Combination Algorithm in C#

Question

I need a combination of n fields where each field can be equal to null or not null. For each combination, the fields cannot be repeated. Basically, there should be a total of 2^n combinations.

Example:

if I have 2 fields A and B, the combinations in the output should be:

A != null and B != null
A != null and B == null
A == null and B != null
A == null and B == null

if I have 3 fields A, B, and C, the combinations in the output should be:

A != null and B != null and C != null
A != null and B != null and C == null
A != null and B == null and C != null
A != null and B == null and C == null
A == null and B != null and C != null
A == null and B != null and C == null
A == null and B == null and C != null
A == null and B == null and C == null

I don't know what this combination is called, so how can I do this in code where the number of fields is a variable?

Thanks!

It's the power set of {A, B, C} if you consider that != null means "present in the subset" and == null means "not present in the subset". — user325117
– user325117, Commented Mar 2, 2016 at 10:52
I don't get what you need, you need the algorithm for generating all those possibilities? another thing is where do you put those returns? — Pier Giorgio Misley
– Pier Giorgio Misley, Commented Mar 2, 2016 at 11:00
@Martin - thanks i think i get an idea about what you mean regarding power sets.. can you give me an example? — pineapple
– pineapple, Commented Mar 2, 2016 at 11:01
@PierGiorgioMisley The output should be exactly what i have shown in my question — pineapple
– pineapple, Commented Mar 2, 2016 at 11:02
This looks like a state machine to me, for given states do something. — Samer Tufail
– Samer Tufail, Commented Mar 2, 2016 at 11:02

Dmitrii Bychenko · Accepted Answer · 2016-03-02 11:25:55Z

3

If you want a generator of such lines you can use Linq:

   int count = 2;

   var lines = Enumerable
     .Range(0, 1 << count) // 1 << count == 2 ** count
     .Select(item => String.Join(" and ", Enumerable
       .Range(0, count)
       .Select(index => ((Char) ('A' + index)).ToString() + 
                        ((item >> index) % 2 == 0 ? " != null" : " == null"))));


   // Let's print out all the lines generated
   Console.Write(String.Join(Environment.NewLine, lines));

For count = 2 the output is

  A != null and B != null
  A == null and B != null
  A != null and B == null
  A == null and B == null

Edit: A small modification lets you put your own names:

  String[] names = new String[] { "A", "B", "C" };

  var lines = Enumerable
    .Range(0, 1 << names.Length) // 1 << count == 2 ** count
    .Select(item => String.Join(" and ", Enumerable
       .Range(0, names.Length)
       .Select(index => names[index] +
                        ((item >> index) % 2 == 0 ? " != null" : " == null"))));

  // Let's print out all the lines generated
  Console.Write(String.Join(Environment.NewLine, lines));

edited Mar 2, 2016 at 11:25

answered Mar 2, 2016 at 11:04

Dmitrii Bychenko

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3 Comments

pineapple Over a year ago

thanks but the fields im using are variables.. they are not literally "A", "B", or "C"..

Dmitrii Bychenko Over a year ago

@Paul Adrian Pena: a small modification (see my edit) lets you provide your own names

pineapple Over a year ago

i think i got it.. thanks a lot.. this is brilliant!

atlaste · Accepted Answer · 2016-03-02 11:37:48Z

0

I usually stick to simple recursion in cases like this, because it's so easy to understand. No explanation, I'll just let the (untested) code talk for itself:

public void Generate()
{
    Create("", 0);
}

private string[] names = new[]{ "A", "B", "C" };

public void Create(string s, int current)
{
    if (current != 0)
    { 
        s += " and ";
    }

    if (current != names.Length)
    {
        string c1 = s + names[current] + " == null"; // case 1
        string c2 = s + names[current] + " != null"; // case 2

        Create(c1, current+1);
        Create(c2, current+1);
    }
    else
    {
        Console.WriteLine(s);
    }
}

answered Mar 2, 2016 at 11:37

atlaste

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