Handling temporary calculation in Linq

Question

When solving an interview question

Question A six digit number need to be found in such a manner when it is multiplied by an integer between 2 and 9 gives the original six digit number when its digits are reversed.

Example:

Suppose I multiply 219978 * 4 i get 879912 ,when reverse 879912 I will get 219978 back.

I solved it using

for (long l = 100000; l < 999999; l++)
{
 var num = l.ToString();

for (int i = 3; i < 9; i++)
{
  var mul = l * i;
  string str = mul.ToString();
  char[] splitDigits = str.ToCharArray();
  string reversedDigit =
  new  string(splitDigits.Reverse().ToArray());

  if (reversedDigit.CompareTo(num) == 0)
    {
      Console.WriteLine("{0} * {1}= {2},
       when multiplied {3} ", num, i, mul,reversedDigit);

    }

 }
}

The original task was to solve it using linq. I have bit confusion in handling temp calculations for example

when i use

   var = from l in Enumerable.Range(100000,999999)
         from i in Enumerable.Range(3,9)

What is the way to handle temporary calculations like var num = l.ToString(),etc in linq.It confused me a lot to finish it in Linq.Help is appreciated.

Answer 1

You want let...

    // NOTE: buggy; see below
    var qry =  from l in Enumerable.Range(100000, 999999)
               from i in Enumerable.Range(3, 9)
               let s = l.ToString()
               let t = (l * i).ToString()
               where s.Reverse().SequenceEqual(t)
               select new { l, i };

    var a = qry.First();
    Console.WriteLine("an answer...");
    Console.WriteLine("{0} x {1} = {2}", a.l, a.i, a.l * a.i);

    Console.WriteLine("all answers...");
    foreach (var row in qry)
    {
        Console.WriteLine("{0} x {1} = {2}", row.l, row.i, row.l * row.i);
    }

with first answer (note the inclusion of 9 is taken from your original version of the code, but it may be desirable to use Range(3,8) instead):

109989 x 9 = 989901

---

Optimised version (and correct range):

    var qry =  from l in Enumerable.Range(100000, 999999 - 100000)
               let s = l.ToString()
               let sReversed = new string(s.Reverse().ToArray())
               let wanted = int.Parse(sReversed)
               from i in Enumerable.Range(3, 8 - 3) 
               where l * i == wanted
               select new { l, i };

This reduces the number of strings created, uses integer equality, and correctly uses the range (the second parameter to Range is the count, not the end).

Answer 2

Here's another solution that matches the problem statement with a few helper methods for clarity (which could be moved into the original linq query):

    private static IEnumerable<int> SixDigitNumbers = Enumerable.Range(100000, (999999 - 100000));
    private static IEnumerable<int> Multipliers = Enumerable.Range(2, 8);

    static void Main(string[] args)
    {

        var Solutions = from OriginalNumber in SixDigitNumbers
                              from Multiplier in Multipliers
                              let MultipliedNumber = (OriginalNumber * Multiplier)
                              where MultipliedNumber < 999999 && ResultIsNumericPalindrome(OriginalNumber, Multiplier)
                              select new { MultipliedNumber, OriginalNumber, Multiplier };


        var AllSolutions = Solutions.ToList();

    }

    private static string Reverse(string Source)
    {
        return new String(Source.Reverse().ToArray());
    }

    private static bool ResultIsNumericPalindrome(int Original, int Multiplier)
    {
        return (Original.ToString() == Reverse((Original * Multiplier).ToString()));
    }

Here are ALL of the solutions:

{ MultipliedNumber = 989901, OriginalNumber = 109989, Multiplier = 9 }
{ MultipliedNumber = 879912, OriginalNumber = 219978, Multiplier = 4 }

Be careful with Enumerable.Range - I see one person responding to this question made the mistake of excluding two numbers requested in the problem statement.