Question

I have a p structure in R memory and I’m trying to access the Rate column of the matrix.

When I type p$6597858$Sample in the console, I get …

 p$`6597858`$Sample
        Rate             Available                 X                    Y
[1,]    1.01              1520.93                0.00                  0.0
[2,]    1.02               269.13                0.00                  0.0
[3,]    1.03               153.19                0.00                  0.0
[4,]    1.04               408.80                0.00                  0.0

and so on …

Within my code when I try to

get("p$`6597858`$Sample[,1]") 

I get this returned …

object 'p$`6597858`$Sample[ ,1]' not found

Is this an apostrophe problem?

Answer 1

Neither the $ nor the [[ operator work within get() (because p[[1]] is not an R object, it’s a component of the object p).

You could try

p <- list(`6597858`=list(Sample=data.frame(Rate=1:3,Available=2:4)))
z <- eval(parse(text="p$`6597858`$Sample[,1]"))

but it’s probably a bad idea. Is there a reason that

z <- p[["6597858"]][["Sample"]][,"Rate"]

doesn’t do what you want?

You can do this dynamically by using character variables to index, still without using get: for example

needed <- 1234
x <- p[[as.character(needed)]][["Sample"]][,"Rate"]

(edit: suggested by Hadley Wickham in comments) or

x <- p[[c(as.character(needed),"Sample","Rate")]]

(if the second-lowest-level element is a data frame or list: if it’s a matrix, this alternative won’t work, you would need p[[c(as.character(needed),"Sample")]][,"Rate"] instead)

This is a situation where figuring out the idiom of the language and working with it (rather than struggling against it) will pay off …

library(fortunes)
fortune(106)

If the answer is parse() you should usually rethink the question.
   -- Thomas Lumley
      R-help (February 2005)

In general,

• extracting elements directly from lists is better (safer, leads to less convoluted code) than using get()
• using non-standard names for list elements (i.e. those such as pure numbers that need to be protected by backticks) is unwise
• [[ is more robust than $