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).