FTGET

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

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

      h - FITS ASCII extension header (e.g. as returned by FITS_READ)
                            or
      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

      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.

       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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