Strange behaviour dropping column from data.frame in R

Question

I\'ve encountered a strange behavior when dropping columns from data.frame. Initially I have:

> a <- data.frame(\"a\" = c(1,2,3), \"abc\" = c(3,2,1));          


        

Answer 1

From the R Language Definition [section 3.4.1 pg.16-17] --

https://cran.r-project.org/doc/manuals/r-release/R-lang.pdf

• Character: The strings in i are matched against the names attribute of x and the resulting integers are used. For [[ and $ partial matching is used if exact matching fails, so x$aa will match x$aabb if x does not contain a component named "aa" and "aabb" is the only name which has prefix "aa". For [[, partial matching can be controlled via the exact argument which defaults to NA indicating that partial matching is allowed, but should result in a warning when it occurs. Setting exact to TRUE prevents partial matching from occurring, a FALSE value allows it and does not issue any warnings. Note that [ always requires an exactmatch. The string "" is treated specially: it indicates ‘no name’ and matches no element (not even those without a name). Note that partial matching is only used when extracting and not when replacing.

Answer 2

While your exact question has already been answered in the comments, an alternative to avoid this behaviour is to convert your data.frame to a tibble, which is a stripped downed version of a data.frame, without column name munging, among other things:

library(tibble)
df_t <- as_data_frame(a)
df_t
# A tibble: 3 × 1
    abc
  <dbl>
1     3
2     2
3     1
> df_t$a
NULL
Warning message:
Unknown column 'a' 

Answer 3

Perhaps it's worth pointing out (since it didn't come up on the previous related question) that this partial matching behavior is potentially a reason to avoid using '$' except as a convenient shorthand when using R interactively (at least, it's a reason to be careful using it).

Selecting a column via dat[,'ind'] if you know the name of the column, but not the position, or via dat[,3] if you know the position, is often safer since you won't run afoul of the partial matching.

Answer 4

From the the help. ?$

name: A literal character string or a name (possibly backtick quoted). For extraction, this is normally (see under ‘Environments’) partially matched to the names of the object.

So that's the normal behaviour because the name is partially matched. See ?pmatch for more info about partial matching.

Cheers