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      Function to return value(s) from specified column in a FITS ASCII table

Calling Sequence

      values = FTGET( h, tab, field, [ rows, nulls ] )
      values = FTGET( ft_str, tab, field. [rows, nulls]


      h - FITS ASCII extension header (e.g. as returned by FITS_READ)
      ft_str - FITS table structure extracted from FITS header by FTINFO
                Use of the IDL structure will improve processing speed
      tab - FITS ASCII table array (e.g. as returned by FITS_READ)
      field - field name or number

Optional Inputs

      rows - scalar or vector giving row number(s)
              Row numbers start at 0. If not supplied or set to
              -1 then values for all rows are returned


      the values for the row are returned as the function value.
      Null values are set to 0 or blanks for strings.

Optional Output

      nulls - null value flag of same length as the returned data.
              It is set to 1 at null value positions and 0 elsewhere.
              If supplied then the optional input, rows, must also
              be supplied.


      Read the columns labeled 'WAVELENGTH' and 'FLUX' from the second
      (ASCII table) extension of a FITS file 'spectra.fit'
      IDL> fits_read,'spectra.fit',tab,htab,exten=2 ;Read 2nd extension
      IDL> w = ftget( htab, tab,'wavelength') ;Wavelength vector
      IDL> f = ftget( htab, tab,'flux') ;Flux vector
      Slightly more efficient would be to first call FTINFO
      IDL> ftinfo, htab, ft_str ;Extract structure
      IDL> w = ftget(ft_str, tab,'wavelength') ;Wavelength vector
      IDL> f = ftget(ft_str, tab,'flux') ;Flux vector


      (1) Use the higher-level procedure FTAB_EXT to extract vectors
              directly from the FITS file.
      (2) Use FTAB_HELP or FTHELP to determine the columns in a particular
              ASCII table.


      coded by D. Lindler July, 1987
      Always check for null values W. Landsman August 1990
      More informative error message W. Landsman Feb. 1996
      Converted to IDL V5.0 W. Landsman September 1997
      Allow structure rather than FITS header W. Landsman May 2000
      No case sensitivity in TTYPE name W. Landsman February 2002

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