R help - Oct 2007 - Help deeded: Does a R graphical function return something special?

Hi All,

  When I tried to catch a returned value from a R graphical 
function or command (e.g., plot, lines, points, abline, title, 
xyplot, etc.), I always get a NULL value if it is executed 
correctly. For example:
  x <- c(1,2,3,4,5,9,13,19,36)
  plot.Comm <- plot(x) # plot.Comm <- try(eval(plot(x)))

then plot.Comm has a NULL value. Obviously, this is not a good way 
to check if or not it is a graphical function or command since 
plot.Comm <- NULL also lets plot.Comm be NULL.

Does anyone knows:

(1) that a R graphical function or command (e.g., plot, lines, 
points, abline, title, xyplot, etc.) could return something 
special if it is run correctly, which is different from those 
returned from executing any R non-graphical functions or 
commands?

(2) that there is an easy way for a R programmer to know that a R 
function or command is a graphical one before executing it?

  Thank you very much for the help in advance.
  Have a good weekend!
  Howard

What exactly are you trying to determine?  You can look at the help
page for a function to see what, if any, value is returned.

If you are 'try'ing to catch an error, then the following might help:
> x <- plot(0)
> x
NULL
> x <- plot()
Error in plot() : argument "x" is missing, with no
default
> x
NULL
> str(x)
 NULL
> x <- try(plot())
Error in plot() : argument "x" is missing, with no
default
> str(x)
Class 'try-error'  chr "Error in plot() : argument \"x\"
is missing,
with no default\n"
>
What you want to do to see if there is an error is to:

if (class(x) =='try-error').....

this will do it.  You can not just check the 'value'.

On 10/19/07, HU,ZHENGJUN <hhu at ufl.edu> wrote:
> Hi All,
>
>  When I tried to catch a returned value from a R graphical
> function or command (e.g., plot, lines, points, abline, title,
> xyplot, etc.), I always get a NULL value if it is executed
> correctly. For example:
>  x <- c(1,2,3,4,5,9,13,19,36)
>  plot.Comm <- plot(x) # plot.Comm <- try(eval(plot(x)))
>
> then plot.Comm has a NULL value. Obviously, this is not a good way
> to check if or not it is a graphical function or command since
> plot.Comm <- NULL also lets plot.Comm be NULL.
>
> Does anyone knows:
>
> (1) that a R graphical function or command (e.g., plot, lines,
> points, abline, title, xyplot, etc.) could return something
> special if it is run correctly, which is different from those
> returned from executing any R non-graphical functions or
> commands?
>
> (2) that there is an easy way for a R programmer to know that a R
> function or command is a graphical one before executing it?
>
>  Thank you very much for the help in advance.
>  Have a good weekend!
>  Howard
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem you are trying to solve?

On 10/19/07, HU,ZHENGJUN <hhu at ufl.edu> wrote:
> Hi All,
>
>   When I tried to catch a returned value from a R graphical
> function or command (e.g., plot, lines, points, abline, title,
> xyplot, etc.), I always get a NULL value if it is executed
> correctly. For example:
>   x <- c(1,2,3,4,5,9,13,19,36)
>   plot.Comm <- plot(x) # plot.Comm <- try(eval(plot(x)))
>
> then plot.Comm has a NULL value. Obviously, this is not a good way
> to check if or not it is a graphical function or command since
> plot.Comm <- NULL also lets plot.Comm be NULL.
>
> Does anyone knows:
>
> (1) that a R graphical function or command (e.g., plot, lines,
> points, abline, title, xyplot, etc.) could return something
> special if it is run correctly, which is different from those
> returned from executing any R non-graphical functions or
> commands?
>
> (2) that there is an easy way for a R programmer to know that a R
> function or command is a graphical one before executing it?
You should always consult the appropriate help page. ?lines mentions
no return value (and returns NULL), but ?hist and ?xyplot do describe
non-trivial return values.

-Deepayan