How to change certain lines in a file?

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Andrew
Andrew on 16 Apr 2015
Edited: Ken Atwell on 18 Apr 2015
Hello, I have a text file:
f2.txt:
1
2
3
4
5
6
7
8
9
10
11
Then, I want to change certain lines of this text file with another condition.
clc;
close all;
clearvars;
%declaring some variables
a=0;
b=1;
c=0;
d=1;
e=0;
f=1;
fin = fopen('f2.txt','r+'); %opening text file f2.txt in w+ mode
lcounter = 0; %lcounter is a line counter variable
while (lcounter<12)
nextline = fgetl (fin); %fgetl() gets nest line of the fin in nextline var
if (lcounter ==1 && a==0) %If both conditions are satisfied, then write **L1 to file
nextline = ['**L1'];
elseif (lcounter ==2 && b==0)
nextline = ['**L2'];
elseif (lcounter ==3 && c==0)
nextline = ['**L3'];
elseif (lcounter ==4 && d==0)
nextline = ['**L4'];
elseif (lcounter ==5 && f==0)
nextline = ['**L5'];
end
lcounter = lcounter + 1;
end
fclose (fin);
Here is the code. What I am trying to do is if both conditions are satisfied, I want to write something to the file. While debugging, I see control goes to the writing statement, but nothing is written to the file. Any idea how to do this?
  5 Comments
per isakson
per isakson on 17 Apr 2015
Or
  1. read one line at a time from f2.txt
  2. modify the line as needed
  3. write the line to a new file, temp.txt
  4. and finally delete f2.txt and rename temp.txt to f2.txt

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Answers (1)

Ken Atwell
Ken Atwell on 17 Apr 2015
As Per states, you can't edit an existing file unless the changes are exactly the same size (number of bytes) as the original, which is almost never the case in a text file. I know text editors give the illusion you're only editing individual lines, but in fact the entire file is likely being rewritten on save.
Honestly, I would write a second file line by line. Start with what you have: Read a line, maybe change it, and write the line to a second file that you fopen (with 'w') just before your for loop. You can always replace the original file with the new file when you're done with copyfile or similar -- but don't do that until your code is throughly debugged, or you'll keep clobbering your input data.
  2 Comments
Ken Atwell
Ken Atwell on 18 Apr 2015
Because most files, and especially plain text files, are just a long sequence of numbers (bytes). If you insert any new byte, every number after the insertion point must be pushed "to the right". If you delete a byte, everything after must be pulled "to the left".
So you're left with the choice of rewriting the file with every modification (ugh), or "stream" from an input file to an output file like Per and I have suggested.

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