Do you use attach() or call variables by name or slicing?

Question

Many intro R books and guides start off with the practice of attaching a data.frame so that you can call the variables by name. I have always found it favorable to call variables with $ notation or square bracket slicing [,2]. That way I can use multiple data.frames without confusing them and/or use iteration to successively call columns of interest. I noticed Google recently posted coding guidelines for R which included the line

1) attach: avoid using it

How do people feel about this practice?

Answer 1

I never use attach. with and within are your friends.

Example code:

> N <- 3 > df <- data.frame(x1=rnorm(N),x2=runif(N)) > df$y <- with(df,{    x1+x2  }) > df           x1         x2          y 1 -0.8943125 0.24298534 -0.6513271 2 -0.9384312 0.01460008 -0.9238312 3 -0.7159518 0.34618060 -0.3697712 >  > df <- within(df,{    x1.sq <- x1^2    x2.sq <- x2^2    y <- x1.sq+x2.sq    x1 <- x2 <- NULL  }) > df           y        x2.sq     x1.sq 1 0.8588367 0.0590418774 0.7997948 2 0.8808663 0.0002131623 0.8806532 3 0.6324280 0.1198410071 0.5125870 

Edit: hadley mentions transform in the comments. here is some code:

 > transform(df, xtot=x1.sq+x2.sq, y=NULL)        x2.sq       x1.sq       xtot 1 0.41557079 0.021393571 0.43696436 2 0.57716487 0.266325959 0.84349083 3 0.04935442 0.004226069 0.05358049