Finding differences within 2 Lists of string arrays

Using linq you can just go:

var differences = orignal.Except(web).ToList();

Reference here

This will give you the values that are in original, that don't exist in web

Sorry didn't read your question properly, to answer your question: You have a nested for-loop. So for each value of original (3) it will loop through all values of web (3), which is 9 loops total.

In 3 cases it doesn't match and therefore outputs 3 times.

I think this is what you want. I use Linq to grab the primary keys, and then I use Except to do original - web. By the way, you can use == instead of Equals with strings in C# because C# does a value comparison as opposed to a reference comparison.

List<string[]> original = new List<string[]>
{
    new string[3] { "a", "2", "3" },
    new string[3] { "x", "2", "3" },
    new string[3] { "c", "2", "3" }
};
List<string[]> web = new List<string[]>
{
    new string[3] { "a", "2", "3" },
    new string[3] { "b", "2", "3" },
    new string[3] { "c", "2", "3" }
};

var originalPrimaryKeys = original.Select(o => o[0]);
var webPrimaryKeys = web.Select(o => o[0]);

List<string> differences = originalPrimaryKeys.Except(webPrimaryKeys).ToList();

Console.WriteLine("The number of differences is {0}", differences.Count);
foreach (string diff in differences)
{
    Console.WriteLine(diff);
}

And here it is without Linq:

var differences = new List<string>();

for (int i = 0; i < original.Count; i++)
{
    bool found = false;
    for (int j = 0; j < web.Count; j++)
    {
        if (original[i][0] == web[j][0])
        {
            found = true;
        }
    }

    if (!found)
    {
        differences.Add(original[i][0]);
    }
}

To answer your question: It is a nested for loop as stated in JanR's answer. This approach will make you reiterate to your web count 9 times, thus listing your mismatched key three times.

What could be a better way to do is this:

 //Check for originals not introduced in web.
        if(original.Count > web.Count)
        {

           for(int y = web.Count; y < original.Count; y++)

           {
              differences.Add(original[y][0]);
           }
        }
 //Check if Web has value, if not, everything else is done on the first for loop
         if(web.Count > 0)
         {
             for(int i = 0; i < original.Count; i++)
                {
                   if(!original[i][0].Equals(web[i][0]))
                      differences.Add(original[i][0]);
                }
         }

Also, the output is in a for loop, when you just need one result, the length of the mismatch. You can do that without a loop.

          Console.WriteLine("SIZE " + differences.Count);

This is, of course to make it kinda simpler if you're not used to using LINQ statements, but if you can do so with LINQ, then by all means, use LINQ as it's more efficient.

You can get the difference by using Except extension method like this:

var originalDic = original.ToDictionary(arr => arr.First());
var webDic = web.ToDictionary(arr => arr.First());
var differences =
    originalDic
    .Except(webDic, kvp => kvp.Key)
    .Select(kvp => kvp.Value)
    .ToList();

The trick here is to first convert your original and web lists into a Dictionary using the first element of each array as key and then perform Except.