| Re: Global Variable [message #26835 is a reply to message #19399] |
Mon, 01 October 2001 02:53   |
R.Bauer
Messages: 1424
Registered: November 1998
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Senior Member |
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Martin Downing wrote:
>
> First comment is to only use global variables sparingly, sometimes they are
> appropriate but try not to rely on them to make programming simpler.
> I know of three ways, the SYSTEM variables are fully global, and may be user
> defined, COMMON BLOCK variables are shared between those routines which
> include a
> declaration for the common block, these are very suitable for those programs
> where you cant find a way of writing your code without creating global
> variables, but do not want the variables to be seen by any old routine.
>
> system variable eg:
> ------------
> IDL>filepaths = {tag:"FILEPATHS", bin:bin)
> IDL>defsysv, '!fpaths',filepaths ; a variable for local paths
>
> now you can use this variable from anywhere in your code as:
>
> IDL> print, !fpaths.bin
> d:\martin\bin\
> ------------
>
> For common block variables you need a block identifier then a list of
> variables in the block. Note that the order is all that is relevent and that
> the first declaration defines the size of the block: eg
>
> pro foo
> COMMON FOO_COMMON_VARIABLES, a,b,c,d
> a = "first"
> b="second"
> c="third"
> d="fourth"
> print_foo
> end
>
> pro print_foo
> ; note later calls do not have to decare all block variables
> COMMON FOO_COMMON_VARIABLES, d,c,var3
> print, d
> print, c,
> print, var3
> end
>
> the result of calling this procedure is demonstrates that there is no
> association between variable names inthe common block just the variable
> order.
> IDL> foo
> first
> second
> third
>
> ------------------------------
> Lastly there is a *really* nice way of passing around all the 'global'
> variables you need in your widget programs using get/set_uvalue which. This
> is my favourite tip learnt from David's books so I shall leave him to
> describe. In essence you call:
> WIDGET_CONTROL, base, SET_UVALUE=info_structure
> WIDGET_CONTROL, event.top, GET_UVALUE=info_structure
>
> cheers
>
The fourth way is to use Pointers
value=10
ptr=ptr_new(value)
help, *ptr
Reimar
--
Reimar Bauer
Institut fuer Stratosphaerische Chemie (ICG-1)
Forschungszentrum Juelich
email: R.Bauer@fz-juelich.de
http://www.fz-juelich.de/icg/icg1/
============================================================ ======
a IDL library at ForschungsZentrum Juelich
http://www.fz-juelich.de/icg/icg1/idl_icglib/idl_lib_intro.h tml
http://www.fz-juelich.de/zb/text/publikation/juel3786.html
============================================================ ======
read something about linux / windows
http://www.suse.de/de/news/hotnews/MS.html
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