R help - Oct 2009 - Simple extraction of level,x,y from contourLines()

A follow-up to my previous query.

Say one of the results returned by contourLines() is

  C.W <- contourLines(....)

Then C.W is a list of (in this case 28) lists,
each of which is a list with components $level (a single number),
$x (the x coords of the points on a contour at that level) and
$y (the y coords).

I can of course get these individually with, for the 5th one for
instance,

  C.W[[5]]$level
  C.W[[5]]$x
  C.W[[5]]$y

But I can't see how to obtain, in one line and without running
a nasty loop, to get all the levels at once!

In other words, I'm looking for an expression which will return
the vector

  c(C.W[[1]]$level,C.W[[2]]$level,...,C.W[[28]]$level)

(This is probably something I should already have learned from
the documentation, but I think I may have been asleep during
that class ... ).

With thanks,
Ted.

--------------------------------------------------------------------
E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 22-Oct-09                                       Time: 19:57:09
------------------------------ XFMail ------------------------------

On 22/10/2009 2:57 PM, (Ted Harding) wrote:
> A follow-up to my previous query.
> 
> Say one of the results returned by contourLines() is
> 
>   C.W <- contourLines(....)
> 
> Then C.W is a list of (in this case 28) lists,
> each of which is a list with components $level (a single number),
> $x (the x coords of the points on a contour at that level) and
> $y (the y coords).
> 
> I can of course get these individually with, for the 5th one for
> instance,
> 
>   C.W[[5]]$level
>   C.W[[5]]$x
>   C.W[[5]]$y
> 
> But I can't see how to obtain, in one line and without running
> a nasty loop, to get all the levels at once!
> 
> In other words, I'm looking for an expression which will return
> the vector
> 
>   c(C.W[[1]]$level,C.W[[2]]$level,...,C.W[[28]]$level)
sapply(C.W., function(x) x$level)

should do it.  (It will contain repeats if your contours have gaps in 
them; unique() can remove those.)

Duncan Murdoch
> (This is probably something I should already have learned from
> the documentation, but I think I may have been asleep during
> that class ... ).
> 
> With thanks,
> Ted.
> 
> --------------------------------------------------------------------
> E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
> Fax-to-email: +44 (0)870 094 0861
> Date: 22-Oct-09                                       Time: 19:57:09
> ------------------------------ XFMail ------------------------------
> 
> ______________________________________________
> 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.

Ted Harding asked:
>
> I can of course get these individually with, for the 5th one for
> instance,
> 
>   C.W[[5]]$level
>   C.W[[5]]$x
>   C.W[[5]]$y
> 
> But I can't see how to obtain, in one line and without running
> a nasty loop, to get all the levels at once!
> 
> In other words, I'm looking for an expression which will return
> the vector
> 
>   c(C.W[[1]]$level,C.W[[2]]$level,...,C.W[[28]]$level)
> 
Hmmmm...

Did you try this?

# reproducible example
C.W <- list(list(level = 1, x = 2), list(level = 2, y = 3), list(level = 10, 
z = 4))
sapply(C.W, function(x) x$level)

Alberto Monteiro