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Re: byte offset in POINT_LUN [message #55266 is a reply to message #55264]

Wed, 08 August 2007 14:36

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

Josh writes:

> as i'm cruising through, reading a large ascii file, I'd like to keep
> track of the beginning of certain "blocks" of code. in the event that
> i find what i'm looking for within the block, I'd like to PRINTF the
> entire block (the beginning of which is now above my file position).
> I've used POINT_LUN to return to the start of a file before, by
> setting position=0, but is there a way I can use POINT_LUN to go to a
> specific line within the file (since I'm not sure how that would
> correspond to a byte offset) ?

I've never used POINT_LUN like this in an ASCII file,
but I had an idea that seems to work in the few tests
I've just made.

Suppose you open an ASCII data file:

OPENR, lun, 'myfile.txt', /GET_LUN

And suppose you read the first 10 lines of the
file:

data = StrArr(10)
READF, lun, data

Now, say you want to come back to this spot later,
you need to know where you are in the file. In an
unformatted file you could probably count and get
the value correctly. An ASCII file is *way* more
complicated, but POINT_LUN can tell us if we use
a "negative" value for the logical unit number:

POINT_LUN, -lun, thisPoint

Now you could read more data:

READF, lun, data

When you want to return to the previous point in the
file, you can use POINT_LUN again:

POINT_LUN, lun, thisPoint

And, viola! You are back! :-)

Cheers,

David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")

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[Message index]

		Re: byte offset in POINT_LUN By: badjelly.witch on Thu, 09 August 2007 15:44
		Re: byte offset in POINT_LUN By: Josh on Wed, 08 August 2007 15:15
		Re: byte offset in POINT_LUN By: David Fanning on Wed, 08 August 2007 14:36
		Re: byte offset in POINT_LUN By: James Kuyper on Wed, 08 August 2007 14:30
		Re: byte offset in POINT_LUN By: Josh on Wed, 08 August 2007 13:05

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