Arrange the numbers in .dat file (C#)

Comments:

0) requires Linq, uses the 'Func Delegate syntax for economy.

1) fortunately, conversion to Double from all the other numeric Types will work: otherwise, this would be a mess to implement.

2) the code here shows examples of extracting converted, filtered, numbers based on a function you supply. left for you to write is taking the sets of filtered data and turning them back into a .dat file you save ... hint: use a StringBuilder

public static class SelectNumbers
{   
    private static char[] split1 = new char[] {'\r', '\n'};

    public static double[] LinesToDouble(string data)
    {
        data = data.Trim();

        string[] lines = data.Split(split1, StringSplitOptions.RemoveEmptyEntries);

        var trimlines = lines
            .Select(line => line.Trim());

        return trimlines.Select(line => line.StringToDouble()).ToArray();
    }

    public static double StringToDouble(this string str)
    {
        double value;

        if (double.TryParse(str, out value))
        {
            return value;
        }

        throw new InvalidCastException($"{str} ... is not a number");
    }
}

usage example:

var ns = SelectNumbers.LinesToDouble(@"
    1
    2
    5
    3.999
    -4
    533
    5
    -6.456
");

    double[] matchlta = GetMatchingValues(4.0, ns, (double d1, double d2) => d1 < d2);

    double[] matcheqa = GetMatchingValues(5.0, ns, (double d1, double d2) => d1 == d2);

Feel free to ask questions.

The spec is unclear on a number of points, including edge cases, such as what if there are no values less than A and what if all the provided values are greater than A. I tried to address this as best I could.

public sealed partial class ParsedDecimal : System.IComparable<ParsedDecimal>
{
  public static System.Globalization.NumberStyles NumberStyles ;

  public static ParsedDecimal MinValue { get ; private set ; }
  public static ParsedDecimal MaxValue { get ; private set ; }

  static ParsedDecimal
  (
  )
  {
    NumberStyles = System.Globalization.NumberStyles.Float ;

    MinValue = new ParsedDecimal ( System.Decimal.MinValue ) ;
    MaxValue = new ParsedDecimal ( System.Decimal.MaxValue ) ;

    return ;
  }

  public decimal Value { get ; private set ; }

  private string original ;

  private ParsedDecimal
  (
    decimal Value
  )
  {
    this.Value = Value ;

    this.original = Value.ToString() ;

    return ;
  }

  public ParsedDecimal
  (
    string Value
  )
  {
    this.Value = System.Decimal.Parse ( Value , NumberStyles ) ;

    this.original = Value ;

    return ;
  }

  public int
  CompareTo
  (
    ParsedDecimal Op1
  )
  {
    int result = this.Value.CompareTo ( Op1.Value ) ;

    if ( result == 0 )
    {
      result = this.original.CompareTo ( Op1.original ) ;
    }

    return ( result ) ;
  }

  public override string
  ToString
  (
  )
  {
    return ( this.original ) ;
  }
}

public static string
Separate
(
  decimal         A
,
  params string[] List
)
{
  System.Text.StringBuilder result =
    new System.Text.StringBuilder() ;

  ParsedDecimal least = ParsedDecimal.MaxValue ;

  System.Collections.Generic.List<ParsedDecimal> lest =
    new System.Collections.Generic.List<ParsedDecimal>() ;

  System.Collections.Generic.List<ParsedDecimal> less =
    new System.Collections.Generic.List<ParsedDecimal>() ;

  System.Collections.Generic.List<ParsedDecimal> same =
    new System.Collections.Generic.List<ParsedDecimal>() ;

  System.Collections.Generic.List<ParsedDecimal> more =
    new System.Collections.Generic.List<ParsedDecimal>() ;

  for ( int i = 0 ; i < List.Length ; i++ )
  {
    ParsedDecimal d = new ParsedDecimal ( List [ i ] ) ;

    if ( least.Value > d.Value )
    {
      least = d ;
    }

    if ( d.Value < A )
    {
      less.Add ( d ) ;
    }
    else if ( d.Value > A )
    {
      more.Add ( d ) ;
    }
    else
    {
      same.Add ( d ) ;
    }
  }

  for ( int i = 0 ; i < less.Count ; i++ )
  {
    if ( less [ i ].Value == least.Value )
    {
      lest.Add ( less [ i ] ) ;
    }
    else
    {
      result.Append ( less [ i ] ) ;
      result.AppendLine() ;
    }
  }

  if ( lest.Count == 0 )
  {
    lest = same ;
  }
  else
  {
    for ( int i = 0 ; i < same.Count ; i++ )
    {
      result.Append ( same [ i ] ) ;
      result.AppendLine() ;
    }
  }

  if ( lest.Count == 0 )
  {
    for ( int i = 0 ; i < more.Count ; i++ )
    {
      if ( more [ i ].Value == least.Value )
      {
        lest.Add ( more [ i ] ) ;
      }
    }
  }

  for ( int i = 0 ; i < lest.Count ; i++ )
  {
    result.Append ( lest [ i ] ) ;
    result.AppendLine() ;
  }

  return ( result.ToString() ) ;
}