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Here is the bug report I got from Professor Hsuan, and my exploration
of it. The cause of the problem is that a character vector is not
coerced to factor by xtabs or model.frame.
Is it a bug? I think yes. Character vectors are so frequently
coerced to factor that it is reasonable to assume that it will happen
here. The documentation is ambiguous as ?xtabs says
formula: a formula object with the cross-classifying variables,
separated by `+', on the right hand side. Interactions are
not allowed.
It says "variables", not "factors", implying to us that the coercion to
factor will take place.
Rich
######################
Date: Mon, 1 Dec 2003 21:53:06 -0500
From: "Francis Hsuan" <[EMAIL PROTECTED]> Add To Address Book
To: <[EMAIL PROTECTED]>
Cc: "Francis Hsuan" <[EMAIL PROTECTED]>
Rich, I got some strange output from R. Does this indicate
a bug in the xtabs() function? Thanks, Francis
> rm(list=ls(all=TRUE))
> Freq <- c(9, 2, 5, 4, 16, 26, 10, 28)
> H <- rep(c("yes", " no"), each=4)
> S <- rep(rep(c("yes", " no"), each=2), 2)
> A <- rep(c("yes", " no"), 4)
> xtabs(Freq~H+S+A)
Error in model.frame(formula, rownames, variables, varnames,
extras, extranames, :
invalid variable type
> Data3 <- data.frame(H, S, A, Freq)
> xtabs(Freq~H+S+A, Data3)
, , A = no
S
H no yes
no 28 26
yes 4 2
, , A = yes
S
H no yes
no 10 16
yes 5 9
######################
The bug is generated in model.frame.default by the line
data <- .Internal(model.frame(formula, rownames, variables,
varnames, extras, extranames, subset, na.action))
####
## Here is one repair
> H <- factor(H)
> S <- factor(S)
> A <- factor(A)
> xtabs(Freq~H+S+A)
, , A = no
S
H no yes
no 28 26
yes 4 2
, , A = yes
S
H no yes
no 10 16
yes 5 9
>
## Here is another way to generate the error
> class(Data3$H)
[1] "factor"
> Data3$H <- H
> class(Data3$H)
[1] "character"
> xtabs(Freq~H+S+A, Data3)
Error in model.frame(formula, rownames, variables, varnames, extras, extranames, :
invalid variable type
>
####
--please do not edit the information below--
Version:
platform = i386-pc-mingw32
arch = i386
os = mingw32
system = i386, mingw32
status =
major = 1
minor = 7.1
year = 2003
month = 06
day = 16
language = R
Windows XP Home Edition (build 2600) Service Pack 1.0
Search Path:
.GlobalEnv, package:methods, package:ctest, package:mva, package:modreg, package:nls,
package:ts, file:c:/HOME/rmh/hh/splus.library/.RData, package:grid, package:lattice,
Autoloads, package:base
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