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  Craig B. Markwardt, NASA/GSFC Code 662, Greenbelt, MD 20770


  Compute SIZE descriptors for each tag in a structure

Calling Sequence

                  STRUCTURE_NAME=structname, STATUS=status, LENGTH=length)


  The TAGSIZE function determines the types and sizes of each tag in
  a structure. This is not as easy as it may seem, because IDL
  makes it very, *very* difficult to find out the true dimensions of
  a structure element.
  Here is a brief explanation. It is known that IDL drops trailing
  dimensions of size 1 in many situations. Also, for structures
  only, arrays of any dimensionality which have only one element are
  RETURNED AS SCALARS. Thus, if you are doing any heavy duty work
  with structures, copying and querying individual elements between
  structures, etc., you will find that you will lose some crucial
  dimensions which you can't normally regain.
  TAGSIZE attempts to work around all of these limitations to
  present the true dimensions of all elements in a structure.
  It returns an 11xNTAGS array, which contains a SIZE-style vector
  for each element in the structure. Eleven elements is the largest
  array size needed to describe any IDL data type using SIZE. Thus,
  to extract information about the second tag in structure X
  (element number 1 starting from zero), you would use the following
    SIZES = TAGSIZE(X) ;; Extract type information from structure X
    SIZE_1 = SIZES(*,1) ;; Extract type information about the 2nd element
    SIZE_1 = SIZE_1(0:SIZE_1(0)+2) ;; Trim the array if desired
  The last command is optional, but trims the resulting array to be
  a true SIZE-style result.
  TAGSIZE also has several convenience keywords to extract other
  relevant information about a structure.


  STRUCTURE - any structure to examine. If the value is not a
              structure then an error is reported.


  N_TAGS - upon return, the number of tags in the structure is
            stored in this keyword.
  TAG_NAMES - upon return, the names of each tag are stored in this
              keyword, as an array of strings.
  STRUCTURE_NAME - upon return, the name of the structure is stored
                    in this keyword. If the structure is anonymous
                    then the empty string ('') is returned.
  LENGTH - upon return, the size in bytes of each tag element in the
            structure is stored in this keyword, as an array of
  STATUS - upon return, the status is stored in this keyword. A
            value of 1 indicates success, 0 indicates failure.


  A two dimensional array, with dimensions LONARR(11,NTAGS),
  containing the size information of all tag elements in the
  structure. SIZES(*,i) is the SIZE-style vector for tag element i.


  Compute the sizes of the elements in X, defined here.
    IDL> x = {a: [1], b: intarr(2,2), c: reform(strarr(2,1),2,1)}
    IDL> help, /struct, x
    ** Structure <818c8b4>, 3 tags, length=28, refs=1:
        A INT Array[1]
        B INT Array[2, 2]
        C STRING Array[2, 1]
    IDL> print, byte(tagsize(x))
        1 [1] 2 1 0 0 0 0 0 0 0
        2 [2 2] 2 4 0 0 0 0 0 0
        2 [2 1] 7 2 0 0 0 0 0 0
    [ Array dimensions are emphasized with brackets ]
  Compare this to the type information returned by HELP, which is
  incorrect for tags A and C.
    IDL> help, x.a, x.b, x.c
    <Expression> INT = 1
    <Expression> INT = Array[2, 2]
    <Expression> STRING = Array[2]

See Also


Modification History

  Written, CM, 13 May 2000
  Documented, 05 Jul 2000
  Small documentation changes, CM, 31 Aug 2000
  Signficant cleanup of HELP parsing, CM, 04 Dec 2000
  Added case for array of structures with new parsing, CM 12 Jan
  $Id: tagsize.pro,v 1.4 2001/02/09 04:57:42 craigm Exp $

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