Let's say I have an abstraction that accepts a file path as a symbol argument.

[loadbang]
|
[symbol $1]

But maybe the user didn't provide a symbol. In that case, I'd like to stop the data flow: if the symbol is not empty, do [open $1( and forward the message on; if the symbol is empty, don't trigger the "open" message at all.

[route] and [select] can match non-empty symbols but it isn't obvious how to match an empty symbol.

In any other programming environment, I could compare to an empty string... but I can't even find how to denote an empty string in Pd.

How to do this?

hjh

This may work for missing filenames in an abstraction, if there is no symbol a bang is passed to the rejection outlet otherwise the symbol passes through

A bit OT and not exactly elegant, but if you need a general solution that also works with floats:

It's the only way I know to see whether an argument (symbol or float) has been provided or not. It takes advantage of a string escape bug, credit goes to the Pd list.

@ddw_music Also [makefilename %s] can tell the difference...... simply not passing on an empty symbol..... nothing.... not even a bang.
It should be the simplest for your purpose, and it doesn't complain.
It keeps the symbol tag for file opening, and filenames with spaces sent from [openpanel] output do make it through complete.
David.

I usually just use [select symbol] with [symbol( sent to the right inlet:

@ddw_music In the case of creation arguments you can also use [pdcontrol] with the [args 0( message. If the args list is empty, [pdcontrol] will output a bang, if it contains a single symbol, it will output this symbol. So a [route symbol] will control for that.

Bonus: If the relevant file path is also relative to the patch, you can use [pdcontrol] to get the path to the directory of the patch and maybe just use the file name as creation argument of the abstraction.

Thanks for all of these. None of them is obvious.

hjh