How to get "unordered choose two" permutation in a list of Strings in C#

Question

i have a list of string

   List< String> lst

 A
 B
 C

i need to add the combination of each item with other items to the list like

 A
 B
 C
 A-B
 A-C
 B-C

now i'm usign nested for loop for this.

is there any way to do this using linq?

thanks in advance

I note that you're not actually getting the Cartesian product here, you are getting the "unordered choose two" permutation. The Cartesian product would be {AA, AB, AC, BA, BB, BC, CA, CB, CC}, nine elements. The "unordered choose two" permutation is only three items, {AB, AC, BC}. I think your question has the wrong title; I don't think you're looking for the Cartesian product at all. — Eric Lippert
– Eric Lippert, Commented Sep 1, 2010 at 18:33

maciejkow · Accepted Answer · 2010-09-01 07:15:33Z

7

var sourceStrings = new List<string> {"A", "B", "C"};

var resultStrings = from a in sourceStrings
                    from b in sourceStrings
                    where a != b
                    select a + "-" + b;

foreach (var result in resultStrings)
    Console.WriteLine(result);

answered Sep 1, 2010 at 7:15

maciejkow

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1 Comment

Chris Over a year ago

I'm not too familiar with LINQ but tryign to learn, mainly from udnerstanding examples on here... Is this going to return A-C as well as C-A? That's what I think it should do but just wanted to check... :)

Thomas Levesque · Accepted Answer · 2010-09-01 08:08:47Z

6

In addition to previous answers, you might be interested in this article by Eric Lippert.

answered Sep 1, 2010 at 8:08

Thomas Levesque

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Comments

Oliver · Accepted Answer · 2010-09-01 07:41:00Z

4

Just for the completeness, here the statement in fluent writing:

var sourceStrings = new List<string> { "A", "B", "C" };

var resultStrings = sourceStrings.SelectMany(a => sourceStrings,
                                             (a, b) => new { a, b })
                                 .Where(n => n.a != n.b)
                                 .Select(n => n.a + "-" + n.b);

foreach (var result in resultStrings)
    Console.WriteLine(result);

answered Sep 1, 2010 at 7:41

Oliver

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Comments

Eric Lippert · Accepted Answer · 2010-09-02 15:14:40Z

As I mentioned in my comment, I think what you want is either a permutation (where you reorder an ordered set) or a combination (where you get unordered subsets of an unordered set)

A general purpose library for doing all those things in C# can be found here:

http://www.codeproject.com/KB/recipes/Combinatorics.aspx

It's still not 100% clear to me what you want. Here are some options:

Let S be the set {A, B, C}

The Cartesian product S x S is AA, AB, AC, BA, BB, BC, CA, CB, CC. That is every possible combination of two elements, order matters, including duplicates.

"S permute 2" is AB, AC, BA, BC, CA, CB. That is, every possible combination of two elements, order matters, but no duplicates.

"S choose 2" is AB, AC, BC. That is, every possible combination of two elements, no order, no duplicates.

"Power set of S" is nothing, A, B, C, AB, AC, BC, ABC. That is, every possible combination of zero, one, two or three elements, no order.

You can generate a power set (if that's what you want to do) by counting in binary. Consider the numbers 0 through 7 in binary:

ABC
000  0  nothing
001  1  C
010  2  B 
011  3  BC
100  4  A   
101  5  AC
110  6  AB
111  7  ABC

So if you can count, you can generate the power set.

What is it you want, exactly?

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