[gradsusr] Can collect, gr2stn(), and coll2gr() be applied to defined variable?

[MY]

 Hi, dear all,

Can collect, gr2stn(), and coll2gr() be applied to defined variables? I use
the opengrads v2.0.a7.oga.3 little-endian version in the MS Windows system.

'define uu=ugrd'                  ;*   the ugrd is variable defined in ctl
file
'collect 1 free'
i=0
while(i<=10)
......
i=i+1
'collect 1 gr2stn(uu,'xlon','xlat')'
......
endwhile
'set x 1 'i
'd coll2gr(1,-u)'

I got error message:
************************
Error from COLL2GR:  Invalid dimension environment
Operation Error:  Error from coll2gr function
DISPLAY error:  Invalid expression
  Expression = coll2gr(1,-u)
************************

But, the following is OK:

.....
'collect 1 gr2stn(ugrd,'xlon','xlat')'
.....
'd coll2gr(1,-u)'


Thank you.

MY
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