When you can't concatenate structures like you expect....

[Paul van Delst <paul.vandelst(at)ssec.wisc.edu>]

Figure this one out.

Create a structure:

IDL>
t={latitude:0.0,longitude:0.0,decimal_time:0.0,c_date:'21-May-1999'}
IDL> HELP, t, /STRUCT
** Structure <10375708>, 4 tags, length=20, refs=1:
   LATITUDE        FLOAT           0.00000
   LONGITUDE       FLOAT           0.00000
   DECIMAL_TIME    FLOAT           0.00000
   C_DATE          STRING    '21-May-1999'

Now replicate it into some structure arrays:

IDL> t1=REPLICATE( t, 5 )
IDL> t2=REPLICATE( t, 10 )
IDL> HELP, t1, t2, /STRUCT
** Structure <10375708>, 4 tags, length=20, refs=3:
   LATITUDE        FLOAT           0.00000
   LONGITUDE       FLOAT           0.00000
   DECIMAL_TIME    FLOAT           0.00000
   C_DATE          STRING    '21-May-1999'
** Structure <10375708>, 4 tags, length=20, refs=3:
   LATITUDE        FLOAT           0.00000
   LONGITUDE       FLOAT           0.00000
   DECIMAL_TIME    FLOAT           0.00000
   C_DATE          STRING    '21-May-1999'

Concatenation is no problem:

IDL> HELP, [ t1, t2 ]
<Expression>    STRUCT    = -> <Anonymous> Array[15]

Now create a different structure with *exactly* the same fields and
*exactly* the same data types and dimensions, and replicate it:

IDL>
s={latitude:0.0,longitude:0.0,decimal_time:0.0,c_date:'21-May-1999'}
IDL> HELP, s, /STRUCT
** Structure <1031e108>, 4 tags, length=20, refs=1:
   LATITUDE        FLOAT           0.00000
   LONGITUDE       FLOAT           0.00000
   DECIMAL_TIME    FLOAT           0.00000
   C_DATE          STRING    '21-May-1999'
IDL> s1=REPLICATE( s, 10 )
IDL> HELP, s1, /STRUCT
** Structure <1031e108>, 4 tags, length=20, refs=2:
   LATITUDE        FLOAT           0.00000
   LONGITUDE       FLOAT           0.00000
   DECIMAL_TIME    FLOAT           0.00000
   C_DATE          STRING    '21-May-1999'

Can't concatenate those two structures now:

IDL> HELP, [ t1, s1 ]
% Conflicting data structures: S1,concatenation.
% Execution halted at:  $MAIN$
IDL>

Bummer. If you note the structure names, the hex strings 10375708 and
1031e108, this seems to distinguish what structures you can and cannot
concatenate - is this what tells IDL about the strucutre *type*?

Anyway, my first workaround didn't work (as I expected, but..):

IDL> HELP, t1, s1
T1              STRUCT    = -> <Anonymous> Array[5]
S1              STRUCT    = -> <Anonymous> Array[10]
IDL> s2 = REPLICATE( s, 5 )
IDL> s3 = [ s1, s2 ]
IDL> HELP, s3
S3              STRUCT    = -> <Anonymous> Array[15]
IDL> s3[10:14]=t1[*]
% Conflicting data structures: <STRUCT    Array[5]>,S3.
% Execution halted at:  $MAIN$

Using "relaxed" structure assignment:

IDL> STRUCT_ASSIGN, s3[10:14], t1

Works fine.

Still, it's a pain in the butt when the internal representation of
structures dictates what you can and cannot concatenate. The discussion
of relaxed structure assignment in the IDL online help explains why it
is needed but the examples are for different structures where the tag
names are the same, but the data types/sizes are all different. Seems to
me that the "relaxed" assignment capability is great for those
situations where structures are quite different, but not for the simple
concatenation of structure arrays.

Anyway,

cheers,

paulv

--
Paul van Delst
Space Science and Engineering Center | Ph/Fax: (608) 265-5357, 262-5974
University of Wisconsin-Madison      | Email:
paul.vandelst@ssec.wisc.edu
1225 W. Dayton St., Madison WI 53706 | Web:
http://airs2.ssec.wisc.edu/~paulv