start to write for a specified location using fprintf

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sermet OGUTCU 2021-7-12

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评论： sermet OGUTCU 2021-7-13

FileName1='data_file';
result_1=23.9;
FileName2='data_file2';
result_2=21.9;

startingFolder='C:\Program Files\MATLAB';
 if ~exist(startingFolder, 'dir')
    startingFolder = pwd;
 end
 defaultFileName=fullfile(startingFolder, '*.txt');
 [baseFileName, folder]=uiputfile(defaultFileName, 'Select a file');
  if baseFileName == 0
    return
  end
  fullFileName = fullfile(folder, baseFileName);   %fullfile=building file
  fid = fopen(fullFileName, 'w');

How can I write a text file (starting with line 2) for the results as following:

result_1 result_2

data_file: 23.9 data_file2: 21.9

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Answer 1

Scott MacKenzie 2021-7-12

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Here's a solution. Make sure you use "r+" when opening the file, otherwise you'll overwrite the first line.

fid = fopen('testdata.txt', 'r+'); % open file for reading and writing
fgetl(fid); % skip over 1st line 
fprintf(fid','data_file: %.1f       data_file2: %.1f\n', result_1, result_2);
fclose(fid);

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Walter Roberson 2021-7-12

My requirement is that I must start writing at line 2 for an empty text file.

That is not possible in any operating system that MATLAB has been supported on in the last 20 years. The only operating system that such an operation was possible, and which MATLAB was available for, was DEC's VAX VMS operating system. However, MATLAB never supported the operation. VMS supported two completely different modes of writing files: RMS ("Record Management System"), or as byte streams (what every supported operating system uses.) MATLAB never supported making RMS calls. However, out of all of the operating systems MATLAB has ever run on, VMS RMS was the only one that ever supported the possibility of creating a line 2 without also creating a line 1.

Scott's solution does not start with an empty text file: it starts with a text file that is assumed to have at least one line. It reads the line, and then writes a new second line, replacing whatever was there. If there were additional lines already in the file, they might be partly or fully overwritten with the new line, or they will be left in place.

Walter Roberson 2021-7-12

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@Scott MacKenzie

When you switch between reading (fgetl() in this case) and writing (fprintf() in this case), it is necessary to perform an fseek() operation to inform the operating system to flush buffers. It is acceptable to fseek() by 0 bytes relative to the current position: you do not have to move the current position, just call fseek() to prepare for the switch.

fid = fopen('testdata.txt', 'r+'); % open file for reading and writing
fgetl(fid); % skip over 1st line
fseek(fid, 0, 'cof');    %prepare to switch from read to write
fprintf(fid','data_file: %.1f       data_file2: %.1f\n', result_1, result_2);
fclose(fid);

sermet OGUTCU 2021-7-13

Dear @Walter, thank you very much for the clarification.

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Answer 2

Scott MacKenzie 2021-7-12

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fid = fopen('testdata.txt', 'w'); % change, as needed
fprintf(fid, '           result_1               result_2\n');
fprintf(fid','data_file: %.1f       data_file2: %.1f\n', result_1, result_2);
fclose(fid);

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Scott MacKenzie 2021-7-12

@Walter Roberson Thanks for pointing this out. Is this documented anywhere? I don't see any mention of this in the fseek documentation.

Walter Roberson 2021-7-12

POSIX requirement.

https://pubs.opengroup.org/onlinepubs/9699919799/functions/fseek.html

https://pubs.opengroup.org/onlinepubs/007904975/functions/fopen.html

When a file is opened with update mode ( '+' as the second or third character in the mode argument), both input and output may be performed on the associated stream. However, the application shall ensure that output is not directly followed by input without an intervening call to fflush() or to a file positioning function ( fseek(), fsetpos(), or rewind()), and input is not directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file.

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