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      Function to determine if a string is a valid numeric value.


      A string is considered a valid numeric value if IDL can convert it
      to a numeric variable without error.

Calling Sequence

      result = strnumber( st, [val, /HEX] )


      st - any IDL scalar string


      1 is returned as the function value if the string st has a
      valid numeric value, otherwise, 0 is returned.

Optional Output

      val - (optional) value of the string. double precision unless /L64 is set

Optional Input Keyword

      /HEX - If present and nonzero, the string is treated as a hexadecimal
            longword integer.
      /L64 - If present and nonzero, the val output variable is returned
              as a 64 bit integer. This to ensure that precision is not
              lost when returning a large 64 bit integer as double precision.
              This keyword has no effect on the function result.
      /NAN - if set, then the value of an empty string is returned as NaN,
              by default the returned value is 0.0d. In either case,
              an empty string is considered a valid numeric value.


      IDL> res = strnumber('0.2d', val)
          returns res=1 (a valid number), and val = 0.2000d


      (1) STRNUMBER was modified in August 2006 so that an empty string is
      considered a valid number. Earlier versions of strnumber.pro did not
      do this because in very early (pre-V4.0) versions of IDL
      this could corrupt the IDL session.
      (2) STRNUMBER will return a string such as '23.45uyrg' as a valid
      number (=23.45) since this is how IDL performs the type conversion. If
      you want a stricter definition of valid number then use the VALID_NUM()


      version 1 By D. Lindler Aug. 1987
      test for empty string, W. Landsman February, 1993
      Hex keyword added. MRG, RITSS, 15 March 2000.
      An empty string is a valid number W. Landsman August 2006
      Added /NAN keyword W. Landsman August 2006
      Added /L64 keyword W. Landsman Feb 2010

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