Odp: A problem of is.list function

Hi
Hi all

I want to use function of is.list and is.data.frame in if-else 
statement,
but I get a trouble. To replicate this trouble, I run codes like that:

set.seed(123)
x <- rnorm(100)
x <- data.frame(matrix(x, 10, 10))
class(x)
[1] "data.frame"
is.list(x)
[1] TRUE
is.data.frame(x)
[1] TRUE

version
               _
platform       i386-pc-mingw32
arch           i386
os             mingw32
system         i386, mingw32
status
major          2
minor          13.1
year           2011
month          07
day            08
svn rev        56322
language       R
version.string R version 2.13.1 (2011-07-08)

The class of x is data frame, but when I use is.list, I also get a TRUE
result. Why does this happen?
R-intro document which shall be included in any installation tells you 
about data frame

6.3 Data frames
A data frame is a list with class "data.frame". There are restrictions on 
lists that may be made into data frames, namely 
The components must be vectors (numeric, character, or logical), factors, 
numeric matrices, lists, or other data frames. 
Matrices, lists, and data frames provide as many variables to the new data 
frame as they have columns, elements, or variables, respectively. 
Numeric vectors, logicals and factors are included as is, and character 
vectors are coerced to be factors, whose levels are the unique values 
appearing in the vector. 
Vector structures appearing as variables of the data frame must all have 
the same length, and matrix structures must all have the same row size. 

Regards
Petr
Lian Peng

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Odp: A problem of is.list function

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