| Re: Data Handling [message #11621 is a reply to message #11548] |
Fri, 24 April 1998 00:00  |
Harald Jeszenszky
Messages: 9
Registered: January 1997
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Junior Member |
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Hello Neil,
I've tried to send you an E-mail several times but it has been returned by
your server
every time, so I answer you in the newsgroup:
> Harald
>
> Thanks for the help, it has been very useful. However, as I'm relatively
> new to IDL, some of the ideas have been hard to follow:
>
> IF (blocks EQ 1) THEN $ ; save block position
> pos = (FSTAT(unit)).CUR_PTR $
> ELSE $
> pos = [TEMPORARY(pos), (FSTAT(unit)).CUR_PTR]
>
> In the above piece of code I can't understand what "pos" does and where
> it is defined in the function.
>
> Many Thanks for your help
>
> Neil
I'm sorry for this rather complicated code but I'll try to explain it
to you:
- The variable "pos" contains the positions of the data blocks within
the data file.
- The function FSTAT(unit) returns an IDL structure containing status
information of the file associated with the logical unit and the
field CUR_POS reflects the current position of the file pointer. To
get access to a field of a structure variable you have to use the
'.' to reference the field.
The statement
pos = (FSTAT(unit)).CUR_POS
is a shorthand for:
info = FSTAT(unit)
pos = info.CUR_POS
- In IDL arrays can be concatenated by the "c=[a, b]" statement. The
statement
pos = [pos, value]
adds one element to the variable "pos". If the initial value of
"pos" is a scalar, the result is a 2-dimensional vector. The
"TEMPORARY()" function is used to speed up the concatenation
procedure and is useful for extending long arrays (see IDL
Reference Guide or Online Help).
After reading the 1st data header (blocks = 1), the variable "pos" is
initialized to a scalar containing the first data block position.
Each new data block (blocks > 1) extends the variable "pos" by one
element. If the data file contains "n" data blocks the variable "pos"
results to an "n"-dimensional vector.
With kind regards
Harald
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