Get Equal Part of Multiple Strings at the Beginning

Question

I've got a few big arrays/lists of filenames that start the same. Like this:

C:\Program Files\CCleaner\...
C:\Program Files\Common Files\...
C:\Program Files (x86)\Adobe\...
C:\Program Files (x86)\Common Files\...

I would like to extract the beginning part that they all have in common.
In this case: "C:\Program Files"

How do I do that?

I thought I might have to compare 2 strings at a time and get the same beginning. I don't even know how to do that without comparing each character manually? Then I'll have to compare each string to every other string? Will it be O(n²)? Is there a better, faster way?

Edit: Is there also a way without Linq?

Answer 1

Quick shot:

List<string> strings = ...;
var minimumLength = strings.Min(x => x.Length);
int commonChars;
for(commonChars = 0; commonChars < minimumLength; commonChars++)
{
  if (strings.Select(x => x[commonChars]).Distinct().Count() > 1)
  {
    break;
  }
}
return strings[0].Substring(0, commonChars);

OR

var minimumLength = strings.Min(x => x.Length);
Enumerable
  .Range(0, minimumLength)
  .Count(i => strings.All(y => y[i] == strings[0][i]));

Without Linq:

List<string> strings = ...;
var minimumLength = strings.Min(x => x.Length);
int commonChars;
for(commonChars = 0; commonChars < minimumLength; commonChars++)
{
  foreach(var str in strings)
  {
    if (str[commonChars] != strings[0][commonChars])
    {
      break;
    }
  }
}
return strings[0].Substring(0, commonChars);

There are a couple of other solutions.

Answer 2

Another Linq solution:

var strings = new List<string> {@"C:\Program Files\CCleaner\...", @"C:\Program Files\Common Files\...", 
                                @"C:\Program Files (x86)\Adobe\...", @"C:\Program Files (x86)\Common Files\..."};

var common = new string(strings.Select(str => str.TakeWhile((c, index) => strings.All(s => s[index] == c)))
                               .FirstOrDefault().ToArray());

Console.WriteLine(common); // C:\Program Files

Answer 3

If you have a very big list, the best is to sort the string, check the number of characters the first and last string has.

I cant really prove that it works, but intuitively, it does. All the middle ones will need to have the same prefix to be sorted that way.

using System;
using System.IO;
using System.Collections.Generic;
namespace StringSameStart
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");

            var files = Directory.GetFiles("/Users/ibrar", "*", SearchOption.AllDirectories);
            foreach (var file in files)
            {
                Console.WriteLine("file : " + file);
            }

            Array.Sort(files);
            var first = files[0];
            var last = files[files.Length - 1];

            List<char> list = new List<char>();

            for (int ctr = 0; ctr < files[0].Length; ctr++)
            {
                if (first[ctr] != last[ctr])
                {
                    break;
                }

                Console.WriteLine("Same : " + first[ctr]);
                list.Add(first[ctr]);
            }


            Console.WriteLine("Match : " + new string(list.ToArray()));
        }
    }
}