Using fprintf to write to multiple files simultaneously

I want to write the same text to the screen and in to a file. So I have to double the code:

fprintf('this is some text with numbers: %d, %d', x,y);
fprintf(fid, 'this is some text with numbers: %d, %d', x,y);

Alternatively I can introduce a text string to avoid double code:

str = sprintf('this is some text with numbers: %d, %d', x,y);
fprintf(str); 
fprintf(fid, str);

But obviously it does not work to use fprintf to write to mutliple files simultaneously? Would be nice if possible.

fprintf([1 fid],'this is some text with numbers: %d, %d', x,y);

Answer 1

[Ameer Hamza]

As far as I know, this feature is not natively supported by MATLAB. But if it is really important for your application, then you can try the following two methods.

First, create a custom function which takes an array of FIDs and call fprintf one by one on each of them. Save the following function in a file printf.m in MATLAB's path.

function printf(fid, varargin)
    for f = fid(:)'
        fprintf(f, varargin{:});
    end
end

Call this function

f = fopen('test.txt', 'w+');
printf([1 f 1 1 f], 'hello world\n');
fclose(f);

Second, overload the fprintf function. Create a folder name @double in your MATLAB's path and save the following function in file fprintf.m

function y = fprintf(fid, varargin)
    y = zeros(size(fid));
    for i = 1:numel(fid)
        y(i) = builtin('fprintf', fid(i), varargin{:});
    end
end

[Important note:] The definition of overloaded function is updated; the first code can break the compatibility in some cases.

Call this function

f = fopen('test.txt', 'w+');
fprintf([1 f 1 1 f], 'hello world\n');
fclose(f);

Note that this overloading will not break compatibility, other programs calling fprintf() will still work normally.

As a side note, these functions can also be written as one-liners without for-loop:

printf.m:

function printf(fid, varargin)
    arrayfun(@(f) fprintf(f,  varargin{:}), fid);
end

fprintf.m

function y = fprintf(fid, varargin)
    y = cell2mat( arrayfun(@(f) {builtin('fprintf', f, varargin{:})}, fid) );
end

[Mirko]

Thank you very much for your detailed and useful response!

[Ameer Hamza]

Glad to be of help.

Answer 2

[Walter Roberson]

You are correct, any one fprintf() can only write to one place at a time.

You should have a look at diary

[Lionel Trebuchon]

Hello Walter!

Do you know of a functionality (even hidden/undocumented) to write diaries/logs to two locations simultaneously? The code of diary is hidden to us.

The context is a slightly unstable system, and we want to enhance our chances to access the logs one way or the other.

We would rather prefer not to brute-force, i.e. copy the full logs, as they can get quite long.

Thanks!

Best, Lionel

[Steven Lord]

You could try launching MATLAB with the -logfile option and also call diary in that session. The two files won't be exactly the same but they may be close enough for your purposes.

But for problems like the original question and for this, if I wanted to log information to multiple places I'd write a function that I could call with a vector of file identifiers (and potentially some "dummy" identifier if I wanted to write to the screen as well) that would write to all those locations.

function logToFilesAndScreen(fid, varargin)
% logToFilesAndScreen Write text to one or more files and to the screen
%
% logToFilesAndScreen(fid, varargin) writes the information stored in
% varargin (which must contain valid inputs to the fprintf function when
% passed to it as a comma-separated list) to the screen and to all the
% files whose file IDs (as opened using fopen) are passed in as the fid
% input.

% Print to screen
fprintf(varargin{:})

% Print to file(s)
for whichfile = 1:numel(fid)
    fprintf(fid(whichfile), varargin{:})
end
end

[Walter Roberson]

Note: identifier 1 always writes to the screen (well, unless MATLAB has been run in non-interactive mode.)