Why does FPRINTF return an error when the file identifier 0 is specified in MATLAB 7.3 (R2006b)?

The FPRINTF function returns an error if the fid value is specified as 0 in MATLAB 7.3 (R2006b). In previous releases we have used a file identifier value of 0 when it was necessary to suppress printing to the command window.

fprintf(1, 'I use this to supress printing to the Command Window\n')

produces an output:

 I use this to supress printing to the Command Window

However, the statement:

fprintf(0, 'I use this to supress printing to the Command Window\n')

produces the following error:

 ??? Error using ==> fprintf

 Operation is not implemented for requested file identifier.

Answer 1

[MathWorks Support Team]

This error was deliberately introduced in MATLAB 7.3 (R2006b) as the previous behavior was incorrect. The documentation for MATLAB 7.2 (R2006a) does not state that a file identifier (fid) value of 0 is allowed by the FPRINTF function. The valid file identifier values are only those returned by the FOPEN function as well as 1 (for stdout) and 2 (for stderr).

A file identifier value of 0 indicates stdin and printing out to stdin is not correct. In previous releases no action was taken when a file identifier value of 0 was specified, but it was not clear to users that an fid value of 0 is in fact incorrect. Thus, the error has been added to make that clear.