Hello, The dataset "cats" contain information about the heart weight
("Hwt"), body weight ("Bwt") and gender ("Sex") of
a group of 144 cats. I write the following piece of code:
library(MASS)attach(cats)ratio <- Hwt/Bwtmale <- ratio[Sex ==
"M"]female <- ratio[Sex == "F"] My question is, when I
look at the object "ratio", it is just a list of 144 numbers with no
information about the gender of the cat that the ratio comes from, and yet the
command "ratio[Sex == "M"]" is able to pick out those
numbers of "ratio" for which the corresponding cat is male. Why is
this? If I write the code like library(MASS)cats$ratio <-
cats$Hwt/cats$Bwtmale <- cats$ratio[cats$Sex == "M"]... it also
works, which I suppose is because there is a correspondence between the
"Sex" variable and the "ratio" variable in the cats dataset.
Regards,Rasmus Hedegaard.
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Hi Rasmus, Things will be much less confusing if you don't use attach. I know that sounds flippent, but I'm quite serious. Best, Ista On Thu, Feb 28, 2013 at 7:50 AM, Rasmus Hedegaard <hedegaard_84 at hotmail.com> wrote:> > > > > Hello, The dataset "cats" contain information about the heart weight ("Hwt"), body weight ("Bwt") and gender ("Sex") of a group of 144 cats. I write the following piece of code: library(MASS)attach(cats)ratio <- Hwt/Bwtmale <- ratio[Sex == "M"]female <- ratio[Sex == "F"] My question is, when I look at the object "ratio", it is just a list of 144 numbers with no information about the gender of the cat that the ratio comes from, and yet the command "ratio[Sex == "M"]" is able to pick out those numbers of "ratio" for which the corresponding cat is male. Why is this? If I write the code like library(MASS)cats$ratio <- cats$Hwt/cats$Bwtmale <- cats$ratio[cats$Sex == "M"]... it also works, which I suppose is because there is a correspondence between the "Sex" variable and the "ratio" variable in the cats dataset. Regards,Rasmus Hedegaard. > [[alternative HTML version deleted]] > > ______________________________________________ > R-help at 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.
Below is what happens when you let hotmail format your message using html. Always use plain text emails. The command "attach(cats)" told R to put the data.frame in the search path so that the variables in cats would be visible without specifying the name of the data frame: detach(cats) data(cats) ratio <- Hwt/Bwt Error: object 'Hwt' not found attach(cats) ratio <- Hwt/Bwt R can find the variables in cats, but it does not automatically put new variables in cats so your command creates "ratio" as a separate variable. You can still use "sex" to subset "ratio" even though they are not in the same data.frame.> str(cats)'data.frame': 144 obs. of 3 variables: $ Sex: Factor w/ 2 levels "F","M": 1 1 1 1 1 1 1 1 1 1 ... $ Bwt: num 2 2 2 2.1 2.1 2.1 2.1 2.1 2.1 2.1 ... $ Hwt: num 7 7.4 9.5 7.2 7.3 7.6 8.1 8.2 8.3 8.5 ...> str(ratio)num [1:144] 3.5 3.7 4.75 3.43 3.48 ... If you want to add ratio to the cats data frame, you need to tell R to put it there:> cats$ratio <- Hwt/Bwt > str(cats)'data.frame': 144 obs. of 4 variables: $ Sex : Factor w/ 2 levels "F","M": 1 1 1 1 1 1 1 1 1 1 ... $ Bwt : num 2 2 2 2.1 2.1 2.1 2.1 2.1 2.1 2.1 ... $ Hwt : num 7 7.4 9.5 7.2 7.3 7.6 8.1 8.2 8.3 8.5 ... $ ratio: num 3.5 3.7 4.75 3.43 3.48 ... ---------------------------------------------- David L Carlson Associate Professor of Anthropology Texas A&M University College Station, TX 77843-4352> -----Original Message----- > From: r-help-bounces at r-project.org [mailto:r-help-bounces at r- > project.org] On Behalf Of Rasmus Hedegaard > Sent: Thursday, February 28, 2013 6:51 AM > To: r-help at r-project.org > Subject: [R] Hidden information in an object > > Hello, The dataset "cats" contain information about the heart weight > ("Hwt"), body weight ("Bwt") and gender ("Sex") of a group of 144 cats. > I write the following piece of code: library(MASS)attach(cats)ratio <- > Hwt/Bwtmale <- ratio[Sex == "M"]female <- ratio[Sex == "F"] My question > is, when I look at the object "ratio", it is just a list of 144 numbers > with no information about the gender of the cat that the ratio comes > from, and yet the command "ratio[Sex == "M"]" is able to pick out those > numbers of "ratio" for which the corresponding cat is male. Why is > this? If I write the code like library(MASS)cats$ratio <- > cats$Hwt/cats$Bwtmale <- cats$ratio[cats$Sex == "M"]... it also works, > which I suppose is because there is a correspondence between the "Sex" > variable and the "ratio" variable in the cats dataset. Regards,Rasmus > Hedegaard. > [[alternative HTML version deleted]] > > ______________________________________________ > R-help at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting- > guide.html > and provide commented, minimal, self-contained, reproducible code.
Hello, Inline. Em 28-02-2013 12:50, Rasmus Hedegaard escreveu:> > > > > Hello, The dataset "cats" contain information about the heart weight ("Hwt"), body weight ("Bwt") and gender ("Sex") of a group of 144 cats. I write the following piece of code: library(MASS)attach(cats)ratio <- Hwt/Bwtmale <- ratio[Sex == "M"]female <- ratio[Sex == "F"] My question is, when I look at the object "ratio", it is just a list of 144 numbers with no information about the gender of the cat that the ratio comes from, and yet the command "ratio[Sex == "M"]" is able to pick out those numbers of "ratio" for which the corresponding cat is male. Why is this?Because Sex == "M" is a logical condition returning TRUE/FALSE. If you use a logical vector as an index, only the TRUE values will be selected. Read An Introduction to R, file R-intro.pdf in the doc directory of your installation of R, section 2.7 - Index Vectors. Hope this helps, Rui Barradas If I write the code like library(MASS)cats$ratio <- cats$Hwt/cats$Bwtmale <- cats$ratio[cats$Sex == "M"]... it also works, which I suppose is because there is a correspondence between the "Sex" variable and the "ratio" variable in the cats dataset. Regards,Rasmus Hedegaard.> [[alternative HTML version deleted]] > > ______________________________________________ > R-help at 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. >