| Re: Reading in to array elements from a file. [message #12272] |
Wed, 22 July 1998 00:00  |
davidf
Messages: 2866
Registered: September 1996
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Senior Member |
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Neil Rogers (nrogers@dera.gov.uk) writes:
> I wonder if anyone could solve the following mystery for me.
> I am trying to read in *individual* array elements from a file using
> the readf procedure. I have an input file called test.txt containing
> (say)
> 1 2 3 4 5 6
> and the following IDL procedure to read them in and print them out
> again.
> ;------------------
> pro test
> array=intarr(6)
> infile='test.txt'
> openr,inlun,infile,/get_lun
> for cnt=0,5 do begin
> readf,inlun,array[cnt]
> endfor
> close,inlun
> free_lun,inlun
> print,array
> return
> end
> ;-----------------
> Unfortunately, although no compile or run-time errors are encountered,
> nothing is assigned to the array elements and the file pointer stays
> at the beginning of the file. The output is then,
> 0 0 0 0 0 0
> Why is this?
The reason you can't read into a subscripted array has to
do with the way IDL passes information into and out of
procedures and functions. Variables are passed by
*reference*, meaning that it is actually the pointer
(a _real_ pointer here) or address that is passed into the
procedure or function. Changes made to the variable inside
the procedure or function are made to the "real thing".
For example, consider this procedure:
PRO JUNK, variable
variable = 10
END
If I call it like this:
thisVar = 5
Junk, thisVar
Print, thisVar
10
We see that thisVar has been changed by the procedure.
But variables are the *only* things that are passed by
reference. Everything else (and this includes things
like structure defererences, expressions, constants,
and--yes--subscripted variables) are passed by *value*.
This means that a COPY of the thing itself is passed
into the procedure or function.
So, for example, in our little example if we subscript
the variable like this then the variable outside the
procedure doesn't change:
thisVar = 5
Junk, thisVar[0]
Print, thisVar
5
Notice that this code doesn't cause any error messages.
In fact it is perfectly valid IDL code to pass a parameter
by value and not by reference.
As it happens, this is *exactly* what happens when you
do this:
array = IntArr(6)
For j=0,5 DO ReadF, 1, array[j]
The value array[j] is passed by VALUE, not by REFERENCE
and thus it's value outside the ReadF procedure can never
change, although internally to the ReadF procedure it is
doing exactly what it is suppose to be doing (i.e., reading
a value from the file into a *copy* of a subscripted
variable).
This behavior is so fundamental to the way IDL works that
it is unlikely to change in our lifetime. :-)
Cheers,
David
-----------------------------------------------------------
David Fanning, Ph.D.
Fanning Software Consulting
E-Mail: davidf@dfanning.com
Phone: 970-221-0438
Coyote's Guide to IDL Programming: http://www.dfanning.com/
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