[R] Why does this work?
Paul Murrell
p.murrell at auckland.ac.nz
Wed May 29 00:25:15 CEST 2002
Hi
> I waxs just checking examples in lattice levelplot and I ran into this
> strange (?) behavior I do not understand.
>
> Here is the example:
>
> x <- seq(pi/4, 5*pi, length = 100)
> y <- seq(pi/4, 5*pi, length = 100)
> r <- sqrt(outer(x^2, y^2, "+"))
> grid <- expand.grid(x=x, y=y)
>
> At this point grid is a 10000*2 data frame and r is a 100*100 matrix. Now
> comes a surprising part.
>
> grid$z <- cos(r^2) * exp(-r/(pi^3))
>
> This does not generate any error. But typing
>
> grid
>
> produces
>
> Error in data.frame(x = c(" 0.7853982", " 0.9361311", " 1.0868641", "
> 1.2375971", :
> arguments imply differing number of rows: 10000, 100
>
> Moreover
>
> dim(grid)
> [1] 10000 3
>
> but
>
> grid[1:5, ]
>
> produces an output with 102 columns named x, y, z.1, ..., z.100
>
> and
>
> grid[101, ]
>
> produces "subscript out of bound" error.
>
> Finally
>
> levelplot(z~x*y, grid, cuts = 50, xlab="", ylab="", main="Weird
> Function", colorkey = FALSE)
>
> generates a proper contour plot. Could somebody explain to me why this
> actually works? Thanks in advance,
I think the problem is that grid$z has a "dim" attribute which means
that it is being treated as a matrix in some of the cases above.
Compare ...
attributes(grid$x)
attributes(grid$y)
attributes(grid$z)
... You can also see this if you just do ...
grid$x
grid$y
grid$z
... grid$x and grid$y are just vectors of 10000 values, but grid$z is a
100x100 matrix of values. Now, if you define grid$z as ...
grid$z <- as.numeric(cos(r^2) * exp(-r/(pi^3)))
... ALL of your examples work (grid$z no longer has a "dim" attribute).
The examples that worked before were presumably doing an explicit
as.numeric() conversion already (or were just not checking for the "dim"
attribute).
Hope that helps.
Paul
-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !) To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._
More information about the R-help
mailing list