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Re: Write to a certain line of a file [message #78957 is a reply to message #78956]

Thu, 19 January 2012 09:49

Michael Galloy
Messages: 1114
Registered: April 2006

Senior Member

On 1/19/12 9:43 AM, Mark Piper wrote:
> On 1/19/2012 5:18 AM, Brian J. Daniel wrote:
>> I haven't done this, but POINT_LUN may do what you want. You'd need
>> to know the number of bytes per line. You will still have to read the
>> entire line, change the bit you want to change, move the LUN back to
>> the beginning of the line, and rewrite it.
>>
>> OpenR, LUN, filename, /Get_Lun
>> Point_Lun, LUN, line_number * bytes_per_line
>> tmp=''
>> ReadF, LUN, tmp
>> ; change the line as you see fit
>> new_line = tmp + 'changed line!'
>> Point_Lun, LUN, line_number * bytes_per_line
>> PrintF, LUN, new_line
>> Free_Lun, LUN
>>
>> Code is UNTESTED!
>
> To add to Brian's idea, if you have a text file, use SKIP_LUN with the
> LINES keyword set. A crude example:
>
> pro replace_line_ex
> compile_opt idl2
>
> f0 = 'ascii.txt'
> f1 = 'ascii-1.txt'
> file_copy, file_which(f0), f1, /overwrite
>
> nskip = 7
> line = ''
> replacement = 'My dog has fleas.'
>
> openu, u, f1, /get_lun
> skip_lun, u, nskip, /lines
> point_lun, -u, mark
> readf, u, line
> nchars = strlen(line)
> point_lun, u, mark
> printf, u, replacement, format='(a-' + strtrim(nchars,2) + ')'
> free_lun, u
> end
>
> Diff 'ascii.txt' in the example/data directory of the IDL distro with
> 'ascii-1.txt' in your working directory.

You are essentially treating the file as a binary file where you have
more options, but also more responsibility for keeping things straight.

Sure, this would work in the case where the new output has the exact
same length in bytes as the old output, but if not everything is going
to get screwed up. You could rewrite the file after the point of
insertion to fix this, but how much is it worth it to get around writing
this file?

Mike
--
Michael Galloy
www.michaelgalloy.com
Modern IDL, A Guide to Learning IDL: http://modernidl.idldev.com
Research Mathematician
Tech-X Corporation

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[Message index]

		Re: Write to a certain line of a file By: David Fanning on Thu, 19 January 2012 11:02
		Re: Write to a certain line of a file By: Michael Galloy on Thu, 19 January 2012 09:49
		Re: Write to a certain line of a file By: Mark Piper on Thu, 19 January 2012 08:43
		Re: Write to a certain line of a file By: Brian Daniel on Thu, 19 January 2012 04:18
		Re: Write to a certain line of a file By: Brian Wolven on Wed, 18 January 2012 18:02
		Re: Write to a certain line of a file By: Matt Francis on Wed, 18 January 2012 17:16
		Re: Write to a certain line of a file By: Michael Galloy on Wed, 18 January 2012 12:08
		Re: Write to a certain line of a file By: natha on Wed, 18 January 2012 12:05
		Re: Write to a certain line of a file By: Mark Piper on Fri, 20 January 2012 08:21

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