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      Add or modify a parameter in a FITS header array.

Calling Sequence

      SXADDPAR, Header, Name, Value, [ Comment, Location, /SaveComment,
                              BEFORE =, AFTER = , FORMAT= , /PDU]


      Header = String array containing FITS or STSDAS header. The
              length of each element must be 80 characters. If not
              defined, then SXADDPAR will create an empty FITS header array.
      Name = Name of parameter. If Name is already in the header the value
              and possibly comment fields are modified. Otherwise a new
              record is added to the header. If name is equal to 'COMMENT'
              or 'HISTORY' or a blank string then the value will be added to
              the record without replacement. For these cases, the comment
              parameter is ignored.
      Value = Value for parameter. The value expression must be of the
              correct type, e.g. integer, floating or string. String values
                of 'T' or 'F' are considered logical values.

Optional Input Parameters

      Comment = String field. The '/' is added by this routine. Added
              starting in position 31. If not supplied, or set equal to
              '', or /SAVECOMMENT is set, then the previous comment field is
              retained (when found)
      Location = Keyword string name. The parameter will be placed before the
              location of this keyword. This parameter is identical to
              the BEFORE keyword and is kept only for consistency with
              earlier versions of SXADDPAR.

Optional Input Keyword Parameters

      BEFORE = Keyword string name. The parameter will be placed before the
              location of this keyword. For example, if BEFORE='HISTORY'
              then the parameter will be placed before the first history
              location. This applies only when adding a new keyword;
              keywords already in the header are kept in the same position.
      AFTER = Same as BEFORE, but the parameter will be placed after the
              location of this keyword. This keyword takes precedence over
      FORMAT = Specifies FORTRAN-like format for parameter, e.g. "F7.3". A
              scalar string should be used. For complex numbers the format
              should be defined so that it can be applied separately to the
              real and imaginary parts. If not supplied then the default is
              'G19.12' for double precision, and 'G14.7' for floating point.
      /PDU = specifies keyword is to be added to the primary data unit
              header. If it already exists, it's current value is updated in
              the current position and it is not moved.
      /SAVECOMMENT = if set, then any existing comment is retained, i.e. the
              COMMENT parameter only has effect if the keyword did not
              previously exist in the header.


      Header = updated FITS header array.


      Add a keyword 'TELESCOP' with the value 'KPNO-4m' and comment 'Name
      of Telescope' to an existing FITS header h.
      IDL> sxaddpar, h, 'TELESCOPE','KPNO-4m','Name of Telescope'


      The functions SXADDPAR() and FXADDPAR() are nearly identical, with the
      major difference being that FXADDPAR forces required FITS keywords
      BITPIX, NAXISi, EXTEND, PCOUNT, GCOUNT to appear in the required order
      in the header, and FXADDPAR supports the OGIP LongString convention.
      There is no particular reason for having two nearly identical
      procedures, but both are too widely used to drop either one.
      All HISTORY records are inserted in order at the end of the header.
      All COMMENT records are also inserted in order at the end of the header
      header, but before the HISTORY records. The BEFORE and AFTER keywords
      can override this.
      All records with no keyword (blank) are inserted in order at the end of
      the header, but before the COMMENT and HISTORY records. The BEFORE and
      AFTER keywords can override this.


      Warning -- Parameters and names are not checked
              against valid FITS parameter names, values and types.

Modification History

      DMS, RSI, July, 1983.
      D. Lindler Oct. 86 Added longer string value capability
      Converted to NEWIDL D. Lindler April 90
      Added Format keyword, J. Isensee, July, 1990
      Added keywords BEFORE and AFTER. K. Venkatakrishna, May '92
      Pad string values to at least 8 characters W. Landsman April 94
      Aug 95: added /PDU option and changed routine to update last occurence
              of an existing keyword (the one SXPAR reads) instead of the
              first occurence.
      Comment for string data can start after column 32 W. Landsman June 97
      Make sure closing quote supplied with string value W. Landsman June 98
      Converted to IDL V5.0 W. Landsman June 98
      Increase precision of default formatting of double precision floating
              point values. C. Gehman, JPL September 1998
      Mar 2000, D. Lindler, Modified to use capital E instead of lower case
              e for exponential formats.
      Apr 2000, Make user-supplied format upper-case W. Landsman
      Oct 2001, Treat COMMENT or blank string like HISTORY keyword W. Landsman
      Jan 2002, Allow BEFORE, AFTER to apply to COMMENT keywords W. Landsman
      June 2003, Added SAVECOMMENT keyword W. Landsman
      Jan 2004, If END is missing, then add it at the end W. Landsman
      May 2005 Fix SAVECOMMENT error with non-string values W. Landsman
      Oct 2005 Jan 2004 change made SXADDPAR fail for empty strings W.L.
      May 2011 Fix problem with slashes in string values W.L.

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