Question

How can I select the second column of a dynamically named variable?

I create variables of the form “population.USA”, “population.Mexico”, “population.Canada”. Each variable has a column for the year, and another column for the population value. I would like to select the second column from each of these variables during a loop.

I use this syntax:

sprintf("population.%s", country)[, 2]

R returns the error: Error in sprintf("population.%s", country)[, 2] : incorrect number of dimensions

Answer 1

See ?get. Here is an example:

> country <- "FOO"
> assign(sprintf("population.%s", country), data.frame(runif(5), runif(5)))
> 
> get(sprintf("population.%s", country))[,2]
[1] 0.2241105 0.5640709 0.5945869 0.1830719 0.1895938

It is critically important to look at the object returned by a function if you get an error. It is immediately clear why your example fails if you just look at what it returns:

> sprintf("population.%s", country)
[1] "population.FOO"

At that point it would be immediately clear, if you didn’t already know or have thought to read ?sprintf, that sprintf() returns a string not the object of that name. Armed with that knowledge you would have narrowed down the problem to how to recall an object from the computed name?