Message-ID: <4E2081C1.4080904@gmail.com>
Date: 2011-07-15T18:06:57Z
From: Duncan Murdoch
Subject: Using str() in a function.
In-Reply-To: <CACk-te2stvgXRvOJz0Uc9TyX-qyPhj+ievtUU2UgyGGdUce=gw@mail.gmail.com>
On 15/07/2011 1:44 PM, Bert Gunter wrote:
> Below.
> -- Bert
>
> On Fri, Jul 15, 2011 at 10:31 AM, andrewH<ahoerner at rprogress.org> wrote:
> > Thanks, everybody, this has been very edifying. One last question:
> >
> > It seems that sometimes when a function returns something and you don't
> > assign it, it prints to the console, and sometimes it doesn't. I'm not sure
> > I understand which is which. My best current theory is that, if the function
> > returns NULL, by itself and not as part of some larger object, it does not
> > print it, but non-null values are printed. Is that correct?
>
> -- No.
> It depends on whether the function uses invisible() in the return,
> ?invisible
>
> If invisible() is not used and the value is not assigned, it's
> printed. Otherwise not.cf:
>
> f<- function()NULL
> g<- function()invisible(NULL)
>
> f() ## NULL is printed
> g() ## nothing printed
>
> z1<- f() ## nothing printed
> z2<- g() ## nothing printed
>
> z1 ## NULL
> z2 ##NULL
>
Right. And what invisible() does is set a flag so that the console is
told "don't print this". You can see the flag if you use the
withVisible() function. For example, with Bert's definitions,
> withVisible(f())
$value
NULL
$visible
[1] TRUE
> withVisible(g())
$value
NULL
$visible
[1] FALSE
Duncan Murdoch