Do You Have to get() Objects?

13/04/2023 /

Original link: https://yihui.org/en/2023/04/get-objects/

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Last month I saw an interesting
question on Stack Overflow, in
which the OP wanted to print a series of data frames as tables, and tried double
loops, which did not work:

 for (i in c("CP", "BK", "IT", "WP")) { for (j in c("DD", "SI")) { data <- get(paste0(i, "_", j, "_comb1")) print(data) } }

There are two reasons why this won’t work:

  1. print() doesn’t create tables. You must create a table either explicitly
    (eg, via knitr::kable() ) or inexplicitly (eg, via the df_print
    option
    in R Markdown).
  2. Tables must be printed in top-level
    expressions     . They won’t be generated
    inside loops by default.

I posted an answer there to create top-level expressions to print the data
frames, base on the chunk option code . However, I think there must be a deeper
problem, and I might have provided a right answer to a wrong question. That is,
if you have to get() a series of objects by their names, and the names share
a pattern (eg, *_*_comb1 in the original question), perhaps these objects
shouldn’t have existed in the first place!

If they have existed, that means the author must have created them in a way like
this:

 CP_DD_comb1 <- ... BK_DD_comb1 <- ... IT_DD_comb1 <- ... WP_DD_comb1 <- ... CP_SI_comb1 <- ... BK_SI_comb1 <- ... IT_SI_comb1 <- ... WP_SI_comb1 <- ...

The code in ... must also have a pattern, like
subset(data, X1 == "CP" & X2 == "DD") , which is likely to violate the DRY
principle (Don’t Repeat Yourself).

When you find yourself create a series of objects with a pattern in their names,
I’d say you are probably doing it wrong. Instead of creating these global
objects, you really should create one object only—a list that contains these
objects. For example, if you want to partition the mtcars data by the cyl
and am variables, you shouldn’t create objects like cyl_8_am_0 . Instead, you
create a list:

 mtcars_list <- split(mtcars, ~ cyl + am)

Then if you want to create a table for each combination of cyl and am , you
simply write one line of code:

 knitr::kable(mtcars_list)

That’s much more elegant and simpler than first creating several global objects
(which can pollutes the global environment), then retrieving them via get()
(which can bite you if you are not careful enough about its envir , mode , and
inherits arguments), and finally printing them one by one.

I’d argue that you should rarely need to use get() in your daily code, just
like you should rarely need eval() .
resort to get() , there is likely to be a deeper problem and you may need to
pause and rethink.

This article is transferred from: https://yihui.org/en/2023/04/get-objects/
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