If it looks like a duck and quacks like a duck, it ought to behave like a duck.
To the user a matrix and a dataframe look alike . . . except a dataframe can
hold non-numeric values. Thus to the users, a matrix looks like a special case
of a DF, or perhaps conversely. If you can address elements of one structure
using a given syntax, you should be able to address elements of the other
structure using the same syntax. To do otherwise leads to confusion and is
counter intuitive.
John
John David Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
(Phone) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing)>>> Petr
PIKAL<petr.pi...@precheza.cz> 3/1/2010 8:57 AM>>>
Hi
r-help-boun...@r-project.org napsal dne 01.03.2010 13:03:24:
< snip>
I understand that 2 dimensional rectangular matrix looks quite
similar to data frame however it is only a vector with dimensions.
As such it can have items of only one type (numeric, character, ...).
And you can easily change dimensions of matrix.
matrix<-1:12
dim(matrix)<- c(2,6)
matrix
dim(matrix)<- c(2,2,3)
matrix
dim(matrix)<-NULL
matrix
So rectangular structure of printed matrix is a kind of coincidence
only, whereas rectangular structure of data frame is its main feature.
Regards
Petr
--
Karl Ove Hufthammer
Petr, I think that could be confusing! The way I see it is that
a matrix is a special case of an array, whose "dimension" attribute
is of length 2 (number of "rows", number of "columns"); and "row"
and "column" refer to the rectangular display which you see when
R prints to matrix. And this, of course, derives directly from
the historic rectangular view of a matrix when written down.
When you went from "dim(matrix)<-c(2,6)" to "dim(matrix)<-c(2,2,3)"
you stripped it of its special title of "matrix" and cast it out
into the motley mob of arrays (some of whom are matrices, but
"matrix" no longer is).
So the "rectangular structure of printed matrix" is not a coincidence,
but is its main feature!
Ok. Point taken. However I feel that possibility to manipulate
matrix/array dimensions by simple changing them as I showed above
together with perceiving matrix as a **vector with dimensions** prevented
me especially in early days from using matrices instead of data frames and
vice versa.
Consider cbind and rbind confusing results for vectors with unequal mode.
Far to often we can see something like that
cbind(1:2,letters[1:2])
[,1] [,2]
[1,] "1" "a"
[2,] "2" "b"
instead of
data.frame(1:2,letters[1:2])
X1.2 letters.1.2.
1 1 a
2 2 b
and then a question why does not the result behave as expected. Each type
of object has some features which is good for some type of
manipulation/analysis/plotting bud quite detrimental for others.
Regards
Petr
To come back to Karl's query about why "$" works for a dataframe
but not for a matrix, note that "$" is the extractor for getting
a named component of a list. So, Karl, when you did
d=head(iris[1:4])
you created a dataframe:
str(d)
# 'data.frame': 6 obs. of 4 variables:
# $ Sepal.Length: num 5.1 4.9 4.7 4.6 5 5.4
# $ Sepal.Width : num 3.5 3 3.2 3.1 3.6 3.9
# $ Petal.Length: num 1.4 1.4 1.3 1.5 1.4 1.7
# $ Petal.Width : num 0.2 0.2 0.2 0.2 0.2 0.4
(with named components "Sepal.Length", ... , "Petal.Width"),
and a dataframe is a special case of a general list. In a
general list, the separate components can each be anything.
In a dataframe, each component is a vector; the different
vectors may be of different types (logical, numeric, ... )
but of course the elements of any single vector must be
of the same type; and, in a dataframe, all the vectors must
have the same length (otherwise it is a general list, not
a dataframe).
So, when you print a dataframe, R chooses to display it
as a rectangular structure. On the other hand, when you
print a general list, R displays it quite differently:
d
# Sepal.Length Sepal.Width Petal.Length Petal.Width
# 1 5.1 3.5 1.4 0.2
# 2 4.9 3.0 1.4 0.2
# 3 4.7 3.2 1.3 0.2
# 4 4.6 3.1 1.5 0.2
# 5 5.0 3.6 1.4 0.2
# 6 5.4 3.9 1.7 0.4
d3<- list(C1=c(1.1,1.2,1.3), C2=c(2.1,2.2,2.3,2.4))
d3
# $C1
# [1] 1.1 1.2 1.3
# $C2
# [1] 2.1 2.2 2.3 2.4
Notice the similarity (though not identity) between the print
of d3 and the output of str(d). There is a bit more hard-wired
stuff built into a dataframe which makes it more than simply
a "list with all components vectors of equal length). However,
one could also say that "the rectangular structure is its
main feature".
As to why "$" will not work on matrices: a matrix, as Petr
points out, is a vector with a "dimensions" attribute which
has length 2 (as opposed to a general array where the length
of the dimensions attribute could be anything). Hence it is
not a list of named components in the sense of "list".
Hence "$" will not work with a matrix, since "$" will not
be able to find any list-components. which is basically what
the error message
d2$Sepal.Width
# Error in d2$Sepal.Width : $ operator is invalid for atomic vectors
is telling you: d2 is an atomic vector with a length-2 dimensions
attribute. It has no list-type components for "$" to get its
hands on.
Ted.
--------------------------------------------------------------------
E-Mail: (Ted Harding)<ted.hard...@manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 01-Mar-10 Time: 12:03:21
------------------------------ XFMail ------------------------------
______________________________________________
R-help@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.
______________________________________________
R-help@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.
Confidentiality Statement:
This email message, including any attachments, is for th...{{dropped:6}}
______________________________________________
R-help@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.
