Hi that is why I consider matrix is just a vector with dimensions and data.frame is a rectangular structure similar to Excel table. That saved me a lot of surprises.
But I must admit I am not a real beginner nowadays although I still learn when using R, reading help list and trying sometimes to help others. Regards Petr "John Sorkin" <jsor...@grecc.umaryland.edu> napsal dne 03.03.2010 16:30:39: > Petr, > On the other hand . . . > > > mat<-matrix(1:12, 3,4) > > dat<-as.data.frame(mat) > > mat > [,1] [,2] [,3] [,4] > [1,] 1 4 7 10 > [2,] 2 5 8 11 > [3,] 3 6 9 12 > > dat > V1 V2 V3 V4 > 1 1 4 7 10 > 2 2 5 8 11 > 3 3 6 9 12 > > What you are demonstrating by your example is the manner in which the data are > organized deep in the guts of R, not the way people, especially R beginners > visualize objects in their mind. When I think of the integer sixty-nine, I > visualize 69, not 1000101 despite the fact that 69, as an integer is > represented in the computer as 1000101. > 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> 3/3/2010 9:44 AM >>> > "John Sorkin" <jsor...@grecc.umaryland.edu> napsal dne 01.03.2010 > 15:19:10: > > > 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 > > Well, matrix looks like a data.frame only on the first sight. > > mat<-matrix(1:12, 3,4) > dat<-as.data.frame(mat) > > > str(dat) > 'data.frame': 3 obs. of 4 variables: > $ V1: int 1 2 3 > $ V2: int 4 5 6 > $ V3: int 7 8 9 > $ V4: int 10 11 12 > > str(mat) > int [1:3, 1:4] 1 2 3 4 5 6 7 8 9 10 ... > > seems to me a pretty different look like. > > Regards > Petr > > > > 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 the sole use of > the > > intended recipient(s) and may contain confidential and privileged > information. > > Any unauthorized use, disclosure or distribution is prohibited. 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