[Rd] Error in "getClass" when calling the function "show"
Prof Brian Ripley
ripley at stats.ox.ac.uk
Tue Jun 10 15:44:10 CEST 2008
On Tue, 10 Jun 2008, John Chambers wrote:
> The function show() is intended as a mechanism for automatic display of
> objects, a vehicle for writing methods that control the automatic display
> of new classes of objects (as noted in its documentation and in
> "Programming with Data").
>
> That's why, unlike print() or plot(), it has no optional arguments to
> control its behavior.
>
> That said, it was always intended to call print() for S3 objects, as
> would be obvious from reading the code for showDefault(). It's just a
> glitch that it doesn't, as Brian inferred. We'll fix it, and your usage
> should then be perfectly OK. (The relevance of efficiency when one is
> printing objects for humans to look at is not compelling.)
Not compelling, but there are more hoops to jump through, and hence more
to fail (as here). One point that is a little more serious is that show()
is in methods, and with S3 objects it might be the case that the methods
package is not loaded and hence show() is not visible.
For completeness, I should point out that you can define S4 methods for
print() and S4 methods for show() for S3 classes. If you do that, things
may not work consistently (not least because you will have an S4 generic
'print' that may not be found in preference to base::print), but we do
discourage it.
I find it is rarely needed to call print() or show() explicitly, and when
I do I follow the same rule as auto-printing - use show() when I know I
have an S4 object and print() otherwise.
>
> John
>
> Laurent Gautier wrote:
>
> 2008/6/10 Prof Brian Ripley <ripley at stats.ox.ac.uk>:
>
>
> On Tue, 10 Jun 2008, Laurent Gautier wrote:
>
>
>
> 2008/6/10 Prof Brian Ripley <ripley at stats.ox.ac.uk>:
>
>
> showDefault has
>
> clDef <- getClass(class(object))
>
> Looks like the showDefault code intended
>
> clDef <- getClass(class(object), .force=TRUE)
>
> However, why are you calling show() on a non-S4 object? I cannot see any
> advtanges in doing so.
>
>
> I'd like *one* printing method for all objects, and the generic "show"
> is registered as working for ANYthing (see below) ?
>
>
> print() calls show() for S4 objects (with no additional arguments).
>
> I agree show() ought to do what it is documented to, but calling it on
> non-S4 objects is inefficient.
>
>
> Fair enough.
> May be that word of caution could appear in the documentation for "show" then ?
>
> A good place could be where the documentation says:
> The 'methods' package overrides the base definition of
> 'print.default' to arrange for automatic printing to honor methods
> for the function 'show'.
>
> which led me to think that "show" is covering more cases than "print" does
> (while apparently the opposite is happening with "print" delegating to "show").
>
>
>
> Thanks,
>
>
>
> Laurent
>
>
>
>
>
>
> Or is defining one's own function currently recommended ?
>
> myPrint <- function(x, ...)
> {
> if (isS4(x)) {
> show(x, ...)
> } else {
> print(x, ...)
> }
> }
>
>
>
> showMethods("show")
>
>
> Function: show (package methods)
> object="ANY"
> object="classRepresentation"
> object="derivedDefaultMethod"
> (inherited from: object="MethodDefinition")
> object="function"
> (inherited from: object="ANY")
> object="genericFunction"
> object="MethodDefinition"
> object="MethodsList"
> (inherited from: object="ANY")
> object="MethodWithNext"
> object="ObjectsWithPackage"
> object="signature"
> object="traceable"
>
>
>
> showMethods("print")
>
>
> Function "print":
> <not a generic function>
>
>
> getMethod("show", "ANY")
>
>
> Method Definition (Class "derivedDefaultMethod"):
>
> function (object)
> showDefault(object, FALSE)
> <environment: namespace:methods>
>
> Signatures:
> object
> target "ANY"
> defined "ANY"
>
>
>
>
> On Tue, 10 Jun 2008, Laurent Gautier wrote:
>
>
>
> Dear List,
>
> Calling "show" on an object of class "summary.lm" gives:
> Error in getClass(class(object)) : "summary.lm" is not a defined class
>
> Is this a miss on my end ?
>
>
>
>
> x <- seq(1, 10)
> show(x)
>
>
> [1] 1 2 3 4 5 6 7 8 9 10
>
>
> y <- runif(10)
> fit <- lm(y ~ x)
> show(fit)
>
>
> Call:
> lm(formula = y ~ x)
>
> Coefficients:
> (Intercept) x
> 1.04938 -0.08869
>
>
>
> show(summary(fit))
>
>
> Error in getClass(class(object)) : "summary.lm" is not a defined class
>
>
> class(summary(fit))
>
>
> [1] "summary.lm"
>
>
> class((fit))
>
>
> [1] "lm"
>
>
> getClass("lm")
>
>
> Virtual Class
>
> No Slots, prototype of class "S4"
>
> Extends: "oldClass"
>
> Known Subclasses:
> Class "mlm", directly
> Class "aov", directly
> Class "glm", directly
> Class "maov", by class "mlm", distance 2
> Class "glm.null", by class "glm", distance 2
>
>
> getClass("summary.lm")
>
>
> Error in getClass("summary.lm") : "summary.lm" is not a defined class
>
>
> sessionInfo()
>
>
> R version 2.7.0 Patched (2008-06-07 r45877)
> i686-pc-linux-gnu
>
> locale:
>
>
> LC_CTYPE=en_US.UTF-8;LC_NUMERIC=C;LC_TIME=en_US.UTF-8;LC_COLLATE=en_US.UTF-8;LC_MONETARY=C;LC_MESSAGES=en_US.UTF-8;LC_PAPER=en_US.UTF-8;LC_NAME=C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=en_US.UTF-8;LC_IDENTIFICATION=C
>
> attached base packages:
> [1] stats graphics grDevices utils datasets methods base
>
>
> Laurent
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>
>
> --
> Brian D. Ripley, ripley at stats.ox.ac.uk
> Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
> University of Oxford, Tel: +44 1865 272861 (self)
> 1 South Parks Road, +44 1865 272866 (PA)
> Oxford OX1 3TG, UK Fax: +44 1865 272595
>
>
>
> --
> Brian D. Ripley, ripley at stats.ox.ac.uk
> Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
> University of Oxford, Tel: +44 1865 272861 (self)
> 1 South Parks Road, +44 1865 272866 (PA)
> Oxford OX1 3TG, UK Fax: +44 1865 272595
>
>
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>
>
>
>
--
Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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