Check if list doens't contain other strings

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Hello.
I am sure, that this question has been already answered, but I just can't find the question with my low expression abilities. So, let's say I have a list named list1:

a, b, c

I want to chech if another list (list2) doesn't contain other elements than these in list1.
So if list2 is:

a, b, c, d

It will return false, but also if some elements of list1 are missing, it will still return true. Hopefully you understood me and sorry for my bad english, still learning. Thanks for reading!

Posted 8-Jan-15 2:40am

Samuel Dudík

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ZurdoDev 8-Jan-15 8:44am

I believe LINQ could help you so you don't have to do nested foreach loops. Where are you stuck?

Samuel Dudík 8-Jan-15 8:49am

Well, I'm stuck at everything about this. I don't know where to start. I try to search in LINQ as you mentioned, thanks.

ZurdoDev 8-Jan-15 8:50am

I googled "c# compare list of strings" and quite a few examples are there.

PIEBALDconsult 8-Jan-15 9:17am

Stick each in a Hashset and use set operations -- is set B a subset of set A. (That's all Linq will do anyway, cut out the middleman.)

BillWoodruff 8-Jan-15 14:17pm

That's a very interesting idea: it would be useful to see some timing comparisons of using Linq vs. using HashTables.

PIEBALDconsult 8-Jan-15 14:28pm

Linq uses Hashsets extensively (data manipulation is largely set-based). In truth, I'm guessing, based on Hashset's appearance in the same version of .net that Linq arrived and it's feature-poor interface.
So, you can implement the OP's task in Linq, which will (likely) create two Hashsets and operate on them -- then throw them both away.
_Or_ you can create the Hashsets yourself and perform _multiple_ operations on them before throwing them out.
Which technique do you think will perform better overall?

Correction: It likely makes only one into a Set then iterates the other.

BillWoodruff 8-Jan-15 10:00am

"if some elements of list1 are missing, it will still return true"

This leaves us unable to make a strategy: how many elements of list1 can be missing ? All of them; one of them ?

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PIEBALDconsult · Answer 1 · 2015-01-08T10:30:00

Using the code below I got the following results:

100000000 00:00:45.9990976
200000000 00:00:21.1352109
300000000 00:00:17.3039397

So, for the given data...
Set.IsSupersetOf is quickest and I recommend it for your needs (whatever they are).
Set.IsSubsetOf is not quite as quick -- I expect it has to iterate more of the List.
And the Linqish List.Except is worst, including the situation that if you just want a boolean then there is no need to produce a collection and then count the items in the collection.

Consider, that when seeking a boolean, the process can be short-circuited as soon as one item is found outside the target. I certainly hope that IsSupersetOf and IsSubsetOf do that. Except certainly does not short-circuit and then it makes you perform another operation (like Count) before you can get your result.

using System.Linq ;
using StringList=System.Collections.Generic.List<string> ;
using StringSet=System.Collections.Generic.HashSet<string> ;

        int reps = 100000000 ;

        StringList list1 = new StringList { "a" , "b" , "c" , "z" } ;
        StringList list2 = new StringList { "a" , "b" , "c" , "d" } ;
        StringSet  set1  = new StringSet  ( list1 ) ;
        StringSet  set2  = new StringSet  ( list2 ) ;

        System.Diagnostics.Stopwatch sw = new System.Diagnostics.Stopwatch() ;


        sw.Restart() ;

        for ( int i = 0 ; i < reps ; i++ )
          result += list2.Except(list1).Count()==0 ? 0 : 1 ;

        sw.Stop() ;

        System.Console.WriteLine ( "{0} {1}" , result , sw.Elapsed ) ;


        sw.Restart() ;

        for ( int i = 0 ; i < reps ; i++ )
          result += set2.IsSubsetOf ( list1 ) ? 0 : 1 ;

        sw.Stop() ;

        System.Console.WriteLine ( "{0} {1}" , result , sw.Elapsed ) ;


        sw.Restart() ;

        for ( int i = 0 ; i < reps ; i++ )
          result += set1.IsSupersetOf ( list2 ) ? 0 : 1 ;

        sw.Stop() ;

        System.Console.WriteLine ( "{0} {1}" , result , sw.Elapsed ) ;

Maciej Los · Answer 2 · 2015-01-08T03:11:00

Solution 1

Based on tis article: How to: Find the Set Difference Between Two Lists (LINQ)[^], you can use Linq to find differences between two string lists:

C#

List<string> list1 = new List<string>{"a","b","c"};
List<string> list2 = new List<string>{"a","b","c", "d"};

var hasMatch = list2.Except(list1);

Returns: d

[EDIT]
Thank you for all valuable comments. It helps me to improve my answer.

Another interestinf method to compare two string list is: SequenceEqual, which returns boolean value.

C#

bool listsAreEqual = list2.SequenceEqual(list1);

For further information, please see: Enumerable.SequenceEqual<TSource> Method (IEnumerable<TSource>, IEnumerable<TSource>)[^].

Posted 8-Jan-15 3:11am

Maciej Los

Updated 8-Jan-15 11:55am

v3

Comments

Praveen Kumar Upadhyay 8-Jan-15 9:29am

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There's no way to "record" it.

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BillWoodruff 8-Jan-15 12:09pm

A person "congratulating themselves" here may sometimes come under increased scrutiny. Not by me, however; I congratulate you for your effort, and hope you will always focus as much on the quality of your responses as you do on the quantity of them. In the long-run, the only lasting achievement lies in true quality.

Namaste, Bill

BillWoodruff 8-Jan-15 14:19pm

+4 Of course this is a solid answer to part of the OP's question; to me the more interesting part is the idea that the comparison occurs with some members of "list1" missing, but, until the OP clarifies that, I don't think anyone can really answer that.

PIEBALDconsult 8-Jan-15 15:14pm

Is B a subset of A? Is A a superset of B?

BillWoodruff 8-Jan-15 20:27pm

Until I know what the OP means by "It will return false, but also if some elements of list1 are missing, it will still return true:" I'm just another dancer in the dark.

PIEBALDconsult 8-Jan-15 15:11pm

How would you turn that into a boolean? I see that I can use .Count()==0 , but I'm wondering whether or not there is a better way.

PIEBALDconsult 8-Jan-15 22:01pm

SequenceEqual won't detect "subset".

Check if list doens't contain other strings

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