[R] Hidden information in an object

[Rui Barradas]

Hello,

Inline.

Em 28-02-2013 12:50, Rasmus Hedegaard escreveu:
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> 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. 		 	   		
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