On 15/07/2011 1:44 PM, Bert Gunter wrote:
Below.
-- Bert
On Fri, Jul 15, 2011 at 10:31 AM, andrewH<ahoer...@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
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