Check empty vs not specified option

[Raffaele Grotti]

Dear Statalist,

I would like to implement an option of the form

program myprog
syntax, [myoption(string)]
...
...
end

How can I check if the option has been specified but left empty such as

myprog, myoption()

or if the option has not been specified such as

myprog

?

In both cases the local `myoption' will be empty, but how can i distinguish between the two cases?

Many thanks.

Raffaele

[Clyde Schechter]

As far as I know, you can't.

[German Rodriguez]

You can't with one option, but Nick Cox came up with a neat trick to get around this by using two options, as explained in Stata Tip 79 You define myoption and MYOPTION2(string) and then check both. If "`myoption'`myoption2'" == "" nothing was specified (case 1), else if "`myoption2'" == "" the option was specified but left empty (case 2), otherwise a string was specified and will be in myoption2 (I guess this would be case 3). Note that you don't say myoption(), instead you say myoption or myoption(string). I used this trick in markstat to allow the options beamer and beamer(theme).

[Nick Cox]

Thanks to German for his generous comments. It's hard for me to imagine that I invented that. It's also very hard for me to remember where I first encountered it or even used it independently. No matter: just use it!

[Raffaele Grotti]

Many thanks!
Very useful trick!