these examples we search for only the first match. If your list of lists contains 'inner' lists with duplicate values, another strategy is indicated.