writing in a pre-existing txt file ?

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I have a question regarding writing my outputs to a pre-existing text file in a specific line
example :
in the text file I have the following :
A =
B =
I want my code to write the results next to A and B in the text file
The results will be ( for example ):
A = 1 2 3 4 5 6
B = 4 5 6 7 8 9
Can we do that in MATLAB ?
  1 Comment
Walter Roberson
Walter Roberson on 15 Jan 2020
Note that it is not possible in any operating system MATLAB is supported on, to append text to a line, except by rewriting everything from that point to the end of file. Only the IBM MVS version of MATLAB (we are talking decades ago!) could put text on the end of a line, and only for files initialized a particular way; the mechanism involved each line being fixed length. DEC's VMS is the only operating system that MATLAB was ever been supported on that really supported inserting variable amounts of text to the end of a line without rewriting to end of file and without the lines being fixed length -- but for technical reasons, MATLAB never supported that particular kind of VMS file.
In all file systems and operating systems after the two I mention above, files are represented as streams of bytes, and the only way to add to a line is to copy the entire rest of file .

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Accepted Answer

Walter Roberson
Walter Roberson on 24 May 2021
S = fileread('YourFile.txt');
parts = regexp(S, '\r?\n', 'split');
if isempty(parts{end}); parts(end) = []; end %trailing newline
parts{1} = [parts{1} ' 1 2 3 4 5'];
parts{2} = [parts{2} ' 4 5 6 7 8'];
fid = fopen('NewFile.txt', 'wt');
fprintf('%s\n', parts{:});
It is not recommended that you write to the same file that is your input: if there were a problem in processing then you might have corrupted the original file.
As I discussed in January, it is not possible in any supported operating system to insert text at the end of a line without reading the entire rest of the file and rewriting it. All file systems on all systems supported by MATLAB are only streams of bytes that have no possible way to insert in place.
Walter Roberson
Walter Roberson on 24 May 2021
fprintf('%s\n', parts{:});
should have been
fprintf(fid, '%s\n', parts{:});
Mahmoud Khadijeh
Mahmoud Khadijeh on 25 May 2021
Thank You sir!
it works.

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More Answers (1)

Akira Agata
Akira Agata on 15 Jan 2020
Assuming the pre-existing text file is data.txt, how about the following?
cInput = readcell('data.txt','Delimiter','\n');
cAppend = {'1 2 3 4 5 6';'4 5 6 7 8 9'};
cOutput = strcat(cInput,cAppend);
Akira Agata
Akira Agata on 24 May 2021
The following error message you encountered is due to your MATLAB version (R2016a).
The readcell function has been introduced in R2019a.
> Undefined function or variable 'readcell'.

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