statar | R package for data manipulation — inspired by Stata 's API | Development Tools library

I am brand-new to R and trying to understand the basic syntax of functions.

In both of the functions f(x1) and g(x,1) below, I would like to generate y=2. Only the former works.

I'm familiar with the str_interp() and paste() functions, but those seem to work only in the context of strings, not variables. E.g., prefixnum <- str_interp("${prefix}${num}") doesn't solve the issue.

My motivation is that I'd like to call a function by specifying components of variable names. My background is in Stata, where placeholders are designated with a backtick and a tick (e.g., `prefix'`num'). I've consulted a few relevant resources, to no avail.

As an aside, I've read varying thoughts about whether variables should be prefixed with its dataframe (e.g., df$var). What is the logic behind whether or not to follow this convention? Why does f(df$x1) work, but writing f(x1) and modifying the function to be y <- df$var*2 not work?