I though duck-typing was about type-independency? I could feed the bird object bread() or carrots(), or any other method, and that's okay as long as the bird doesn't die. And since ducks don't like carrots [at least, afaik]....
Quaaack! ;-) Albert-Jan ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In the face of ambiguity, refuse the temptation to guess. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ --- On Mon, 3/1/10, Patrick Burns <pbu...@pburns.seanet.com> wrote: From: Patrick Burns <pbu...@pburns.seanet.com> Subject: Re: [R] two questions for R beginners To: r-help@r-project.org Date: Monday, March 1, 2010, 5:08 PM If it looks like a duck and quacks like a duck, you ought to treat it like a duck. That is, use two subscripts: x[i, j] If you are an ornithologist, then you will know more precisely what can be done. Pat On 01/03/2010 14:19, John Sorkin wrote: > 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 ...{{dropped:28}}
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