• Status: Solved
  • Priority: Medium
  • Security: Public
  • Views: 640
  • Last Modified:

string combinatorics, generate nCr strings from n strings

I have a set of n strings. I would like to extract all combinations of strings from these n strings . (all nCr combinations). How can  I do this programmatically.
For example :
Input : a,b,c,d,e
Output :
a,b,c,d
a,b,c,e
a,b,d,e
a,c,d,e
b,c,d,e
a,b,c
a,c,d
a,d,e
......and so on till

a,b
b,a
and so on till
a
b
c
d
e
Please help thanks
0
TrialUser
Asked:
TrialUser
  • 5
  • 2
1 Solution
 
TommySzalapskiCommented:
The best way is to make a function that finds all string combinations of length N and use that recursively.
Then you just call it for each length.

  
ArrayList FindAllStrings(string input)
{
  ArrayList strings;

  for(int i = 0; i<=input.Length(); i++)
  AddStrings(strings, input, i, "");
}

AddStrings(ArrayList strings, string input, int N, string leading = "")
{
  if(N == 0)
    strings.Add(leading);
  else
  {
    for(int i = 0; i + N < input.Length(); i++)
      addStrings(strings, input.SubString(i+1), N-1, leading + input[i]);
  }

Open in new window

0
 
TommySzalapskiCommented:
Tested and fixed
        private ArrayList FindAllStrings(string input)
        {
            ArrayList strings = new ArrayList();

            for (int i = 0; i <= input.Length; i++)
                AddStrings(strings, input, i, "");

            return strings;
        }

        private void AddStrings(ArrayList strings, string input, int N, string leading = "")
        {
            if (N == 0)
                strings.Add(leading);
            else
            {
                for (int i = 0; i + N <= input.Length; i++)
                    AddStrings(strings, input.Substring(i + 1), N - 1, leading + input[i]);
            }
        }

Open in new window

0
 
TommySzalapskiCommented:
If you want the commas, change line 18 to
AddStrings(strings, input.Substring(i + 1), N - 1, leading + (leading.Length > 0 ? ",":"") + input[ i]);
0
Cloud Class® Course: Ruby Fundamentals

This course will introduce you to Ruby, as well as teach you about classes, methods, variables, data structures, loops, enumerable methods, and finishing touches.

 
Mike TomlinsonMiddle School Assistant TeacherCommented:
Use the libaray here written by Adrian Akison:
http://www.codeproject.com/KB/recipes/Combinatorics.aspx

Simple Demo: Idle-Mind-502573.flv

Click on Project --> Add Reference --> Browse --> Facet.Combinatorics.dll

Then you want to use the Variations class like this:
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            listBox1.DataSource = Variations(textBox1.Text);
        }

        private List<string> Variations(string input)
        {
            List<string> values = new List<string>();
            for (int i=0; i < input.Length; i++)
            {
                values.Add(input.Substring(i,1));
            }

            List<string> results = new List<string>();
            for (int i = input.Length; i >= 1; i--)
            {
                Facet.Combinatorics.Variations<string> vars = new Facet.Combinatorics.Variations<string>(values.AsReadOnly(), i);
                foreach (IList<string> set in vars)
                {
                    results.Add(String.Join(",", set));
                }
            }
            return results;
        }
    }
}

Open in new window


Here's the output:
a,b,c,d
a,b,d,c
a,c,b,d
a,d,b,c
a,c,d,b
a,d,c,b
b,a,c,d
b,a,d,c
c,a,b,d
d,a,b,c
c,a,d,b
d,a,c,b
b,c,a,d
b,d,a,c
c,b,a,d
d,b,a,c
c,d,a,b
d,c,a,b
b,c,d,a
b,d,c,a
c,b,d,a
d,b,c,a
c,d,b,a
d,c,b,a
a,b,c
a,b,d
a,c,b
a,d,b
a,c,d
a,d,c
b,a,c
b,a,d
c,a,b
d,a,b
c,a,d
d,a,c
b,c,a
b,d,a
c,b,a
d,b,a
c,d,a
d,c,a
b,c,d
b,d,c
c,b,d
d,b,c
c,d,b
d,c,b
a,b
a,c
a,d
b,a
c,a
d,a
b,c
b,d
c,b
d,b
c,d
d,c
a
b
c
d

Open in new window

 
0
 
TommySzalapskiCommented:
Idle_Mind, that does all the possible permutations of all the possible combinations. It's also using another library and even then, the code looks just about as long as mine.

TrialUser, I assume your inclusion of both a,b and b,a was accidental and you just wanted the combinations. To do all permutations in mine you would just change
input.Substring(i + 1)
to
input.Substring(0,i) + input.Substring(i + 1)
0
 
TommySzalapskiCommented:
You do need to make sure to have
using System.Collections
at the top to use the ArrayList or you could modify the code to use whatever collection type you prefer.
0
 
Mike TomlinsonMiddle School Assistant TeacherCommented:
The library can also do Permuations and Combinations as well...it's pretty flexible.
0
 
RyanProject Engineer, ElectricalCommented:
Just last week I was solving a Permutation problem on ProjectEuler.net and came across this website. http://www.bearcave.com/random_hacks/permute.html

In particular, I found this graphic interesting, http://www.bearcave.com/random_hacks/permute_diagram.jpg

Merely implementing that, with an otter loop that iterates through removing letters solves it.
0
Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

Join & Write a Comment

Featured Post

Cloud Class® Course: MCSA MCSE Windows Server 2012

This course teaches how to install and configure Windows Server 2012 R2.  It is the first step on your path to becoming a Microsoft Certified Solutions Expert (MCSE).

  • 5
  • 2
Tackle projects and never again get stuck behind a technical roadblock.
Join Now