Question

I need help on an algorithm. I have randomly generated numbers with 6 digits. Like;

123654
109431

There are approximately 1 million of them saved in a file line by line. I have to filter them according to the rule I try to describe below.

Take a number, compare it to all others digit by digit. If a number comes up with a digit with a value of bigger by one to the compared number, then delete it. Let me show it by using numbers.

Our number is: 123456
Increase the first digit with 1, so the number becomes: 223456. Delete all the 223456s from the file.
Increase the second digit by 1, the number becomes: 133456. Delete all 133456s from the file, and so on…

I can do it just as I describe but I need it to be “FAST”.

So can anyone help me on this?

Thanks.

Answer 1

This algorithm will keep a lot of numbers around in memory, but it will process the file one number at a time so you don’t actually need to read it all in at once. You only need to supply an IEnumerable<int> for it to operate on.

    public static IEnumerable<int> FilterInts(IEnumerable<int> ints)
    {
        var removed = new HashSet<int>();

        foreach (var i in ints)
        {
            var iStr = i.ToString("000000").ToCharArray();

            for (int j = 0; j < iStr.Length; j++)
            {
                var c = iStr[j];

                if (c == '9')
                    iStr[j] = '0';
                else
                    iStr[j] = (char)(c + 1);

                removed.Add(int.Parse(new string(iStr)));

                iStr[j] = c;
            }

            if (!removed.Contains(i))
                yield return i;
        }
    }

You can use this method to create an IEnumerable<int> from the file:

    public static IEnumerable<int> ReadIntsFrom(string path)
    {
        using (var reader = File.OpenText(path))
        {
            string line;
            while ((line = reader.ReadLine()) != null)
                yield return int.Parse(line);
        }
    }